Phosphonium yhdes

The reaction affords in variable yields (19-85%) a stabilized phosphonium yhde exhibiting two electron withdrawing substituents on the ylidic carbon a to the phosphorus atom, and requires apparently a dialkylarylphosphine oxide incorporating an overcrowded 2,4,6-trisubstituted aryl group. 

Among various modifications of the side chain of stabilized ylides [13,14] can be pointed out the preparation and transformation of the phenyliodonio a-substi-tuted phosphonium yhdes (Scheme 3) [15]. These compounds represent a potentially useful class of reagents, in which the iodonium group can be further substituted by nucleophiles such as PhSLi. 

Other works have shown that phosphonium yhdes a-stabilized by a cyano or keto group can behave as ambidentate hgands towards palladium complexes (Scheme 21) [85-89]. 

Figure 11.1 shows the parent compound Me3P -CH2 of the phosphonium yhdes as A and the most simple triarylphosphonium ylide, Ph3P CII2 , as B. The ionic representation of the ylides in Figure 11.1 represents only one of two conceivable resonance forms of such... 

A few years after this first report, a chiral non-racemic phosphonium yhde 4 having a stereogenic phosphorus center was prepared and used for asymmetric olefination (Scheme 7.2). Here, it was reported that monocarbonyl substrates 1 were converted into the olefinic products in enantiomerically enriched form, al-... 

In 2005, Abdou et al. reported that the reaction of A-aryl imines with stabilized phosphonium yhdes in chloroform under reflux gave a,p-unsaturated nitriles, esters, and ketones with exclusive E selectivity but in only about 20% yield [226]. Recently, Tian et al. have found that the employment of N-(p-toluenesulfonyl) imines to react with (cyanomethylene)triphenylphosphorane in... 

Fluonnated ylides have also been prepared in such a way that fluonne is incorporated at the carhon P to the carbamonic carbon Vanous fluoroalkyl iodides were heated with tnphenylphosphine in the absence of solvent to form the necessary phosphonium salts Direct deprotonation with butyUithium or hthium dusopropy-lamide did not lead to yhde formation, rather, deprotonation was accomparued by loss of fluonde ion Flowever deprotonation with hydrated potassium carbonate in thoxane was successful and resulted in fluoroolefin yields of45-S0% [59] (equation 54) P-Fluorinated ylides may also be prepared by the reaction of an isopropyli-denetnphenylphosphine yhde with a perfluoroalkanoyl anhydnde The intermediate acyl phosphonium salt can undergo further reaction with methylene tnphenylphosphorane and phenyUithium to form a new yhde, which can then be used in a Wittig olefination procedure [60] (equation 55) or can react with a nucleophile [6/j such as an acetyhde to form a fluonnated enyne [62] (equation 56)... 

Since the pioneering work of H. J. Bestmann and coworkers in the 1960s, acylation of phosphonium ylides is a well known process for the preparation of P-ox-ophosphonium yhdes. The classical way (a) using acylating agents such as acyl chlorides and in situ transylidation is still useful (Scheme 4) [16,17]. 

A different approach to the modulation of steric and electronic properties has been reported using NHC moieties as ancillary ligands. A very rich chemistry has been developed around this topic in the last few years [145-148]. Complexes (67) have been prepared by deprotonation of the corresponding phosphonium-imidazo-lonium salt. The analysis of several Rh derivatives (67) shows that the yhde behave as a very strong CT-donor, even more than the NHC ligand, and that Pd complexes... 

Stabilized species in the phosphorus yhde category are normally generated in a two-step sequence beginning with the formation of the quaternary phosphonium salt. This is usually accomplished with ease by the reaction of triphenylphosphine with the appropriate haloalkane. Salt formation is followed by deprotonation at the carbon adjacent to phosphorus using an appropriate base to generate a zwitterionic species stabilized by the adjacent functionality (illustrated in equation 20). The resultant phosphorus species reacts with an introduced carbonyl compound to generate an intermediate oxaphosphatane that undergoes decomposition to produce alkene and phosphine oxide at relatively low temperatures (equation 21). 

Carbonyl-stabilized phosphorus ylides are less nucleophilic and hence do not react with (phosphine)gold(I) halides, but their gold(I) complexes can be generated from precursors such as [Au(acac)(PPh3)] or [AuCl(SC4Hg)] by reaction with phosphonium salts and ylides, respectively, and again both mono- and bis(yhde) complexes have been obtained (equation 48 and Scheme 8)206 207. The ylide carbon atoms in these complexes are centres of chirality, but no stereospecificity was observed in the coordination process and racemic mixtures are formed throughout. 

Construction of the polyene chain first involves condensation of the aldehyde 2-2 with the yhde from treatment of the phosphonium salt 2-1 with phenyllithium (Scheme 6.2). The trans configuration of the new double bond in the product... 

These highly air-sensitive complexes apparently owe their high stability to the incorporation of chelating phosphorus yhde ligands. The neutral complexes show dynamic behavior in solution producing dimeric, trimeric and polymeric species in addition to the monomers. This could be demonstrated with the di-tert-butyl phosphonium derivative of lutetium (Schumann and Reier, 1982a) ... 

In the first step, acid-catalyzed azomethine formation (113 115) takes place. In the second step, the phosphonium salt 115 is deprotonated to give the P-yhde 116, which (in analogy to an aza-WiTTic reaction) undergoes intramolecular nucleophihc addition to the imine function. The resulting phosphonium betaine 117 eliminates PPh3, thus establishing the indole system 114. 

In the first step, the yhde is alkylated by the benzylhalide 1 to give the phosphonium salt 3 its deprotonation by the second yhde moiety leads to another (benzylated) P-ylide, whose intramolecular Wittig reaction with the adjacent ester C=0 group (4 2) affords the 4H[Pg.336]

The single recently published application of the amide form was in the synthesis of the histone deacetylase inhibitor vorinostat (1691 (Figure 6.52). Phosphonium salt 166 was prepared by the reaction of triphenylphosphine with chloro[l- C]acetanilide, itself prepared via chloro [l- " C]acetyl chloride. Olefination of aldehyde 167 with the yhde produced from 166 furnished the a,/3-unsaturated amide 168 in 85% yield °. The tributylphosphonium analog was employed as a starting material for the synthesis of [ " C]GV 1550526, a glutamate co-agonist . 

When compared with phosphonium and sulfonium ylides, the corresponding selenonium and telluronium yhdes frequently exhibit better reactivity and nucle-ophilicity, while the use of selenides and tellurides in catalytic yUde reactions is far less reported. In 2001, Metzner and coworkers reported an asymmetric epoxi-dation reaction of aldehydes using a C2-symmetric selenide 42 resembling their sulfide catalyst (Scheme 20.32). High yields (65-97%) and enantioselectivity (76-94% ee) were obtained with a range of aromatic aldehydes and cinnamalde-hydes. However, no diastereoselectivity (trans cis = 1 1) was observed in these reactions, while the corresponding sulfide gave around 80% diastereoselectivities, which could be rationalized as the formation of a less diastereoselective early transihon state with the more reactive selenonium ylides. The reactions of more electron-deficient aryl aldehydes (p-Cl and p-CFs) were less enantioselective (76% ee and 83% ee, respectively) [58]. 

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