1982 J. Org. Chem., Vol. 44, No. 12, 1979
Zimmerman, Gannett, and Keck
Stereochemistry of the Aryl-Vinyl Version of the Di-?r-methane Rearrangement 1,2 H. E. Zimmerman,* T. P. Gannett, and G. E. Keck Chemistry Department, University of Wisconsin, Madison, Wisconsin 53706 Received November 9, 1978 Of the three known versions of the di-n-methane rearrangement-the divinylmethane case, the aryl-vinylmethane variety, and the oxa-di-n-methane variation-the reaction stereochemistry has been established in only two. The present study deals with stereochemistry at the methane carbon in the'aryl-vinylmethane rearrangement. The rearrangement proceeds with inversion of configuration a t this center in analogy to the divinylmethane rearrangement but in contrast to the oxa-di-n-methane process where stereochemistry is sometimes lost. Thus, direct irraled to (+)-trans-2-ethyl-2-methyl-3,3-didiation of (-)-3-(p-methoxyphenyl)-3-methyl-l,l-diphenyl-l-pentene phenyl-1-(p-methoxypheny1)cyclopropane and to the (-)-cis isomer. By correlation of configurations of reactant and products it was shown that the methane carbon was inverted in the reaction. Extrapolation of the photochemistry to zero percent conversion revealed 100.1 f 1.7% optical purity of photoproduct. The example is one where extrapolation is required, and even runs as low as 6%conversion lead to error. In the ordinary divinylmethane version inversion of configuration is readily predicted using a six-orbital cyclic array of atomic and hybrid orbitals. In the present aryl-vinylmethane case a cyclic array including one molecular orbital allowed theoretical treatment. Again inversion was predicted.
The di-7r-methane rearrangement has become one of the most general and useful of photochemical reactions. A thorough understanding of all of its facets has been a goal of our research for some years3 Our research has concentrated on two of the three variations of the reaction. Thus, of the divinylmethane version, the aryl-vinylmethane variety and the oxa-di-7r-methane type, the first two have been of special interest to us. While structurally similar, the three versions do differ in a number of respects. The divinylmethane and the aryl-vinylmethane rearrangements utilize both singlets and triplets while the oxa-di-7r-methane modification proceeds by the triplet excited state.4 The aryl-vinylmethane singlets tend to rearrange more slowly than their divinylmethane counterp a r t ~ . Still ~ - ~ another difference is that the divinylmethane rearrangement requires central (i.e., methane carbon) substitution while the aryl-vinylmethane6 and oxa-di-.rr-methane varieties do The reaction stereochemistry has been investigated in two of the three rearrangements. Here, again, there is a difference. Thus, our earlier efforts8 have shown that the methane carbon is inverted in the divinylmethane rearrangement. In contrast, the oxa-di-7r-methane rearrangement sometimes proceeds with loss of methane carbon stereo~hemistry,~ and sometimes proceeds with inversion.1° Our present study was aimed at determining the stereochemistry of the aryl-vinylmethane version of the rearrangement. The most difficult question deals with the nature of the stereochemistry at the methane carbon. In the cases of l-methylene-4,4-diphenyl-2-cyclohexene (1)" and 5,5-diphenyl-l,3-~yclohexadiene (3),12 there is constrained geometry; nevertheless, the di-x-methane rear-
I
Q2
Ph"
Ph
3
2b
2a
F:+ QL.: 'Ph
Ph
Ph
40
4b
12)
rangement is of the aryl-vinyl type and is instructive. It is seen that the major products from each of these photolyses is the trans- or endo-phenyl stereoisomer (i.e., 2a and 4a).Reference to structure 5 in Figure 1 reveals that as one phenyl group migrates from the methane carbon, overlap of the anti lobes of the orbitals at carbons 2 and 4 with disrotatory twisting leads to the preferred product. Disrotatory twisting with syn overlap affords the minor product. Accordingly, the major reaction course involves inversion of configuration at the methane carbon (Le., C-4) and anti overlap with the vinyl p orbital (i.e., a t C-2) in this phenyl-vinylmethane system. This stereochemistry is then the same at the methane carbon as in divinylmethane systems,8 and the anti preference for bonding with the vinyl moiety also parallels our findings in divinylmethane systems.13 However, the systems in eq 1 and 2 are constrained, and thus the natural preference in absence of constraints is still uncertain. For example, Mariano and co-workers14 have presented an unusually elegqnt example of enforced syn-disrotatory stereochemistry in the divinylmethane example in eq 3.
