Sept. 5 , 1963 COMMUNICATIONS TO THE EDITOR 267 1

TABLE I1 PHOSPHORESCENCE MAXIMA, EPA AT 77°K. IN C M . ~

21,097 21,097 21,044

19,700 19,708 19,635

I I1 1-Methylnaphthalene

20,576 20,584 20,534

19,547 19,547 19,500

emission from benzophenone could have been detected (Fig. 2).

Apparently, efficient transfer of triplet excitation from the benzophenone moiety to the naphthalene moiety of I occurs. Singlet excitation transfer from a benzo- phenone to a naphthalene is unlikely since the first excited singlet level of benzophenone lies about 3000 cm.-l lower than the first excited singlet level of naph- thalene. The lowest triplet state of benzophenone lies about 3000 cm.-' higher than the lowest triplet state of naphthalene and efficient triplet energy trans- fer can occur. Transfer of singlet excitation from the naphthalene unit to the benzophenone moiety is energetically possible and must occur with high efficiency" when I is excited with light adsorbed by the naphthalene nucleus. Fluorescence from l-methyl- naphthalene (0-0 band 31,397 cm.-l) is nearly ten times more intense than fluorescence from I (0-0 band 31,348 cm.-l). Direct involvement of the carb- onyl group of I in fluorescence quenching is indicated by the fact that fluorescence of I1 (0-0 band at 31,505 cm.-l) is as intense as that of 1-methylnaphthalene.

OH

H (0) u

I1 The large difference between the intensities of fluores-

cence from I and I1 is attributed to loss of singlet ex- citation from the naphthalene nucleus of I to the ketonic part of the molecule Ik'hile the excitation is in the carbonyl-containing unit, efficient intersystem crossing to a triplet state occurs and the excitation then returns to the naphthalene unit.

Acknowledgment.-The work performed a t Wesleyan University was supported by Grant No. 1467-B5 from the Petroleum Research Fund of the American Chemi- cal Society. The work carried out a t the California Institute of Technology was supported by the National Science Foundation.

(11) 0 Schnepp and M Levy, J A m Chem Soc ,84 , 172 (1962), and ref

(12) National Science Foundation Predoctoral Fellow erences therein

HALL LABORATORY OF CHEMISTRY PETER A LEERMAKERS WESLEYAN UNIVERSITY GARY W BYERS MIDDLETOWN, CONNECTICCT CONTRIBUTION X o 2988 GATES ASD CRELLIN LABORATORIES ANGELO A L A M O L A ~ ~

OF CHEMISTRY GEORGE S HAMMOND CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CALIFORNIA

RECEIVED JUNE 13, 1963

The Role of n - x* Triplet in the Photochemical Enolization of o-Benzylbenzophenone'

Sir: The identification of reactive excited states in photo-

chemistry is a subject of current interest. The n -+x* triplet state of the aromatic ketones has been shown to be the intermediate in intermolecular photochemical

(1) The work was presented in part a t the Photochemistry Symposium in Rochester, N. Y., on March 27, 1963.

- 1.00 - 6 - e .- m .80 -

- 0 - 0 .- c 8.60-

-

.40 - -

340 4 20 500 A m p .

Figure 1

reactions, 2-6 while in the intramolecular type I1 process the singlet excited state was suggested as the reactive intermediate.' o-Benzylbenzophenone undergoes an intramolecular photoenolization, a reaction analogous to the type I1 process,8 and the photoenol thus formed reverts back to benzylbenzophenone in a dark reaction. On the basis of the two possible reactive excited states, the photoenolization may proceed directly throbgh the n + x* singlet excited state (reactions 1 and 2) or it may proceed through the n + ir* triplet as the inter- mediate (reactions 1, 3, and 4). By flash spectroscopy we have now demonstrated that the triplet is the reactive intermediate in this intramolecular reaction.

