Arsonium ylides mechanism

We postulated that the mechanism for the formation of the cyclopropanes was similar to that for the reaction of arsonium ylides with a,i3-unsaturated esters. 

Current results indicate that stabilized arsonium ylides such as phenacylide, carbomethoxymethylide, cyanomethylide, fluorenylide, and cyclopentadienylide afford only olefinic products upon reaction with carbonyl compounds. Nonstabilized ylides such as ethylide afford almost exclusively epoxides or rearranged products thereof. However, semi-stabilized arsonium ylides, such as the benzylides, afford approximately equimolar amounts of olefin and epoxide. Obviously, the nature of the carbanion moiety of the arsonium ylide greatly affects the course of the reaction. It is reasonable to suppose that a two-step mechanism is involved in the reaction of heteronium (P, S, and As) ylides with carbonyl compounds (56). 

Chiral arsonium ylides containing the benzylidene group react with aromatic aldehydes to produce trans-2,3-diaryloxiranes with optical purities of up to 41%. The degree of asymmetric induction depends upon the nature of the substituents on the ylide, the substrate, and upon the reaction conditions. For the variety of arsonium ylides investigated, however, the yields of recovered optically active arsines were almost quantitative with complete retention of configuration at arsenic in each case. Thus, apart from the value of stereoselective benzylidene transfer for the asymmetric synthesis of trans-2,3-diaryl oxiranes, the reaction provides the cleanest route to the recovery of optically active arsines from resolved benzylarsonium salts (see Section III.B.5). The mechanism of the reaction, which must take into account the exclusive formation of trans-diaryloxiranes, is believed to occur via the stereoselective decomposition of a pair of (R, S )-betaines (Scheme 16). 

SCHEME 6. Proposed mechanism for the thermolysis of arsonium ylides ... 

Some arsonium ylides have been reported to decompose when heated in solution. For example, when triphenylarsonium benzylide was heated in a boiling benzene-ether mixture it decomposed to give triphenylarsine and a mixture of stilbenes. Trimethylar-sonium methylide likewise undergoes thermal decomposition to give trimethylarsine and ethylene A likely mechanism for these reactions involves carbenic decomposition of the ylide followed by attack of the carbene on unchanged ylide with expulsion of an arsine fragment (equation 2). 

It has been speculated" that, by analogy with the mechanism proposed for the extensively studied hydrolysis of phosphonium ylides, the steps involved in hydrolysis are protonation, followed by formation of a pentacovalent arsenic species and finally loss of a carbanion (equation 5). Presumably for arsonium ylides, as for phosphonium ylides, the... 

Triphenylarsine oxide reacts with a number of electrophilic acetylenes having electron-withdrawing substituents in what are, in effect, reverse-Wittig reactions, thereby providing stable arsonium ylides (equation 31) Reaction is presumably initiated by Michael-type reaction of the oxide with the alkyne, as exemplified in equation 32. As would be expected from such a mechanism, use of an unsymmetric alkyne, as in the foregoing example, results in virtually regiospecific attack by the oxide to give the product shown. 

In 2005, Gall, Mioskowski, and coworkers reported the organoborane-initiated polymerization of allylic arsonium ylides, which proceeds in a mechanism closely related to that of 11 [76]. However, in this process the polymer main chain is not constructed from one carbon unit but three carbon unit, which results in the formation of a series of poly(2-substimted-l-propenylene)s. When 2-substituted allylic arsonium ylide generated by the reaction of the corresponding arsonium salt with base... 

The Cu(I)-catalyzed cyclization for the formation of ethyl ( )-tetrahydro-4-methylene-2-phenyl-3-(phenylsulfonyl)furan-3-carboxylate 82 has been accomplished starting from propargyl alcohol and ethyl 2-phenylsulfonyl cinnamate. Upon treatment with Pd(0) and phenylvinyl zinc chloride as shown in the following scheme, the methylenetetrahydrofuran 82 can be converted to a 2,3,4-trisubstituted 2,5-dihydrofuran. In this manner, a number of substituents (aryl, vinyl and alkyl) can be introduced to C4 <00EJO1711>. Moderate yields of 2-(a-substituted N-tosyIaminomethyl)-2,5-dihydrofurans can be realized when N-tosylimines are treated with a 4-hydroxy-cis-butenyl arsonium salt or a sulfonium salt in the presence of KOH in acetonitrile. The mechanism is believed to involve a new ylide cyclization process <00T2967>. 

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