computational and ultraviolet photoelectron spectroscopic studies of 5-substituted cyclopentadienes....

Computational and ultraviolet photoelectron spectroscopic studies of 5-substituted cyclopentadienes. Evidence that n-n orbital mixing in the HOMO is not the source

of the syn n-facial selectivity in Diels-Alder reactions

NICK H. WERSTIUK' AND JIANGONG MA Department of Chemistry, McMaster University, Hamilton, Ont., Canada L8S 4Ml

AND

JOHN B. MACAULAY AND ALEX G . FALLIS The Ottawa-Carleton Chemistry Institute, Department of Chemistry, University of Ottawa,

Ottawa, Ont., Canada KIN 6N5

Received April 14, 1992

NICK H. WERSTIUK, JIANGONG MA, JOHN B. MACAULAY, and ALEX G. FALLIS. Can. J. Chem. 70, 2798 (1992). A series of C5 monomethyl (la-If) and pentamethylcyclopentadienes (2a-2f) bearing stereogenic C5 heteroatom

substituents (-NH,, -OH, -OCH3, and -SCH3) have been studied computationally (ab initio and AM1) and with ultraviolet photoelectron (pe) spectroscopy. In the case of compounds Id-lf and 2d-2f, the conformations 3 and 4 of C, symmetry with the lone paris anti to the cyclopentadiene ring are computed to be the most stable geometries. On the other hand, the twisted-anti conformers 5a and 56 are the most stable geometries of l c and 2c. Analysis of the computed MO eigenvalues (orbital energies), MO eigenvectors (orbital coefficients), and the pe spectra of cyclopentadienes 2a-2f established that n-n orbital mixing is not important in the HOMO'S of lc-ld or 2c-2d. That the ionization energy of the HOMO is found to be virtually independent of the substituent at C5 in the series 2a-2f, provides support for the computational results. Because 2c-2d undergo Diels-Alder reactions selectively, syn to the heteroatom substituent, n-n orbital mixing in the HOMO cannot be the source of the n-facial selectivity observed for these compounds.

NICK H. WERSTIUK, JIANGONG MA, JOHN B. MACAULAY et ALEX G. FALLIS. Can. J. Chem. 70, 2798 (1992). Utilisant des calculs (ab initio et AM1) et la spectroscopie photo&lectronique ultraviolette, on a CtudiC une sene C5

. de rnonomCthy1 (la-lf) et de pentamCthylcyclopentadienes (2a-2f) portant des substituants hCtCroatomiques stCrCoginiques en C5 (-NH,, -OH, -OCH3 et -SCH3). Dans les cas des composds Id-lf et 2d-2f, les calculs ont permis d'Ctablir que les conformations 3 et 4 de symttrie C,, dans lesquelles les paires libres sont anti par rapport au cycle cyclopentadibne, possbdent les gtomCtries les plus stables. Par ailleurs, les conformbres anti dCformCs, 5a et 56, correspondent aux gComCtries les plus stables des composes l c et 2c. Une analyse des valeurs propres calculCes pour les OM (Cnergies des orbitales), des vecteurs propres des OM (coefficients des orbitales) et des spectres pe des cyclopentadibnes 2a-2f a permis d'ttablir qu'un melange d'orbitales n-n n'est pas important dans les OM hautes occupCes des produits lc-ld ou 2c-2d. Le fait que 1'Cnergie d'ionisation de I'OM haute occupCe soit virtuellement independante de la nature du substituant en C5, dans la sene 2a-2f, confirme les rCsultats des calculs. Compte tenu du fait que les composCs 2c-2d sub- issent des reactions de Diels-Alder ~Clectivement syn par rapport au substituant hCtCroatomique, on en conclut que le melange d'orbitals n-n dans I'OM haute occupee ne peut pas &tre la source de la sClectivitC T-faciale observee pour ces composCs.

[Traduit par la redaction]

Introduction

The origin of the r-facial diastereoselectivity observed in Diels-Alder (DA) reactions of C5 heteroatom-substituted cyclopentadienes is still imperfectly understood. Conse- quently, to determine the key factors responsible, a wide range of substrates have been studied both experimentally (1- 11) and computationally (1 1-18). These investigations re- vealed that a number of interrelated factors are important. Various suggestions have been advanced as possible sources for the pattern observed. These include steric effects, com- plexation between the diene and the dienophile, secondary orbital interactions, polarizability, electrostatic interactions, orbital mixing, and transition-state hyperconjugation. This latter interpretation, proposed originally by Cieplak (19) for reactions of cyclohexanones, is based on the concept of transition-state stabilization by a-electron donation from the adjacent bond into the vacant a orbital associated with the incipient bond. This explanation accounted for the syn approach of a butadiene to 5-fluoroadarnantane-2-thione where the electrostatic model failed (20). Macaulay and Fallis and

' ~ u t h o r to whom correspondence may be addressed.

co-workers extended these ideas to Diels-Alder reactions of 2,5-dimethylthiophenes oxides (21) and C-5 substituted cyclopentadienes (22) to rationalize the preference observed for cycloaddition anti to the antiperiplanar a bond that is the better donor.