CsHs I

CsHs I y I11

n+n* interstate n + n *

singlet crossing triplet (3) I1

o-Benzylbenzophenone (I), the n + n* triplet state (II), and the enol (111) will all exhibit measurable elec- tronic absorption spectra under the experimental condi- tions, while the n --+ x* singlet will have too short a lifetime to be detected. The flash photolysis and spec- troscopy of I were carried out in cylindrical Pyrex cells ( I = 26 crn.) with full intensity of the flash light source

(2) G. S . Hammond, et ai., J . A m . Chem. SOC., 8s. 2789 (19611, and related papers.

(3) J. N. Pit ts , H . W. Johnson, Jr . , and T. Kawana, J . Phys . Chem. , 66 , 2456 (1962).

(4) G. Porter and F. Wilkinson, Trans. F a r a d a y Soc., 67, 1686 (1981) ( 5 ) J. A. Bell and H. Linschitz, J . A m . Chem. Soc., 86, 528 (1963). ( 6 ) H. L. BBckstrom and K . Sandros, Acta Chcm. Scand. , 14, 48 (1960). (7) V. Brunet and W. A. Noyes, Jr., BULL. SOL. chrm. France , 121 (1958). (8) N. C. Yangand C. Rivas, J . A m . Chem. Soc., 89, 2213 (1961).

2672 COMMUNICATIONS TO THE EDITOR VOl. 85

in an apparatus previously described.$ When the spectrum of 0.3 m M o-benzylbenzophenone in cyclo- hexane is measured several minutes after the flash ex- citation, i t deviates only to a small extent from the initial absorption (A,,, 338 and 342 mp, E 140).1° However, 50 psec. after the flash excitation the absorp- tion spectrum is markedly different with a new maxi- mum a t 500 mp and a minimum a t 344 m p (Fig. I ) . By studying the optical density a t various wave lengths with kinetic spectrophotometry two transient inter- mediates were identified separately. A fast transient, the absorption maximum of which a t 500 mp is very similar to the triplet-triplet absorption of benzo- phenone," was assigned as the n + H* triplet of o- benzylbenzophenone (IT). I ts rate of decay measured a t 436 mk and 380 mp follows first-order kinetics with k = 1.9 f 0 . 2 X 103 sec.-I which further substantiates the triplet assignment. l 1 The slow transient, detected by the decay of optical density a t 405 mp, is tentatively identified as the enol (111). The decay is also of first order with k = 9.4 0.6 X set.-'.

The interconversion relationship of the two inter- mediates was established by following the variation of the optical density a t 345 mk. The optical density a t 345 mp decreases to a minimum value immediately after the flash (Fig. l), then it increases rapidly to a maximum a few msec. thereafter with a first-order rate constant, k = 3.2 >( l o3 set.-' ( * joyo), which is of the same order of magnitude as that of the triplet decay. The maximum optical density attained a t this wave length is approximately 0.1 unit higher than the final absorption. The decay of this maximum absorption to the final value also follows first-order kinetics with k = 8.7 X 1C)-2 sec.-I, which is the same as the decay of the slow transient within experimental error. The above observations establish that (1) the fast transient is converted into the slow transient, i . e . , then -+ H* triplet is the precursor of the enol, and (2) the enol is not formed directly from the n --t H* singlet. If the n + H* singlet were the immediate precursor of the enol, there would be a strong absorption a t 345 mk immedi- ately after the flash.

Our observations demonstrate that photoenolization of o-benzylbenzophenone, an intramolecular photo- chemical reaction analogous to the type I1 process, in- volves a reaction triplet intermediate. Although the enolization may occur theoretically within the lifetime of the singlet excited state, i t i s actually slower than the interstate crossing of the singlet to the triplet (reaction 3). This work supports the theory that the triplet state may be a reactive state in other similar in- tramolecular photochemical reactions. I 2 , l 3

(9) E. F. Zwicker and I,, I. Grossweiner, J . Phys. Chem., 87, 519 (1963); L. I . Grossweiner and E F. Zwicker, J C h e m Phys , 34, 1411 (1961).

