Arsonium ylides reaction with carbonyl compounds

Arsonium ylides were discovered near the turn of the century, but their reactions with carbonyl compounds did not become elucidated until the 1960s. In a broad sense, arsonium ylides are midway in chemical behavior between ylides of phosphorus and those of sulfur. Stabilized arsonium ylides react with carbonyl compounds to afford alkenes, whereas the unstabilized analogs give rise to epoxides. More subtly, the nature of the substituents on either the ylide arsenic or carbon atom can alter the course of the reaction the choice of solvent can exert a similar effect. ... 

Among the heavy main group elements arsenic has received the most attention. Arsonium ylides react with carbonyl compounds to yield either the alkenes or epoxides, depending upon the structure of the ylide and the reaction conditions. )espite their toxicity, arsenic compounds can be handled safely. 

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). 

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

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. 

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... 

By far the most important reactions of ylides are those of the Wittig type, especially those with carbonyl compounds, and they will be dealt with first other reactions of this sort are those with other C = X functions and with nitroso-compounds. Other carbanionic reactions are then considered and a final section deals with the formation of cyclic compounds from arsonium ylides. Hydrolysis has already been discussion in Section III.B. 

Reaction of triphenylarsonium benzylide with benzylideneaniline gave 1,2,3-triphenylaziridine. This reaction is analogues to that of reactive arsonium ylides with carbonyl compounds. 

Reactions of arsonium ylides with carbonyl compounds take place much more readily than with phosphonium or sulphonium ylides. The nature of the products depends upon the character of the substituents on the ylide carbon atom, where electron-withdrawing substituents favour alkene formation, and of substituents on the arsenic atom, where... 

The reactions of substituted arsonium ylides with carbonyl compounds can be carried out with high stereoselectivity in favor of tra/w-disubstituted epoxides (equation 14). Equatorial attack is observed for addition to 4-t-butylcyclohexanone. Good stereoselectivity (S9 l) was observed for the addition of triphenylarsonium methylide to some (N,)V-dibenzyl)amino aldehydes at -78 C in THF. Interestingly, the initial hydroxy tetraalkylarsonium adducts were isolated under these conditions, and had to be cyclized under the action of sodium hydride in a separate step. 

A useful method for the direct epoxidation of carbonyl compounds involves reaction of an unstabilized arsonium ylide (10) with an aldehyde or ketone, analogous to the reactions of sulphur ylides. Unlike previous methods, the reaction of (10) to form (11) proceeds with a high degree of stereochemical control, and most aldehydes react cleanly giving frans-oxirans. Drawbacks to the method are (i) the toxicity of arsenic, and (ii) the fact that since PhaAs is... 

It is well known that aziridination with allylic ylides is difficult, due to the low reactivity of imines - relative to carbonyl compounds - towards ylide attack, although imines do react with highly reactive sulfur ylides such as Me2S+-CH2-. Dai and coworkers found aziridination with allylic ylides to be possible when the activated imines 22 were treated with allylic sulfonium salts 23 under phase-transfer conditions (Scheme 2.8) [15]. Although the stereoselectivities of the reaction were low, this was the first example of efficient preparation of vinylaziridines by an ylide route. Similar results were obtained with use of arsonium or telluronium salts [16]. The stereoselectivity of aziridination was improved by use of imines activated by a phosphinoyl group [17]. The same group also reported a catalytic sulfonium ylide-mediated aziridination to produce (2-phenylvinyl)aziridines, by treatment of arylsulfonylimines with cinnamyl bromide in the presence of solid K2C03 and catalytic dimethyl sulfide in MeCN [18]. Recently, the synthesis of 3-alkyl-2-vinyl-aziridines by extension of Dai s work was reported [19]. 

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). 

A standard method for the preparation of arsonium and sulphonium ylides involves the reaction of an arsine oxide or a sulphoxide with a compound having an acidic methylene group in the presence of either acetic anhydride or of a mixture of triethylamine and phosphorus pentoxide. This method has been applied to a number of cyclopentadienes substituted either by phenyl groups [105,131,142] or carbonyl groups [20,142,143], When acetic anhydride is used, acetylation of the cyclopentadiene ring may also take place, e.g. 

With HMPA, Wittig reactions that give ( )-alkenes were also observed (eq 18), as was the directed selectivity of a semista-bilized arsonium ylide towards carbonyl compounds. The arsenic ylide was generated from LDA in THF or THF/HMPA solution to give exclusively epoxide (eq 19) or diene (eq 20), respectively. ... 

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