Arsonium ylide

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). 

Of the quaternary arsonium compounds, methyltriaryl derivatives are important as precursors of arsonium ylides, e.g. 

Vasishtha [56] also reportedp-acetyl benzylidene triphenyl arsonium ylide radically initiated bulk polymeriza-... 

Diarylprolinol ether 30 has also been used to accelerate the cyclopropanation of a,p-unsaturated aldehydes with arsonium ylides with excellent levels of asymmetric induction (95-98% ee) [157]. 

There is a comprehensive review of this area2 so only a few recent developments will be mentioned here. Organoarsenic intramolecular coordination compounds, e.g. (1), have also recently been reviewed,3 and organoarsenic chemistry is reviewed annually.4 There is a review containing 102 references on arsonium ylides.5 The canonical structures of the arsenic ylides are... 

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. 

The chemistry of arsonium ylides was first reviewed by Johnson (52) in 1966, by Samaan (78) in 1978, and by Bansal and Sharma (7) recently. In the present chapter, emphasis is placed on studies of the chemistry of arsonium ylides at our Institute and at the University of Science and Technology of Shanghai. However, our work was interrupted and some of our findings were delayed in publication for about 15 years. 

Most of the arsonium ylides reported have been prepared by the so-called salt method, which involves the quatemization of a suitable arsine... 

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). 

Arsonium ylides have also been prepared from the decomposition of diazonium compounds in the presence of a tertiary arsine. Thus tet-raphenylcyclopentadiene triphenylarsorane (13) was obtained by heating diazotetraphenylcyclopentadiene at its melting point in the presence of triphenylarsine (66). 

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). 

Arsonium ylides can be represented generally by the canonical forms 31a and 31b. By the application of X-ray diffraction to 2-acetyl-3,4,5-triphenylcyclopentadiene triphenylarsorane, Ferguson and Rendle (21) established that the canonical form 7c makes a significant contribution to the ground state structure. 

The pKa values of a series of conjugated acids of the arsonium ylides have been determined by potentiometric titration (73). A decrease in the basicity of the ylides with an increase in substituent electronegativity was observed. The effect of the substituents agrees well with the Hammett equation. Arsonium ylides are more basic (200-230 times) than the corresponding phosphonium compounds. 

The ultraviolet spectra of some stable arsonium ylides have been studied (32). The absorption maxima of the ylides occurred in the region... 

The photoelectron spectra of the arsonium ylides (CH3)3As=CH2 and its derivatives (CH3)3As=CHSi(CH3)3 and (CH3)3As=C[Si(CH3)3]2 have been studied (86). Vertical ionization potentials of arsonium and phos-phonium ylides are listed in the following tabulation. 

The 7r-ionization energies of arsonium ylides are very low, as is typical for ylides. The arsonium ylides are slightly more easily ionized than the phosphorus ylides, the difference amounting to about 0.1 eV AIEn(As= CH2/I CH2) = 0.1 eV. The differences in cr-ionization energies between As—C and P—C are also significant AIE As—C/P—C) 0.2-0.4 eV. 

Only a few reports on studies of the mass spectra of arsonium ylides have appeared in the literature. Gosney and Lloyd (32) studied the mass spectra of bis(carbethoxy)methylene triphenylarsorane and nitromethy-lene triphenylarsorane and found that preliminary fragmentation resulted in loss of the carbanionic moiety. We studied the mass spectra of 11 arsonium ylides and of triphenylarsine difluoride (30). 

For arsonium ylides 1 and 2 the molecular ion peak and (M - 1)+ peak were characteristic, whereas for the fluorinated arsonium ylides (3 to 9) they were not discernible at all. 

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

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