The synthetic utility of the Diels-Alder reaction is well established (23). Nevertheless, the ability to control the facial selectivity without the use of a chiral auxiliary is often limited. Complete facial control renders the reaction enan- tioselective and with asymmetric, planar 1,3-dienes there exists a latent capacity to establish five or more chiral centres in one operation. Thus it is desirable to identify the factors that affect the r-facial diastereoselectivity so the stereo- chemistry of Diels-Alder reactions of C5 substituted cyclopentadienes can be predicted with certainty and the adducts utilized for the total synthesis of complex natural products.

We undertook a study of two groups of substituted cyclopentadienes. One group consisting of the C5 disubstituted cyclopentadienes l a - l e was studied computationally with a b initio and semiempirical methods. The other class, 2a- 2f, were synthesized earlier (9, 22) to establish experimentally the diastereoselective preferences in this series. The

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WERSTIUK ET AL.

TABLE 1. Diastereoselectivities for Diels-Alder re- A A -- actions of pentamethylcyclopentadienes with maleic

anhydride

Addition ratio

Compounda Reaction timeb syn anti

2c <30 s 10 0 -. 2d <I0 min 10 0 2e 3.5 h 10' 0 2f 27.5 h 1 9

"See ref. 9. ' ~ ~ ~ r o x i m a t e time for diene disappearance (TLC); reac-

tions were run at 22°C; ratios determined by integration of 'H NMR spectra of the total reaction mixture. SCHEME 1

'N-Phenylmaleimide adduct.

results are shown in Table 1". This group has been examined both computationally and experimentally with the aid of ultraviolet photoelectron spectroscopy (UPS) measurements. The objectives of the combined computational/UPS study were to determine the conformational preferences of the heteroatom substituents at C5 and gain information on the magnitude of the n-T orbital mixing in the HOMO in both series of compounds. This is important since it has been postulated that the latter effect is important in determining the syn-facial selectivity observed for reactions of cyclopentadienes bearing oxygen and chlorine substituents at C5 (13). According to the orbital mixing rule (13), for cyclopentadienes bearing heteroatom substituents at C5 the T- HOMO combines out of phase with the n- and a-orbitals in such a way that the n-orbital mixes with the T-HOMO (Scheme 1). This produces a nonequivalent extension of the T-MO in the direction of the substituent and leads to pref-

erential syn attack of the incoming dienophile. In view of the facial diastereoselectivities observed for 2c-2f (9, 22), we anticipated that a computational study of compounds la - l f and 2a-2f, coupled with a UPS study of 2a-2f, would establish whether n-T orbital mixing occurs in the HOMO of these compounds and thereby establish its importance in determining the T-facial selectivity of DA reactions of C5 heteroatom-substituted cyclopentadienes (cp's). Methyl substitution at C1, C2, C3, and C4 of the cp ring should differentially destablize the HOMO and lead to an increase in the E H ~ ~ o , E,, gap (E,,,, > E,,) in the series of compounds 2c-2f relative to the 5,5-disubstituted cp's, l a - l f . In principle, this should reduce the magnitude of the n-~,~,, interaction and, according to the orbital mixing model, result in a loss of facial diastereoselectivity.

In this paper we document and discuss the results of the ab initio and semiempirical calculations and analyze the

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2800 CAN. J . CHEM. VOL. 70. 1992

( R )n TABLE 3. Torsional angles o f twisted cyclopentadiene conformers

H3C-C5-X-R torsional angle (deg)

Diene Ab irzitio (4-3 1 G) AM 1

Ip - syn (n=l or 2) lp - anti (n=l or 2) l c 175.4, -51.4 177.4, -58.6 Id 66.4 67.0

SCHEME 2 l e 69.9 64.3 I f 66.9 66.5 2c (I 175.0, -65.5

TABLE 2. Ab initio relative energies o f 5,s-disubstituted cyclopentadiene conformers

2d 53.1 70.0 2e (1 42.0 2f 37.5 Relative energy (kcal mol-')"

51.4

"Not computed at the ab initio level. Diene Cs sYn Cs anti Twisted-syn

'The total energy (hartrees) of the most stable conformation is given in parentheses. All calculations were done at the 4-3 1G*"//4-3 1 G level.

T h i s is the twisted-atlri conformation of lc.

photoelectron (pe) spectra of 2u-2f. From the pe spectral data and the results of the ab initio and semiempirical computational studies, information was gained on the magnitude of the n-n interactions in dienes 2c-2f. This has led to new insights and improved our predictive capability.

Results and discussion Ab initio calculations on 5,5-disubstituted

TABLE 4. Ab initio orbital energies o f 5,5-disubstituted cyclopentadienes

Orbital energy (eV)"

Diene HOMO HOMO-1 HOMO-2 HOMO-3

l b l c syn

anti Twisted-anti

Id syn anti Twisted-syn

l e syn

" . cyclopentadienes anti 8.23 9.22 10.51 11.69

Ab initio calculations (24. 251 were camed out on cvclo- Twisted-syn 8.37 9.05 10.58 11.82 \ , ,

pentadienes la-le (i) to determine the preferred orienta- tion, syn or anti to the cp ring, of the lone pairs of X (Scheme "Computed with the 4-3 1GS*//4-31 G basis sets.

