Arsonium ylides reactions

Another arsonium ylide reaction involves a notable transylidation reaction between a phosphorus and an arsenic ylide (Scheme 6). A useful arsenic ylide which provides a hydroxymethyl epoxide has been reported (equation IS) note the use of biphasic reaction conditions for ylide generation. 

Arsonium Ylides. Arsonium ylides were first prepared by reaction between an arsonium halide and phenyUithium. Thus methyitriphenyiarsonium iodide [1499-33-8], C H gAsI, and phenyUithium give tripbenylarsonium metbylide [19365-61-8], C H yAs ... 

In this case the ylide was not isolated but allowed to react with ben2ophenone to give, after hydrolysis with hydrochloric acid, 1,1-diphenylethylene, diphenylacetaldehyde, and triphenylarsine (160). An excellent method for preparing arsonium ylides involves the reaction between a stable dia2o compound and triphenylarsine in the presence of a copper catalyst such as bis(acetylacetonato)copper(II) (161). Rather than a dia2o compound, an iodonium yhde can be used again a copper catalyst is necessary for an optimum yield of product. An example of the use of a dia2o compound is shown in the formulation of triphenyl arsonium 2,3,4-triphenylcyclopentadienyLide [29629-32-17, C H As ... 

FYedictably, fluoroketones undergo olefination reactions with more reactive arsonium ylides [35] (equation 28). 

However, certain phosphonium ylides, such as those with an electron-withdrawing substituent in the alkylidene moiety, are relatively unreac-tive toward certain substrates such as ketones (22, 77, 95). This led us to consider whether arsonium ylides might be preferable to phosphonium ylides in certain reactions (48, 94). The overlap of the p orbitals of carbon with d orbitals of arsenic is less effective than with d orbitals of phosphorus. Therefore the covalent canonical form (la) should make a smaller contribution to the overall structure of arsonium ylides than to that of the corresponding phosphonium ylides. 

Although, in principle, any arsonium ylide could be prepared via the slat method as demonstrated above, attempts to prepare a ylide from dimethyldibenzylarsonium salt (4) with ethereal phenyllithium failed. The product of the reaction after quenching with water was stilbene (5). Apparently, a Stevens rearrangement occurred during the reaction (101). 

A facile method for the preparation of a variety of stabilized arsonium ylides in good yield has been developed by the action of active methylene compounds with tertiary arsine oxide or tertiary arsine dihalide. Thus triphenyl-arsine dihalides react with a number of active methylene compounds in the presence of a tertiary amine to afford arsonium ylides (6) (40). The reaction of triphenylarsine oxide with active methylene compounds in the presence of either acetic anhydride or triethylamine-phos-phorus pentoxide gave rise to arsonium ylides (6) (32, 36. 65, 67). 

In the reversible Wittig reaction, triphenylarsine oxide reacted with electron-deficient acetylene derivatives to form stable ylides. Thus triphenylarsine oxide reacted readily with methyl propiolate, ethyl phenylpropiolate, dimethyl acetylenedicarboxylate, and hexafluoro-2-butyne as well as dicyanoacetylene to give arsonium ylides (12). The reaction temperatures required ranged from -70°C in the case of dicyanoacetylene to 130°C in the case of ethyl phenylpropiolate (15). 

This method was extended to different diazonium salts and several arsonium ylides (14) were prepared (23, 32). The reaction is greatly facilitated by the presence of copper, copper-bronze, or copper salts. For example, attempts to prepare the bis(carbethoxy)methylene ylide by thermolysis of diethyl diazomalonate in the presence of triphenylarsine without the presence of a catalyst proved abortive, whereas this ylide was obtained in 61% yield if the reactants were heated at 150°C with copper-bronze (32). 

A. Reactions of Stabilized Arsonium Ylides with Carbonyl Compounds... 

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. 

Cavicchio c-Y al. (14) studied the reaction of stabilized ylides and related onium salts with tropylium ion and found that the reaction between a tropylium salt and an arsonium ylide led to tra .v-chalcone (48) along with triphenylarsine. 

Trippett and Walker (97) found that the reaction of para-substituted benzylidene triphenylarsoranes with carbonyl compounds led to olefins when the benzylidene para substituent is highly electron withdrawing, but otherwise to epoxide. KumaricY al. (64) prepared two new semistabilized arsonium ylides, p-bromo- and p-iodobenzylidene triphenylarsorane, which were treated with a series of carbonyl compounds to yield exclusively trans-olefins. In no case was an epoxide obtained. 

Allen et at. found that high yields of the dissymmetric trans-2,3-diaryloxirans result from the reaction between semistabilized arsonium ylides derived from the benzyl salts of the enantiomers of racemic o-phen-ylenebismethyl phenylarsine and of methyl a-naphthyl-p-tolylarsine upon reaction with prochiral aromatic aldehydes. Optical yields of between 4.7 and 38% are obtained using optically active arsonium salts (2). 

Toward carbonyl compounds, the behavior of nonstabilized arsonium ylides such as methylene or ethylidene triphenylarsorane is similar to that of sulfonium ylides (10, II, 13, 97). When an arsonium ylide was reacted with aminoketones in a cold 1 1 DMSO-THF solution a smooth reaction took place and the corresponding 3-substituted indoles were obtained in fair to good yields (II). 

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). 

In comparison to the stabilized phosphonium ylides which fail to react with cv,/3-unsaturated esters or ketones, the corresponding arsonium ylides react smoothly with ,/3-unsaturated esters or ketones (e.g., PhCO—CH=CH—CH3) to form cyclopropanes in fair to good yields. Furthermore, the reaction is stereospecific (45, 45). 

E)-ct, -Enones. The arsonium ylides formed from these salts react with aldehydes under phase-transfer conditions to form (E)-a,(3-enones in 71-99% yield (equation I). This Wittig olefination is particularly useful for reactions of unstable aldehydes because of the mild conditions. 

Arsonium ylides. Semistabilized allylic arsonium ylides are generally not useful in synthesis because they react with carbonyl compounds to form mixtures of epoxides and alkenes. Unexpectedly, the reaction of the semistabilized ylide triphenylarsonium... 

Indeed, in diethyl ether, lithium dimethylcuprate usually reacts with the a-enone group to give a methyl-substituted bromo ketone. Addition of hexamethylphosphoric triamide (HMPT), however, slows down this reaction to such an extent that displacement of the bromo substituent takes place [698], Another remarkable example of the influence of HMPT on chemoselectivity is the reaction of an arsonium ylide, Ph3As= CH-CH=CH-Ph, with benzaldehyde in tetrahydrofuran solution, yielding either an epoxide (in THE) or an alkene (in THF/HMPT) [699],... 

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