With this background, it was clear that a test of the stereochemistry of an unconstrained aryl-vinylmethane needed investigation. The aim of the present investigation was determination of the methane carbon behavior. For our study we selected the 3-(p-methoxyphenyl)-3methyl-1,l-diphenyl-1-pentene (9). Our approach required resolution of the reactant, study of its photolysis, and then correlation of configurations of reactant and products. Results. Synthesis of Photochemical Reactant. The synthesis employed is outlined in Scheme I and detailed in the Experimental Section. One requisite for the synthesis was that one of the intermediates be resolvable; the synthesis selected utilized anisyl acid 12 for this purpose. A point of interest is the facile fragmentation of the anisyl carbinol 14 to give diphenylethylene and alkene derived from the 2-anisyl-2-butyl cation under acidic dehydration conditions. Thus, the phosphorus oxychloride-pyridine method was required. Results. Exploratory Photochemistry. In a typical exploratory run irradiation of 430 mg of anisyl-vinylmethane
0022-3263/79/1944-1982$01.00/00 1979 American Chemical Society
J. Org. Chem., Vol. 44, No. 12, 1979 1983
Aryl-Vinyl Version of the Di-8-methane Rearrangement Scheme I. Synthesis of Photochemical Reactant
A
EtOOC,
/CN
Me An
)tc.
KOH E tlhyycloeln, e G
1 , COOEt R e f l u x
10
0
Et
l ) S O C l * , CgHg
' 12
COOH
Ph
1
Z)MeOH, R e f l u x
.14.
13
Ph'
9
5 Figure 1. Phenyl-bridged species showing syn (S) and anti (A) disrotatory twisting illustrated for diene 1.
reactant 9 in a 450 W immersion apparatus for 45 min led to 185 mg of a mixture of cis- and truns-2-ethyl-Z-methyl-3,3diphenyl-1-(p-methoxypheny1)cyclopropane (15), 112 mg of Scheme 11. Interrelation Scheme Correlating Photochemical recovered aryl-vinylmethane reactant (9),and 54 mg of a Reactant 9 and Product 15a fraction consisting of a 10:1 mixture of reactant and cyclopropane product. The product stereoisomers were separable by recycling HPLC and were found to be present in a 3:l ratio favoring the trans isomer. The product structure derives from \ \ COOH Come a degradation described below in connection with corielation (-)-I2 (-)-13 of configurations. The overall reaction is given in eq 4. HowJ ::-c
M%ph&
A
...
A Messn Et a
Ph Ph
Ph
Hooc Ph
ever, thus far, the relative configurations of reactant and products remain unknown and are depicted arbitrarily. Results. Reactant Resolution, Product Degradation, and Correlation of Configurations. In order to elucidate the stereochemical course of the reaction as pertains to the methane carbon, it was necessary to obtain optically active aryl-vinylmethane 9. It was also necessary to develop a scheme relating the configuration of this photochemical reactant to the cis and trans isomers of photoproduct 15. The first objective was achieved by resolution of 3-anisyl3-methylpentanoicacid (12).This was then used as depicted in Scheme I1 to synthesize optically active aryl-vinylmethane 9. The second objective was to correlate the configuration of resolved aryl-vinylmethane 9 to the configurations of cis- and trans- 2-ethyl-2-methyl-3,3-diphenyl-l-anisylcyclopropane (Le., 15a and 15b). Again, this is depicted in Scheme 11. It was found that aryl-vinylmethane 9, cis photoproduct 15a, and trans photoproduct 15b with the relative configurations shown in Scheme I1 have negative, negative, and positive rotations, respectively. Scheme I1 depicts (S)-9 as having a rotation opposite in sign to 15b having an ( R ) configuration at the methane carbon but a rotation the same in sign as 15a with an ( R )configuration at the methane carbon. In the above configurational interrelating efforts i t seemed particularly important to ascertain that each compound was free of contaminating impurities. Therefore, for each active compound rotations were taken at five wavelengths. This promised that the correlations would be error free and that the relative optical purity of photochemical reactant and products could be determined with maximum accuracy. Table I lists the optically active compounds and their rotations. Finally, we note that to obtain the rotation of optically pure cis and trans anisylcyclopropane photoproducts, active photoproducts of known rotation were ozonized as in Scheme I1 to diphenyl acids 20a and 20b whose activity then could be compared with the optically pure acids.' Results. Irradiation of Optically Active Anisyl-Vin-
(-)-16
(+)-17 ph
(-)-90
TOH.
Hx;
EtOH
Me-
+ M4