2), aid (ii) to gain information on the degree of the interaction of the lone pairs with the n-system from computed eigenvectors (orbital coefficients) and eigenvalues (orbital energies) with no methyl substituents on the carbons of the n-system. Calculations were first carried out on conformations with the lone pair(s) syn and anti to the cp ring, with forced C , symmetry in both cases. Optimizations were carried out with 3-21G and 4-31G basis sets, but the geometries obtained with the 4-31G calculations were used for single point calculations at the 4-31G** level. This was so because the differences in energy between the C , anti and C , syn conformers were computed to be smaller (in the order of 1 kcal mol-') with the 4-31G basis set than the 3-21G basis set. In fact, this led to a switch in the computed relative sta- bilities of the C , anti and C , syn conformers in the case of l c . When the 3-21G basis set was used the C s anti conformer was found to be slightly lower in energy (0.03 kcal mol-') than C s syn conformer; with the 4-3 1G basis set the C , syn conformer was 0.1 kcal mol-' lower in energy than the anti conformer. Single point calculations at the 4-31G** level on the 4-3 1G geometries gave the results documented in Table 2. Polarization functions were added to hydrogens in addition to the heavy atoms simply to balance the basis set. When the 4-3 1 G C , geometries of the anti conformations of Id , l e , and If were used as starting points for full optimizations in the absence of the symmetry constraint, C , sym-

metry was maintained. To establish with certainty that the Cs anti conformation is the global minimum for Id-lf , the CH3-C(5)-0-CH, dihedral angle of l e was set at 170" and the geometry was optimized with the 4-31G basis set without any symmetry constraints. That the dihedral angle was 179.9" in the optimized geometrical structure indicates that the C s anti conformation is the global minimum of l e . Furthermore a FORCE calculation on the 4-31G geometry gave no imaginary frequencies. Because the C , anti conformations are computed to be the most stable geometrical structures of Id and l f as well, frequency calculations were not carried out on these compounds. When the syn conformations of these compounds were optimized without the C , symmetry constraint, twisting occurred about the C5-X bond to relieve torsional strain. Table 2 lists the total and relative energies of three conformations of l c , Id , l e , and I f . Table 3 lists the torsional angles of the twist conformations and Table 4 documents the orbital energies of la- l f . As is seen from the data in Table 2, except for the amino compound, the Cs anti conformers of Id, l e , and I f are computed to be more stable than the C , syn and twisted-syn conformers, which would be considered to be the most re- active in the orbital mixing model. For the amino compound l c , the twisted-syn conformer 5a was computed to be

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FIG. 1. Plots of the 0.025 au contours of the HOMO'S of the C, FIG. 2. Plots of the 0.025 au contours of the HOMO'S of the C, syn (a) and twisted-anti (b) conformers of lc. syn (a) and anti (b) conformers of Id.

lower in energy than the Cs anti conformer with the 4- 3 1G**//4-3 1G basis sets.

Of importance is the nature of the three highest occupied MO's of lc, Id, le, and If. According to the ab initio (4-3 lG**) calculations, there is no significant interaction between the nitrogen and the T-system in the HOMO of the Cs syn conformer of lc; the orbital coefficients at N are <0.03.* A plot of the HOMO that is diene T-type is given in Fig. l(a). This plot, as well as the others obtained from ab initio calculations, was generated with PSI/88(26) and GAUSSIAN 90" checkpoint files; in this and all other ex- amples, the 0.025 au contour is plotted. Because PSI/88 uses only STO-3G, 3-21G, and 6-3 1G basis sets, the MO's were obtained from single point calculations with the 3-21G"' basis set on the 4-3 1G optimized geometry. It is important to note here that the orbital coefficients of the heavy atoms were virtually identical to the coefficients obtained with the 4-31G and 4-31G** basis sets. While the HOMO-1 is predominantly localized on N (coefficient 0.49) there are sizeable coefficients on the methyl carbon (0.20) and C5 (0.17) of the cp ring. There is little mixing with the cp T orbitals; the coefficients at C1, C2, C3, and C4 are less than 0.06. HOMO-2 predominantly cp T-type (orbital coefficients at Cl(C4), 0.17; at C2(C5), 0.23), with a sizeable coefficient on N (0.11). For the twisted-anti conformer of l c there is mixing of the N lp and the T orbitals in the HOMO as is seen in Fig. l(b). The coefficients on N, Cl(C4), and C2(C3) are 0.12, 0.28, and 0.23, respectively. For HOMO-1 the coefficients are 0.44 on N, 0.20 on the carbon (Cl) anti to the lp, 0.14 on C4, and 0.1 on C5. As was the case for the C, syn conformer, the HOMO-2 is predominantly cp ring T-type. The presence of an n-T interaction in the twisted conformation is supported by the fact that the HOMO is destabilized (0.16 eV) and the N lp is stabilized (0.29 eV) relative to the Cs anti conformer (Table 4). For the Cs anti conformer there is no significant mixing between the N lp and the T system in the HOMO. In accord with this result is the fact that the orbital energies (Table 4) do not change significantly relative to the syn conformer.

In the case of the Cs anti conformer of alcohol Id, a weak interaction between the oxygen (orbital coefficient = 0.1) and the T-system in the HOMO is indicated by the ab initio calculations with the 4-3 l **//4-3 1G basis sets. This interaction is purely T-type. HOMO-1 is predominantly diene T+

with some involvement of the oxygen (0.12) and CH, orbit-

'since 4-31G** is a split-valence basis set, only the largest coefficient of the pairs of orbitals is given in each case.

