Arsonium ylides reactivity

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

Many of the stable arsonium ylides are crystalline solids which may be kept in air without significant decomposition. More reactive arsonium ylides need to be made as required and used in situ. Their decomposition usually arises from hydrolytic attack most arsonium ylides appear to be thermodynamically stable at room temperature. 

Many arsonium ylides dissolve in acids to form salts, from which they can be re-obtained by treatment with a base, as in the salt method for their preparation. The basicity of the ylides indicates the relative stabilities of the ylides and their salts and in so doing gives some guide to the stability of the ylides. Thus stable ylides are less readily protonated than are reactive ylides and require weaker bases for their generation from salts. 

The general pattern which emerged was that stable arsonium ylides provided alkenes whilst reactive arsonium ylides gave epoxides . This was attributed to stabilization of the transition state leading to alkene formation being provided by those same electron-withdrawing groups which stabilized the ylides ... 

Allylic arsonium ylides show a similar pattern of reactivity. Ethoxycarbonylallyl ylides, wherein the ester group is conjugated with the ylidic carbon atom, gave dienes in reactions with aldehydes or ketones , whereas other allylic ylides lacking such an electron-withdrawing substituent gave vinylic epoxides in high yield e.g. equation 8. In the... 

Semi-stabilized arsonium ylides are intermediate in behaviour between stable and reactive ylides, and may provide alkenes and/or epoxides " °. In these cases factors such as the substituent groups on arsenic, and the nature of the solvent and of the base, may become important in determining the nature of the product this will be considered in more detail later (Section V.A.4). Small changes in the structure of the ylidic moiety may also have a marked effect for example, whereas triphenylarsonium jS-napththylmethylide reacts to give epoxide, the presence of a bromine atom at the adjacent a-position of the naphthalene ring results instead in the formation of alkenes . ... 

Reactivities of arsonium ylides compared to those of phosphonium and sulphonium ylides... 

Comparative studies involving acylylides , fluorenylides and cyclopentadienyl-ides , , show that arsonium ylides are markedly more reactive than their... 

The reactions of a series of arsonium ylides with p-nitrobenzaldehyde have been shown to be first order for each reagent and there is a general tendency for the more basic ylides to be the more reactive" ". The correlation is not, however, complete, since factors other than basicity, e.g. steric, and interactions between ylidic substituents and the arsenic atom, also affect the reactivity " , but as a generalization it is largely valid and also must be a significant factor in the greater reactivity of arsonium compared to phosphonium and sulphonium ylides. A fair correlation has also been noted between the chemical shift of the signal from the methine proton in a series of ylides and their rates of reaction with p-nitrobenzaldehyde" ". ... 

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

Indoles have been prepared from reactions of o-aminophenylketones with reactive , or stable " arsonium ylides. Oxo-stabilized ylides reacted with 2-chloro-oximes to give trans-5-acyl-A -isoxazolines, and isoxazoles have been obtained from reactive arsonium ylides and a-isonitrosoketones, and from triphenylarsonium methylide and nitrile oxides The latter ylide reacts similarly with nitrile imines to give pyrazoles. With triphenylarsonium benzylides and benzoylylides,benzene diazonium salts give 1,3,4,6-substituted 1,4-dihydro-1,2,4,5-tetrazines in a reaction in which initial coupling of the reagents is followed by a dimerisation. ... 

The first account of the preparation of an arsonium ylide (1) (see Section I) involved what is known as the salt method , namely the reaction of a halogeno compound with triphenylarsine to form an arsonium salt, which is then treated with a suitable base to provide the ylide (see Scheme 1). Stable ylides are frequently made and isolated by using aqueous alkali. Reactive ylides need anhydrous conditions and the use of a suitable strong base, and are used in situ. Thus triphenylarsonium methylide (25) has been prepared in solution " " it was isolated by using as base sodium amide in tetrahydrofuran under an atmosphere of nitrogenTrimethylarsonium methylide (26) has been made indirectly, by desilylation of the trimethylsilyl methylide (27) with trimethylsilanoP. Ylide... 

A publication discussing the uses of reactive arsonium ylides for the stereospecific preparation of epoxides draws attention to the fact that arsonium salts are less readily prepared than phosphonium salts because of the poorer nucleophilicity of arsenic compared to phosphorus, and suggests methods for obtaining them. Primary salts were made from alkyl triflates, while a-branched salts were prepared from alkyldiphenylarsines, obtained from iodo compounds as, for example, in equation 23. Reaction of alkyl halides with arsines to form arsonium salts is also promoted by the presence of silver tetra-fluoroborate . 

