J . Org. Chem. 1988,53, 716-719
716
Photo-Stevens Rearrangement of 9-Dimethylsulfonium Fluorenylide Jian-Jian Zhang and Gary B. Schuster* Department of Chemistry, Roger Adams Laboratory, University of Illinois, Urbana, Illinois 61801 Received J u n e 22,1987
The direct irradiation of 9-dimethylsulfonium fluorenylide (DMSF) in acetonitrile or THF gives primarily and the dimeric bis[9-(methylthio)fluorenyl]. the Stevens rearrangement product 9-methyl-g-(methylthio)fluorene The mechanism of this reaction was investigated by kinetic, product, and isotope tracer techniques. The results indicate that the primary photochemical step is bond homolysis from an nu* singlet state of the ylide. Oxidation of the ylide does not lead to chemical reaction.
In 1928 Stevens and co-workers reported that treatment of phenacylbenzyldimethylammonium bromide with base results in its rearrangement to 2-(dimethylamino)-3phenylpropiophenone (eq l ) . l This transformation has come to be generally known as the Stevens rearrangement. n
0
II
OH -
+
,
Ph CC H N (CH3) Br
I
*
li
PhCCHN(CH,),
I
CH,Ph
(I)
CH,Ph
Subsequent research has shown that the Stevens rearrangement proceeds through an intermediate ylide, and that it also occurs in sulfonium and phosphonium compounds.* Special attention was drawn to the mechanism of the Stevens rearrangement when it appeared that it might be an exception to the principles of conservation of orbital symmetry. The [1,2]sigmatropic shift in the ylide proceeds with (partial) retention of the configuration of the migrating group in defiance of the allowed (but sterically impossible) inversion for a concerted process and the expectation of racemization for the simplest stepwise pathways. This dilemma was resolved by experiments that supported formation of intermediate short-lived radicals that coupled to form products in competition with their rotation and diffusion apart.3 It is well-known that the topological requirements for rearrangement of electronically excited species are in general precisely opposite those of the corresponding ground states. Thus, an electronically excited ylide undergoing the Stevens rearrangement might follow an uninhibited, concerted path to the product of the retained configuration. The photochemistry of sulfonium ylides has not been studied in detail. Trost reported that irradiation of dimethylsulfonium phenacylide gives products primarily from carbene formation and a low yield of propiophenone from presumed secondary photolysis of the Stevens rearrangement product (eq 2).4 It was also found that irraR R,+
-
S-CH-X
hv_
A : R=CH,;
X=COPh
B : R=Ph;
X=CH=CH,
R’
ECH.1 Carbene
I
+ R-SCH-X
(2)
Stevens Rearrangement
(1)Stevens, T.S.; Creighton, E. M.; Gorden, A. B.; MacNical, M. J . Chem. SOC.1928,3193. (2) Morris, D.G. Suru. Prog. Chem. 1983,10,189.Fava, A.Stud. Org. Chem. (Amsterdam) 1985,19,299. (3)Baldwin, J. E.; Erickson, W. F.; Hackler, R. E; Scott, R. M. Chem. Commun. 1970,576. Iwamura, H.; Iwamura, M.; Nishida, T.; Yoshida, M.; Nakayama. Tetrahedron Lett. 1971, 63. Ollis, W.D;Rey, M.; Sutherland, I. 0. J . Chem. SOC.,Perkin Trans 1 , 1983, 1009. (4) Trost, B. M. J . Am. Chem. SOC.1966, 88, 1587; 1967, 89, 138.
0022-3263/88/1953-0716$01.50/0
diation of diphenylsulfonium allylide gives both carbenederived and Stevens rearrangement product^.^ Caserio and co-workers studied the photochemistry of some cyclic, carbonyl-stabilized sulfonium ylides and reported products characteristic of the Stevens rearrangement.6 Maki and Hiramitsu observed that irradiation of some heterocyclic ylides gives products derived from Stevens rearrangement7 More recently, Griller and co-workers reported that irradiation of diazofluorene in the presence of sulfides gives Stevens rearrangement products presumably by secondary photolysis of the first-formed ylide.8 Since some ylides that do not rearrange thermally do undergo facile photoStevens reaction, this route could prove to be a useful, general, synthetic transformation. The mechanism of the thermal Stevens rearrangement has been carefully investigated. Homolysis of a carbonheteroatom bond to form a neutral radical pair is believed to be the critical initial step in most examplesg of this process (eq 3). This conclusion is supported by isotope tracer results, CIDNP evidence, stereochemical probes, and solvent viscosity studiesS3An alternative path considered first by Lepley has single-electron oxidation of the ylide as the key initial step.1° This process yields a radical cation, which could rearrange to give characteristic Stevens products (eq 4). This possibility was considered again R\+ S-CH-X R’
-
1
SCH-X
7
R
I RSCH - X
(3)
--l
more recently by Radom and co-workers who calculated theoretical reaction paths for some ionized ylides, which they call “ylidions”.” Their findings suggest that the prototype sulfonium ylidion (H2SCH2”) exists in a potential minimum and will not spontaneously rearrange. (5) Trost, B. M.; LaRochelle, R. W. J . Am. Chem. SOC.1970,92,5804. LaRochelle, R.W.; Trost, B. M.; Krepski, L. J. Org. Chem. 1971,86,1126. (6)Fish, R.H.; Chow, L. C.; Caserio, M. C. Tetrahedron Lett. 1969, 1259. (7) Maki, Y.; Hiramitsu, T. Chem. Pharm. Bull. 1977,25, 292. (8)Alberti, A.; Griller, D.; Nazran, A. S.; Pedulli, G. F. J. Am. Chem. SOC.1986,108,3024. (9) A possible exception: Schollkopf, U.; Schafer, H. Justus Liebigs Ann. Chem. 1963,22,663. (10)Lepley, A. R. J . Am. Chem. SOC.1969,91, 1237. (11)Yates, B.F.;Bouma, W. J.; Radom, L. J. Am. Chem. SOC.1984, 106,5805. Yates, B. F.; Bouma, W. J.; Radom, L. J . Am. Chem. SOC.1987, 109,2251.
0 1988 American Chemical Society
J. Org. Chem., Vol. 53, No. 4,
9-Dimethylsulfonium Fluorenylide We report herein the examination of the photochemistry of 9-dimethylsulfonium fluorenylide (DMSF); its irradiation gives the Stevens rearrangement product by a dissociative mechanism, but its one-electron oxidation to the ylidion does not initiate this reaction.
& \
DMSF
Results (1) Absorption Spectroscopy. The ylide DMSF was prepared according to the literature procedure by deprotonation of the sulfonium salt.12 It is a stable, air-sensitive, pale yellow solid. The UV-visible spectrum of DMSF in acetonitrile shows two absorption features, a strong band (t = 3.0 X lo4 M-l cm-') a t 255 nm and a weaker band (t = 4.3 X 103M-I cm-l) with a maximum at 362 nm. In some samples there is an additional very weak absorption band with a maximum in the visible region (ca. 450 nm) due to contamination by a trace (