FIG. 3. Plots of the 0.025 au contours of the HOMO (a), and HOMO-1 (b) of the C, syn conformer and the HOMO (c) of the Cs anti conformer of le .

als. The third MO involves mixing of oxygen (0.42) with the cr orbitals of the C1-C5 and C4-C5 bonds of the cp ring; at the 4-31G**//4-31G level en- > en+ > en. A plot of the HOMO of Id computed with the 3-21~" ' basis set (the orbital coefficients were virtually identical to the coefficients computed with the 4-31G**//4-31G basis sets) is given in Fig. 2(a). For the anti conformer the coefficients on oxygen and the carbons of the HOMO are virtually identical to the coefficients of the HOMO of the syn conformer, as is seen from a comparison of Fig. 2(a) and Fig. 2(b).

In going from the Cs syn conformer of Id to the Cs syn conformer of the methoxy compound le, the computed energy of the HOMO increases only by 0.08 eV. Based on the magnitude of the coefficients on oxygen (0.12) and carbon, the interaction of the lp with the T-system of the diene (see Fig. 3(a)) is similar to the interaction in Id. The HOMO-1 of l e involves mixing of the oxygen orbitals within the framework of the cp ring (see Fig. 3(b). The shift of the HOMO- 1 from 1 1.65 to 10.96 eV (Table 3) is as expected for methyl substitution on oxygen. For le, HOMO-2 is

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2802 CAN. J . C H E M . VOL. 70, 1992

FIG. 4 . Plots o f the 0.025 au contours o f the HOMO (a) and HOMO-1 (b) o f the Cs syn conformer o f I f .

TABLE 5. AM1 relative energies o f 5,5-disubstituted cyclopentadiene conformers

Relative heat o f formation (kcal mol-')

Diene Cs syn Cs anti Twisted-syn

"This is the heat of formation of the twisted-anti conformation of lc .

qhese values were obtained using mixed parameters (C,H(AMl), S(MND0)).

similar to the HOMO-2 of Id: there is a minor degree of mixing of the oxygen and T carbon orbitals with the carbon coefficients predominating. For the C s anti conformer, the oxygen orbital coefficient is reduced (0.06) relative to the C s syn conformation. This is seen in a comparison of the HOMO'S given in Fig. 3(a) and Fig. 3(c). The HOMO and HOMO-2 are stabilized by 0.24 and 0.22 eV, respectively and the HOMO- 1 is stabilized by 0.13 eV.

For the C s syn conformer of If the orbital coefficients define a significant interaction between the sulfur lone pair and the T-system of the cp ring in the HOMO (S, 0.37), and HOMO- 1 (S, 0.5) (see Figs. 4(a) and 4(b). HOMO-2 and HOMO-3 also show.considerable mixing of S and c p ~ i n g orbitals. An interaction between S and the cp ring is also computed for HOMO, HOMO- 1, HOMO-2, and HOMO-3 of the anti conformer, as indicated by the coefficients on S and the ring carbons. In fact, based on the coefficients, the mixing is virtually identical to the mixing in the syn conformer.

AM1 calculations on 5,5-disubstituted cyclopentadienes AM1 calculations (27) were also carried out on cyclo-

pentadienes lc-lf as well as l a and lb. As seen from the data in Table 5, the conformational preferences of X-R computed by AM1 are the same as those computed for the series at the 4-31G** level. The anti conformations are the lowest-energy geometrical structures of Id, le , and If, but the twisted-anti conformer of l c is lower in energy than the anti or syn conformations. The orbital energies are given in Table 6.

TABLE 6 . AM1 orbital energies o f 5,5-disubstituted cyclopentadiene conformers

Orbital energy ( e V )


l a 9.07 10.93 l b 9.04 11.04 l c syn 9.08 9.90 11.42

anti 9.30 9.88 11.32 Twisted-anti 9.21 9.95 11.32

Id syn 9.16 11.43 11.65 anti 9.39 1 1 .OO 11.55 Twisted-syn 9.27 11.03 11.52

l e syn 9.10 10.38 11.19 anti 9.29 10.50 1 1.43 Twisted-syn 9.22 10.50 11.32

I f SYn 8.31" 9.37 10.62 11.57 anti 8.47" 9.36 10.62 Twisted-syn 8.48" 9.23 10.62

I f SYn 8.85b 9.43 10.62 anti 9 . 05~ 9.41 10.88 11.51

"These orbital energies were obtained using the AM1 parameters for C, H, and S.

qhese orbital energies were obtained using mixed parameters (C,H(AMl); S(MND0)).

TABLE 7 . Ab initio relative energies o f pentamethylcyclopentadiene conformers

Relative energy (kcal mol-')"

Diene syn anti Twisted-syn

2d 3.27 0(-462.40932) 2.63 2f 5.16 0(-823.72279) 4.12

T h e total energies (hartrees) of the most stable conformations are given in parentheses.

TABLE 8 . Ab initio orbital energies o f pentamethylcyclopentadiene conformers

-ei (eV)"


2b 7.38 10.15 2d syn 7.53 10.44 11.23

anti 7.67 10.59 11.25 Twisted-syn 7.59 10.48 11.36

2f SYn 7.53 9.00 10.00 11.30 anti 7.66 8.97 9.98 11.32 Twisted-syn 7.57 8.95 9.99 11.33

"Computed with the 4-31G**//4-31G basis sets.