In the presence of triethylamine, triphenylarsine dichloride reacts with a variety of compounds having reactive methylene groups to give arsonium ylides (equation 25). This method is limited to compounds in which X, Y are electron-withdrawing groups, i.e. to the preparation of stable ylides. 

Compounds having reactive methylene groups also react with triphenylarsine oxide, either in acetic anhydride, or in triethylamine with phosphorus pentoxide also present, to give arsonium ylides. First applied to cyclopentadienes bearing either phenyl or acyl substituents its use was extended to prepare a range of stable arsonium ylidesWhen the reaction is carried out in acetic anhydride, acetylation may accompany the condensation reaction, e.g. equation 26 ° ... 

In contrast to the above described reactivity of dimethylsulfonium trimethylsilylmethylide (2), triphenylarsonium trimethylsilylmethylide (5) is able to cyclopropanate the C-C double bond of chalcone [( )-l,3-diphenylpropenone] and some of its derivatives. The arsonium ylide is prepared in situ from (trimethylsilylmethyl)triphenylarsonium tetraphenylborate (4) by deprotonation with phenyllithium. The reactions with various substituted chalcones were carried out at room temperature and at — 70 "C. Although the yields achieved for cyclopropanes 6 are almost equal, the formation of the -isomer predominates at lower temperature whereas at room temperature the isomer ratio is nearly 1 1. 

In Wittig-type reactions with aldehydes and ketones, arsonium ylides have been shown to give either epoxides or alkenes or mixtures thereof (Scheme 3.88) [140]. However, the semi-stabihzed yHde 457 can be directed onto either pathway by tuning the basicity of the solvent [141]. In pure TH F, the epoxide 459 was formed, whereas in THF/HMPA mixtures the conjugated diene 461 was obtained. This complete switch was observed for a variety of aldehydes and ketones. Rationalization lies in the assumption of zwitterionic intermediates 458 and 460, which react via different conformations. Thus, the anti-conformer 458 is reactive in unipolar solvents, presumably via aggregate formation to give the epoxide, whereas in the presence of HMPA the monomeric species 460 is formed, which undergoes syn-elimination. 

Generally, arsonium ylides [62] are more reactive but less accessible than phos-phonium ylides. Recently, the chiral arsonium reagent 30 has appeared, and has been applied in asymmetric Wittig-type carbonyl olefinations. This first chiral arsonium reagent also bears 8-phenylmenthyl as a chiral auxiliary on its carboalkoxy portion [63], and gave moderate chemical yields and diastereoselectivities in the conversion of 4-substituted cyclohexanone derivatives to axially chiral non-racemic alkylidene cyclohexanes under the same reaction conditions as used for the related reactions with phosphorus reagents (Scheme 7.15). On the other hand, the corre-... 

Such ylides are unstable and react with carbonyl compounds to give both the Wittig product (p. 545) as well as AsPh3 and an epoxide. However, this very reactivity is sometimes an advantage since As ylides often react with carbonyl compounds that are unresponsive to P ylides. Substituted quaternary arsonium compounds are also a useful source of heterocyclic organoarsanes, e.g. thermolysis of 4-(1,7-dibromoheptyl)trimethylarsonium bromide to l-arsabicyclo[3.3.0]octane ... 

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]. 

The mixed phosphonium-iodonium ylides (Section 2.1.10.1), such as the tosylate 796, represent a useful class of reagents that combine in one molecule the synthetic advantages of a phosphonium ylide and an iodonium salt [1091-1100]. Specifically, phosphorane-derived phenyliodonium tosylate 796 reacts with soft nucleophiles, such as iodide, bromide, benzenesulfinate and thiophenolate anions, to form selectively the respective a-functionalized phosphonium ylides 797 (Scheme 3.315), which can be further converted into alkenes (e.g., 798) by the Wittig reaction with aldehydes [1092]. The analogous arsonium-iodonium ylides have a similar reactivity toward nucleophiles [1091, 1094, 1101]. 

Triaryl-phosphonium or -arsonium fluorenylides are markedly more stable to hydrolysis than their trialkyl analogues [171-174], This can again be attributed to the more polar character of the latter ylides [173], In keeping with this the trialkyl derivatives are also more reactive in Wittig reactions than are their triphenyl analogues [172,173,176,177],... 

Arsenic, Antimony, and Bismuth.- (Diphenylarsino)methyl-lithium (203) has been prepared by either halogen-lithium or tin-lithium exchange from (204) or (205) respectively. Interestingly the reactivity of (203) depends on the method of preparation used, although the reasons for this are not known. A stereoselective synthesis of JE-a, -unsaturated aldehydes has been achieved by reacting aldehydes with the arsonium salt (206) in the presence of a weak base. The ylide derived from (206) shows a reasonable... 

---