Epoxide, Diol, and Double Bond Reactivity

Epoxide, Diol,and Double Bond Reactivity.—Berti and co-workershave examined the reactivity of the conformationally rigid epoxides (134) and (135) with HCl. The two epoxides react in dry CHCI3 to yield almost exclusively (136) and (137), the products of syn-attack by chloride ion on the benzylic carbon. However, with HCl in propan-2-ol, (134) gives (136), (22%) and (138) (78%), 

Pagliardini, G. Torri, L. Elegant, and M. Azzaro, Tetrahedron Letters, 1972, 4241. 

Stereospecificity of reaction of diols with equatorial tertiary hydroxyl, (140) and (141), is in accord with an SN- -type mechanism. When the tertiary OH group is axial approach of the acid is hindered and reaction only occurs at all when the p-hydroxyl is axial (142). The inertia of (143) is considered to arise from the pronounced rate-retarding inductive effect of the P-hydroxy-group which prevents reaction in the absence of a favourable conformational situation. 

A significant difference occurs between the steric courses of reactions of aziridine (145) and epoxide (146) under acidic conditions. The former exhibits 

Camici, B. Macchia, F. Macchia, and L. Monti, Tetrahedron Letters, 1972, 2591. 

Nucleophilic attack on cis-intermediate (244) occurs preferentially at C-1 through a chair-like transition state with BrQ the regioselectivity is lower than would be expected on the basis of analogy. In particular, a large percentage of parallel attack at C-2 anti to t-butyl was noted in CHjClj, CHClj, and benzene, and factors other than direct steric or inductive effects of ring substituents are thought responsible. 

A study of the influence of allylic hydroxy- or methoxy-substituents on the steric course of bromine addition to cyclohexenes has been carried out with substrates (245)—(250). The hitherto unknown compounds (249) and (250) were obtained (in a ratio of 1 9) by LiAlH reduction of the corresponding ketones HQO -catalysed equilibration changed this ratio to 36 64, and pure materials were obtained by chromatographic separation. The equilibrium is similar to that for (248) and (247), 

Data for (245) and (246) indicate that polar factors play only a minor role in determining product stereochemistry. With analogy to the data for HOBr addition, the weightings of the pathways were a -H a = 75 b -I- b = 0 c 4- c = 5 d -f- d = 20. The 75 % electrophilic attack syn to allylic substituent, presumed irreversible, has been tentatively ascribed to an effect outlined in (267) the similar extent of syn-attack in 3-methylcyclohexene was regarded as coincidental. 

Compounds (247) and (248) gave similar diaxialrdiequatorial dibromide product ratios (94 6) with 4-t-butylcyclohexene, a result relatively unperturbed by polar and steric effects. This was unexpected and has been attributed to compensatory steric and polar effects in paths a —d. Using (247) and (248) as models, it was concluded that the conformationally mobile (245) experiences an 88 % contribution of pathways involving bromonium ions with equatorial OH. 

The rate constants (lO k /l min ) for bromination of cyclopentene, cyclohexene, and cis-cyclo-octene are 37.5, 6.84, and 0.73 the corresponding values for the 1,2-dimethyl derivatives are 17300, 2080, and 730, The bromination rates for 1-methylcyclohexene, 1-methyl-cis-cyclo-octene, 1-ethylcyclopentene, and 1-ethyl- 

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