Ab initio calculations on pentamethylcyclopentadienes Ab initio calculations were also canied out on 2b, 2d, and

2f with the 4-3 1G**//4-3 1G basis sets. As is seen from the data in Table 7, the hydroxyl group of 2d has the same conformational preference as Id; the anti conformer is more stable than the syn or twisted-syn conformers. The key point is that the HOMO is destabilized by 0.8 eV relative to I d (Table 8), but HOMO-2 is affected only marginally (0.1 eV). The orbital energies are shown graphically in Fig. 5. For the

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FIG. 5. Orbital energies of the syn (s), anti (a), and twist (t) conformers of 2b (HOMO and HOMO-l), 26 (HOMO, HOMO-1, and HOMO-2), and 2f (HOMO, HOMO-1, HOMO-2, and HOMO- 3) computed with the 4-31G**//4-31G basis sets.

FIG. 6. Plots of the 0.025 'au contours of the HOMO (a), HOMO-1 (b), and HOMO-2 (c) of the Cs syn conformer of 2d.

anti conformation the coefficient on oxygen is smaller (0.06) than the coefficient for the anti conformation of I d (0.10). Figures 6(a)-6(c) give the plots of HOMO, HOMO-1, and HOMO-2 of the Cs syn conformer of 2d and clearly show that there is no appreciable n-T orbital mixing in the HOMO and HOMO-1. On the other hand, the plot of HOMO-2 also shows that the mixing of the lp orbitals is n-u, not n-T. Figures 7(a)-7(d) show the plots of the HOMO, HOMO-1, HOMO-2, and HOMO-3 of the syn conformer of 2f. In this case there is n-T mixing in the HOMO (S, 0.20; C 1(C4), 0.26; C2(C3), 0.20). HOMO-1 is essentially ns (S, 0.62; C1 (C4), 0.098). HOMO-2 and HOMO-3 show a substan- tial amount of n-T mixing. The coefficients of the four highest occupied MO's of 2f are virtually identical to the

FIG. 7. Plots of the 0.025 au contours of the HOMO (a), HOMO-1 (b), and HOMO-2 (c), and HOMO-3 (d) of the Cs syn conformer of 2f.

TABLE 9. AM1 relative energies of pentamethylcyclopentadiene conformers

Relative energies (kcal mol-I)"

Diene syn anti Twisted-syn

2c 1.98 1.15 0(8. 151b 2d 3.16 0(-38.95) 1.58 2e 3.29 0(-31.79) 2.05 2f 2.01 O(8.31) 0.82 2 1.86 O(10.92) 0.94'

T h e heat of formation of the most stable conformer is given in parentheses.

?he heat of formation of the twisted-anti conformation.

These values were obtained with mixed AM1 parameters (C, H(AM1); S(MND0)).

coefficients of the HOMO, HOMO-1, HOMO-2, and HOMO-3 (plots not shown) of syn-2f, as are the orbital energies (Table 8).

AM1 calculations on pentamethylcyclopentadienes As is seen from the data in Table 9, the twisted-anti con-

former of 2c was computed as the most stable geometry with AM 1, in keeping with the results of the ab initio and AM 1 calculations on lc. The orbital energies given in Table 10 are shown graphically in Fig. 8. For 2d, 2e, and 2f the anti conformations are computed to be the most stable geometrical structures. These results are in accord with the ab initio/AMl calculations on Id, l e , and If and the ab initio calculations on 2d and 2f. Plots of HOMO, HOMO-1, and HOMO-2 of twisted-anti - 2c (not shown), syn-2d (Figs. 9(a)-9(c), and syn-2c (not shown) were obtained with PSI/88 and the AMPAC gpt files. It is seen that the MO's (the 0.025 au contours are plotted) of syn-2d are indistinguishable from

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CAN. J. CHEM. VOL. 70, 1992

--

2 a 2b 2 c 2d 2 e 2f 2 f (mixed)

FIG. 8. Orbital energies of 2a (HOMO and HOMO-l), 26 (HOMO and HOMO-l), and the syn (s), anti (a), and twist (t) conformers of 2c (HOMO, HOMO- 1, and HOMO-2), 26 (HOMO, HOMO- I , and HOMO-2), 2e (HOMO, HOMO- 1, and HOMO-2), and 2f (HOMO, HOMO-1, HOMO-2, and HOMO-3) computed with AM1. H1 and H2 are the values for HOMO-1 and HOMO-2 of 2e. The values for 2f (mixed) were obtained with mixed parameters (C,H(AMI); S(MND0)).

TABLE 10. AM1 orbital energies of pentamethylcyclopentadiene conformers

Orbital energies (eV)


2a 8.35 9.95 26 8.33 9.99 2c syn 8.38 9.71 10.26

anti 8.57 9.74 10.24 Twisted-anti 8.50 9.78 10.21

2d syn 8.43 10.18 10.59 anti 8.62 10.34 10.66 Twisted-syn 8.53 10.24 10.91

2e syn 8.39 10.12 10.16 anti 8.55 10.27 10.27 Twisted-syn 8.43 10.14 10.29

2f sYn 8.18 8.63 9.96 10.95 anti 8.36 8.60 10.02 10.91 Twisted-syn 8.30 8.55 9.96 10.91

2f sYn 8.4 1 " 9.02 10.02 11.91 anti 8.58" 9.02 10.13 10.99 Twisted-syn 8.46 9.02 10.02 11.11

'These values were obtained using mixed AM1 parameters (C, H(AM1): S(MND0)).