( IO) After several Hash irradiations or a prolonged irradiation with a high intensity light source, the solution acquired a yellow tint with a blue fluores~ cence. From the reaction medium there was isolated a small yield of a dimer whose structure is being elucidated.

(11) D S hfcClure and P. Hanst. J . C h m . Phys. , 2.9, 1772 (195.5). (12) P. Ausloos and R . E. Rebbert . J . A m . Chem. SOL. , 83, 4897 (1981). (13) A' H. Urry and U. J Trecker, % b i d . 84, 118 (19021. (11) The work a t the Illinois Institute of Technology is supported by the

United States Public Health Service, nepar tment of Health, Education and Welfare

(15) The work a t the University of Chicago is supported by the United States Atomic Energy Commission

( l i i ) Fellow of the Alfred P Sloan Foundation.

I)EPARTME\.T OF PEiYSICS" E F ZIVICKER ILr-rsors I SSTITLTE O F TECHS~ILKY I, I . GROSSWEINER CHICAGO 16. II.:.INOIS

S C Y A S G ~ ~

The Synthesis of the Anomeric 7-~-Ribofuranosyladenines and the Identification of the

Nucleoside from Pseudovitamin BIZ1 Sir:

The final identification of the nucleoside moiety of vitamin Bl2 as l-a-~-ribofuranosyl-5,6-dimethylbenz- imidazole was achieved by comparing its picrate with a synthetic specimen. Later a nucleoside was isolated from pseudovitamin BIZ and identified as 7-D- ribofuranosyladenine. Although the configuration of the glycosyl linkage of this nucleoside was thought to be a,4 this point has not been established by chemical means in the years since the isolation of the nucleoside.

Using a recently developed methods for the synthesis of 7-glycosylpurines, we have now synthesized both 7-p-~-ribof uranosyladenine (I) and 7 - a-D-ribofuranosyl- adenine (11) and established the identity of the nucleo- side from pseudovitamin Blz and 11.

"2 I "2 I

I 3-Benzyladenine (III), prepared by the benzylation of

adenine in N,N-dimethylacetamide in the absence of base,7 was allowed to react with benzoic anhydride to give N6-benzoyl-3-benzyladenine (IV), which was con- verted to its mercury derivative (V) in the usual manner.8 The mercury derivative V was coupled with tri-0- acetyl-D-ribofuranosyl chloride by refluxing a xylene suspension of the two materials for 1 hr. Removal of the acyl groups from the blocked nucleoside VI was accomplished by refluxing for 30 min. a methanol solu- tion of it containing sodium methoxide. A 51% yield

c ~ H ~ C H ~ I V

AcO ?Ac HO OH

AcOCH~ Q HOCHz Q CsHsCH2 c ~ H ~ C H ~

VI VI1

(1) This work was supported by funds from the C. F. Kettering Founda- tion and the Cancer Chemotherapy National Service Center, National Cancer Institutes, National Insti tutes of Health, Contract So. SA-4R-:>h- 1740

( 2 ) F W Holly, C H Shunk, E. W. Peel, J . J Cahill, J B. Iav igne . and K Folkers. J . A m . Chem S O C . , ~ ~ , 4521 (1952)

13) W Friedrich and K . Bernhauer, Chem R e i , 89, 2.507 (19313). (41 X - R a y analysis o f the structure of the BII vitamins suggests the a con

( 5 ) I ) . C Hodgkin, Biochem. SOC. Symp . (Cambridge, England), 13, 28

(17) J A hlonl.gomery and H . J . Thomas, J . Oug. C h e m . . 28, 2804 (19I;:j). : 7 ) J. W Jonesand R. K. Robins, J . A m . C h e m . S o t . . 84, 1914 (1962) (81 J J Fijx, S Yung, I . Wempen, and I . I>, l loerr, i b i d . , 79, 50110 (1967)

figuration a s well as attachment a t 5-7.5

(footnote) ( i9 . i . i ) .