Heterolysis with -rearrangement

The reaction of crotonaldehyde and methyl vinyl ketone with thiophenol in the presence of anhydrous hydrogen chloride effects conjugate addition of thiophenol as well as acetal formation. The resulting j3-phenylthio thioacetals are converted to 1-phenylthio-and 2-phenylthio-1,3-butadiene, respectively, upon reaction with 2 equivalents of copper(I) trifluoromethanesulfonate (Table I). The copper(I)-induced heterolysis of carbon-sulfur bonds has also been used to effect pinacol-type rearrangements of bis(phenyl-thio)methyl carbinols. Thus the addition of bis(phenyl-thio)methyllithium to ketones and aldehydes followed by copper(I)-induced rearrangement results in a one-carbon ring expansion or chain-insertion transformation which gives a-phenylthio ketones. Monothioketals of 1,4-diketones are cyclized to 2,5-disubstituted furans by the action of copper(I) trifluoromethanesulfonate. ... 

In contrast to the thermal solvolysis, a rearranged enol ether 45 (and also the hydrolysis product, acetophenone) is formed in addition to the unrearranged product 44. The rearrangement is more apparent in less nucleophilic TFE. The results are best accounted for by heterolysis to give the open primary styryl cation 46 (Scheme 8). This cation gives products of substitution 44 and elimination 30 by reaction with the solvent. Alternatively, 46 can rearrange to the a-phenyl vinyl cation 47 via 1,2-hydride shift, which gives rise to 45 and 30. 

In parallel with the development of the heterolysis of b-substituted alkyl radicals, a rearrangement reaction was observed and extensively studied in organic solvents. This rearrangement was first noted for b-(acyloxy)alkyl radicals (Scheme 5) by Surzur et al. [48] and, later, for b-(phosphatoxy)alkyl radicals by the Crich and Giese groups [49,50]. 

At one time considered as two distinct reactions occurring by different mechanisms [51], the fragmentations of Scheme 2 and the rearrangments of Scheme 5 are now seen as different facets of the same fundamental heterolysis of -substituted alkyl radicals into alkene radical cations, with the eventual outcome determined by the reaction conditions [52],... 

Mechanisms that are probably associated, respectively, with these processes are (i) the formation of betaine intermediates (306) (Fig. 3) 103,143,149,197,200 homolysis or heterolysis of the X—Z bond (304) or the X—Z bond (305) giving diradicsd (307) or dipolar (308) intermediates, (iv) 1,3-dipolar cycloaddition yielding intermediate adducts (e.g, 309), The base-catalyzed rearrangements (ii) present very interesting mechanistic problems suitable for speculation and experimental enquiry. 

Other Reactions of Olefinic Steroids.—Reaction of cholest-5-en-3-one with air and acetic acid shows that isomerization to the A -3-oxo-compound is accompanied by autoxidation to the 6a- and 6/8-hydroxy-3-oxo-A -compounds and the 3,6-dioxo-A -compound. The oxidation appears to be controlled by heterolysis of the 4/3-proton and formation of the intermediate ion pair (73). Sitosterol was autoxi-dized at C-7 to give the 7-oxo- and the epimeric 7-hydroxy-derivatives. Oxidation of a 17-methylene steroid with Pb, Tl" , and Hg acetates in methanol gave a wide variety of products. The reaction with Pb(OAc)4 gave the rearranged products (74), (75), and (76) whereas the Tl and Hg products retained the... 

The chemistry of radical sites adjacent to phosphatoxy centers elicited interest because of the involvement of such species in DNA degradation processes. These species can give rise to rearrangement, elimination, and substitution products, and for some time concerted eliminations and migrations as well as heterolysis to a radical cation and a phosphate anion were considered to be involved (Scheme 2). Recently, experimental studies of the l,2-dibenzyl-2-(diphenylphosphatoxy)-2-phenylethyl radical and complementary theoretical studies of l,l-dimethyl-2-(dimethylphosphatoxy)ethyl radical have been interpreted as indicating that a radical cation/anion pathway with initial formation of 49 is favored. ... 

The initial transient formed, rearranges in a reaction that involves the ring contraction step in reaction (74). The lifetime of this intermediate is considerably longer than that reported for any other intermediate with a copper(II)-carbon bond in aqueous solution (85-87,101,136), suggesting the stabilized structure featuring the metallocycle. This intermediate decomposes via heterolysis of one of the copper(II)-carbon -bonds followed by homolysis of the second to form the cyclopentyl-methanol radical in reactions (75) and (76), which reacts with Cu + to form the final product cyclopentanecarbaldehyde (89). 

The rearrangement (Scheme 26) seems to be due to heterolysis of the C3a—C4 bond and rotation about the C7—Cla bond with subsequent formation of zwitterionic intermediate 51. 

The solvolysis of 4,4-dimethyl-e -cholesteryi tosylate (5) is interesting in that a simple elimination reaction leading to a conjugated 3,5-diene is impossible. Ionisation of the 3 a-tosylate, which derives no anchimeric assistance from the 5,6-double bond, is followed instead by migration of the 4/9 methyl group to C(3). The data available are consistent with Shoppee s postulate [y8] that the heterolysis occurs as the slow step, and is followed by rapid rearrangement of the classical ion... 

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