the 3-21~" ' MO's (Figs. 6(a)-6(c)). This is the case (not shown) for syn-2e as well. The HOMO and HOMO-1 of 2f computed with AM1 parameters for C, H, and S are shown in Figs. 10(a) and 10(b). Based on the orbital energies (Fig. 5) and MO's computed with the 4-3 1G**//4-3 1G and 3-21G"' basis sets, these AM1 calculations overestimate the ns-T interaction, primarily because the 11, orbital energy is too high. This analysis is supported by the pe spectium of 2f (vide infra). On the other hand, calculations with mixed AM 1 /MNDO parameters (C,H(AM l);S(MNDO) yielded

FIG. 9. Plots of the 0.025 au contours of the HOMO (a), HOMO-1 (b), and HOMO-2 (c) of the Cs syn conformer of 2d computed with AM1 .

results that compare closely to those obtained with ab initio calculations. The HOMO, HOMO-1, HOMO-2, and HOMO-3 or syn-2f are shown in Figs. 1 l(a)-1 l(d). The T-

type S lp is localized more when mixed parameters are used and the MO's closely resemble the 3-21~" ' MO's shown in Figs. 7(a)-7(d). This study was carried out to ascertain which set of parameters gives the best description of the orbital energies and MO's of 2f before a comprehensive AM1

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useful for studying conformational preferences, n-IT orbital mixing in other heteroatom substituted cp's, and the DA reactions of 2a-2e with maleic anhydride. Photoelectron spectra of cyclopentadienes

The pe spectra of 2a-2f are given in Figs. 12(a-12(f), respectively. The IE's are tabulated in Table 1 1 and shown graphically in Fig. 13. The T,, T - splittings of 2a and 2b are 2.07 2 0.05 and 2.04 2 0.05 eV, respectively. These values are similar to the splitting of 2.10 eV observed for 5,5- dimethylcyclopentadiene (28). In going from 2a to 2b there is a decrease of 0.08-0.10 eV in the IT IE's. Relative to cy-

FIG. 10. Plots of the 0.025 au contours of the H O M O ( a ) and clopentadiene (28) the IE's of the HOMO and HOMO-1 dk- H O M O - I ( b ) of the Cs syn conformer of 2f computed with A M 1 crease by 1.20 and 1.28 eV for 2a and 1.27 and 1.38 for 2b. parameters. Relative to 5,5-dimethylcyclopentadiene the shifts of the IT-

and IT, IE's of 2b are 1.17 and 1.23 eV, respectively. These results show that the addition of five or six methyls to the cyclopentadiene ring does not produce a significant differ- ential destabilization of IT, or IT-. According to the ab initio/AMl calculations the HOMO'S of 2a and 2b are basically diene IT-type mixed with IT-type orbitals of the methyl groups at C 1, C2, C3, and C4. No On the other hand, the HOMO- 1 does involve orbital mixing with the C-H and C-C bonds at C5.

For the amino compound 212, the IE of the HOMO does not shift significantly relative to 2a, but HOMO-2 is stabilized by 0.23 eV (Fig. 12(c)). According to the ab initial AM1 calculations, the band at 8.88 eV is due to the nitrogen lone pair. These experimental results are in accord with the AM 1 calculations, which showed that there is no significant interaction between the lone pair and the IT-system in the HOMO of 2c in either of the three conformations studied computationally.

For alcohol 2d the HOMO is stabilized relative to 2a and 2b by 0.17 and 0.24 eV (Fig. 12(d)). This is due to an in- ductive stabilization by oxygen. That the HOMO is stabilized is in accord with the calculations, which showed that there is no significant n-IT interaction in the HOMO. The next two bands are closely spaced, as predicted by the ab initio calculations. According to the ab initio/AM 1 calcu-

FIG. 1 1 . Plots of the 0.025 au contours of the H O M O ( a ) , lations there is no significant n-IT mixing in the HOMO-1, H O M O - 1 ( b ) , H O M O - 2 ( c ) , and H O M O - 3 (d) of the C, syn con- but HOMO-2 involves mixing of the lone pair orbitals and former of 2f computed with A M 1 using mixed parameters. the u (Cl-C5 and C4-C5) orbitals of the cp ring. This

explains why the bands due to HOMO-1 and HOMO-2 overlap rather than being split.

computational study3 of the Diels-Alder addition of maleic 1, going from 2d to ze, the HOMO ( ~ i ~ . lqe)) is slightly anhydride to 2a, 2c, 2d, 2e, ,and 2f was initiated. destabilized (0.1 eV), in keeping with the results of the

The ab itzitio and semiempirical computational studies computationa~ studies, HOMO-2 is stabilized by 0.28 e~ clearly establish that tz-IT mixing is not important in the relative to 2a, a consequence of an indictive effect and a weak HOMO of 2c, 2 4 and 2e. Furthermore, a comparison of the interaction with the oxygen lone pairs. ab initio and semiempirical results shows that AM1 calcu- In the case of 2f, the lowest IE band (Fig. 12(f) is de- lations give conformational preferences, eigenvalues (or- stabilized by 0.09 eV relative to the HOMO of 2a. This re- b i d energies), and eigenvectors (MO's) that compare sult is in keeping with he ab initio/AM ] calculations, which favourably with the ab itzitio calculations at the 4-3 1G** showed that there is an n-IT interaction in the HOMO of 2f. level. This result suggests that AM1 calculations will be Confirmation that this interaction is weak is provided by the

fact that the second band, computed to be predominantely 3 ~ e ( N . H . W. and J.M.) have found that syn approach of ma- r ~ , , remains sharp (FWHM = 0.2 eV) like the ns band of

leic anhydride to 2f is preferred when A M 1 parameters are used for C, H , and S; on the other hand, when mixed parameters are used, (CH3)2S (FWHM = 12 eV) (29). The other sulfur lone-pair

ant i addition is computed to be the low-energy pathway in keep- band is at lo. l9 eV. ing with the experimental results (Table 1 ) . A manuscript docu- Because the computed orbital energies, both ab initio and menting an A M 1 computational study of the D A reaction of 2b- AM1 (Tables 8 and 10) are insensitive to the dihedral angle 2f with maleic anhydride is being written. as shown graphically in Figs. 5 and 8, the predicted confor-

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2806 CAN. J. CHEM. VOL. 70. 1992

7 8 9 10 11 12 13 14 15 16 17 7 8 9 10 11 12 13 14 15 16 17

eV eV

FIG:' 12. Ultraviolet photoelectron spectra of 2a (a ) , 26 (b ) , 2c ( c ) , 2d ( d ) , 2e ( e ) , and 2f ( f ).

mational preferences, twisted-anti for 2c and anti for 2 4 2e, and 2f, cannot be corroborated with the pes data.

As far as the correlation between the computed and experimental n- ,n+ splittings of the dienes is concerned (Table 12), the ab initio calculations overestimated the splittings. Nevertheless, the IE's of the HOMO's are computed with good accuracy (Tables 8 and 11). AM1 overestimated the IE's of the HOMO's and underestimated the splittings by 0.4-0.5 eV. The key point is that both methods, assuming that Koopmans' theorem (30) holds, computed the insensi- tivity of energy of the HOMO to the nature of X at C5 of the cp ring, and this was corroborated by experiment.

Conclusions The results of the computational studies on la-lf and 2u-

2f, the pes studies on 2a-2f and the study of the stereo- chemistry of the DA reactions with maleic anhydride/ N-phenylmaleimide clearly show that n-n orbital mixing in the HOMO proposed by Inagaki et al. (13) cannot be the source of the n-facial selectivity observed in the reactions of cyclopentadienes 2c-2f. According to ab initiolAM1 calculations and pe spectroscopy there is no significant mixing of the lone-pair orbitals and the n-system in the HOMO's of 2c, 2d, and 2e, a requirement in the orbital mixing model, yet syn n-facial selectivity is observed in the DA reactions

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TABLE 1 1 . Vertical ionization energies of substituted cyclopentadienes

Ionization energy (eV)"


T h e estimated errors are k0.025 eV

FIG. 13. Vertical ionization energies of 2a-2f.

TABLE 12. Computed and experimental n,, n- splittings of substituted cyclopentadienes

Compound Ab initioa AMla UPSa.'

Cyclopentadiene 2.31' 1.79 2.15 (ref. 28) 5,s-Dimethylcyclopentadiene 2.41' 2.00 2.10 (ref. 28) 2b 2.77 1.60 2.04 2c twisted-anti 1.71 2.33 2d-anti 2.92 2.04 -2.4 2e-anti 1.72 2.28 2f-anti 2.32 1.66 2.16

"In electron volts (eV). bThe estimated errors are k0.05 eV. 'Calculations done with the 4-31.**//4-31G basis set; this work dNot computed at the ab initio level.

of these compounds (Table 1). Moreover, anti and not syn facial selectivity is observed for the sulfur compound 2f where the ns orbital energy is close to .rr and n-.rr mixing occurs in the HOMO.

Experimental

Computational studies Calculations were carried out with the ab initio packages

GAUSSIAN 90 (24) and GAMESS (25) running on Multiflow Trace 14/300 and IBM RS/6000 computers, respectively. AM 1 calculations were canied out with AMPAC version 2.1 (27) ported to an IBM RS/6000 computer. The keyword PRECISE was used to tighten the convergence criteria. The MO plots were obtained with PSI/88 (26) ported to a SUN 3 / 6 0 computer.

Ultraviolet photoelectron spectroscopic studies The pe spectra of the dienes synthesized previously ( 9 , 22) were

obtained on a non-commercial spectrometer described previously (31) by signal averaging 20-50 scans. The spectra were calibrated with argon.

Acknowledgements We thank the Natural Sciences and Engineering Research

Council of Canada for financial support. The PS/2, 70/486, 80/386, and RS/6000 computers used in this study were obtained under the auspices of an IBM Canada - McMaster University cooperative project. We also thank Professor W. L. Jorgensen for providing the PSI/88 computational package and Mr. S. Urqhart for porting the software to a Sun 3/60 computer.

1 . S. Winstein, M. Shatavsky, C. Norton, and R. B. Woodward. J. Am. Chem. Soc. 7 7 , 4183 (1955).

2 . D . W. Jones. J. Chem. Soc. Chem. Commun. 739 (1980). 3 . (a ) K. L. Williamson, Y. L. Hsu, R. Lacko, and C. H. Young.

J. Am. Chem. Soc. 91, 6129 (1969); (b ) K . L. Williamson and Y. L. Hsu. J. Am. Chem. Soc. 92 , 7385 (1970); ( c ) G. Bianchi, C . De Micheli, A. Gamba, and R. Gandolfi. J. Chem. Soc. Perkin Trans. 1 , 137 (1974).

4 . (a ) R. Breslow and J. M. Hoffman, Jr. J. Am. Chem. Soc. 94, 2110 (1972); ( b ) R. Breslow, J. M. Hoffman, Jr., and C. Perchonock. Tetrahedron Lett. 3723 (1973).

5 . M. Franck-Neumann and M. Sedrati. Tetrahedron Lett. 1391 (1983).

6 . ( a ) D. J. Burnell and Z. Valenta. J. Chem. Soc. Chem. Com- mun. 1247 (1985); ( b ) F. K. Brown, K. N. Houk, D. J. Burnell, and A. Valenta. J. Org. Chem. 52 , 3050 (1987).

7 . E. J. Corey, N. M. Weinshenker. T. K. Schaat. and W. Huber. J. Am. hem. Soc. 91 . 5675 (1969). L. A. Paquette, C. ~ a n l c c i , &d R. D. Rogers. J. Am. Chem. SOC. 111, 5792 (1989). J. B. Macaulay and A. G. Fallis. J. Am. Chem. Soc. 110,4074 (1988). M. Ishida, T. Aoyama, and S . Kato. Chem. Lett. 663 (1989). A. G. Fallis and Y-F. Lu. Adv. Cycloaddit. In press. N. T. Anh. Tetrahedron, 29 , 3227 (1973). S. Inagaki, H. Fujimoto, and K. Fukui. J. Am. Chem. Soc. 98, 4054 (1976). R. Gleiter and L. A. Paquette. Acc. Chem. Res. 16, 328 (1983). ( a ) F. K. Brown and K. M. Houk. J. Am. Chem. Soc. 107, 1971 (1985); ( b ) M. H. Padden-Row, N. G. Rondan, and K. N. Houk. J. Am. Chem. Soc. 104,7162 (1982).

16. (a ) S. D . Kahn and W. J. Hehre. J. Am. Chem. Soc. 109, 663 (1987); ( b ) J. Org. Chem. 53 , 301 (1988).

17. N. Kaila, R. W. Frank, and J. J. Dannenberg. J. Org. Chem. 54 , 4206 (1989).

18. M. Ishida, Y. Beniya, S. Inagaki, and S. Kato. J. Am. Chem. SOC. 112, 8980 (1990).

19. ( a ) A. S . Cieplak. J. Am. Chem. Soc. 103, 4540 (1981); ( b ) A. S. Cieplak, B. D. Tait, and C. R. Johnson. J. Am. Chem. SOC. 111, 8447 (1989).

20. W.-S. Chung, N. J. Turro, S . Srivastava, H. Li, and W. J. le Noble. J. Am. Chem. Soc. 110, 7882 (1988).

21. A. M. Naperstkow, J. B. Macaulay, M. J. Newlands, and A. G. Fallis. Tetrahedron Lett. 5077 (1989).

22. J. B. Macaulay and A. G. Fallis. J. Am. Chem. Soc. 112, 1136 (1990).

23. ( a ) G . Desimoni, G. Tacconi, A. Bario, and G. P. Pollini. Natural product syntheses through pericyclic reactions. ACS Monograph 180. American Chemical Society, Washington, D.C. 1984. Chap. 5 ; ( b ) A. G. Fallis. Can. J. Chem. 62 , 183 (1984).

Can

. J. C

hem

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nloa

ded

from

ww

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/10/

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2808 CAN. J . CHEM. VOL. 70, 1992

24. M. J. Frisch, M. Head-Gordon, G. W. Trucks, J. B. Foresman, H. B. Schlegel, K. Raghavachari, M. A. Robb, J. S. Binkley, C. Gonzalez, D. J. De Frees, D. J . Fox, K. A. Whiteside, R. Seeger, C. F. Melius, J. Baker, R. L. Martin, L. R. Kahn, J. J. P. Stewart, S. Topiol, and A. J. Pople. GAUSSIAN 90. Gaussian Inc., Pittsburgh, Pa. 1990.

25. M. W. Schmidt, K. K. Baldridge, I. A. Boatz, J. H. Jensen, S. Koseki, M. S. Gordon, K. A. Nguyen, T. L. Windus, and S. T. Elbert. GAMESS. Version April 4, 1991. QCPE Bull. 10, 52 (1990).

26. W. L. Jorgensen and D. L. Severence. PSI/88 Version 1 .O. Department of Chemistry, Yale University, New Haven, Conn.

27. Dewar Research Group and J. J. P. Stewart. Austin Method 1 Package 1.0. QCPE 506. QCPE Bull. 6, 2 (1986).

28. C. Guimon, G. Pfister-Guillouzo, J. Dubac, A. Laporterie, G. Manuel, and H. Iloughmane. Organometallics, 4, 636 (1985).

29. K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, and S. Iwata. Handbook of He1 photoelectron spectra of fundamen- tal organic molecules. Japan Scientific Societies Press, Tokyo; Halsted Press, New York. 1981. p. 126.

30. T. A. Koopmans. Physica, 1, 104 (1933). 31. N. H. Werstiuk, D. N. Butler, and E. Shahid. Can. J. Chem.

65, 760 (1986).

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computational and ultraviolet photoelectron spectroscopic studies of 5-substituted cyclopentadienes....

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