Ylid Anions

The ylid anion (67) with chloroaminophosphines gave the phosphino ylids (68) and (69), and a small amount of the product (70) arising from -attack on (67). Both (68) and (69) with dichlorophosphines gave five-membered ring compounds, e.g. (71). The first air-stable a-diazophosphine (72) has been prepared and characterized by an X-ray crystal structure determination. ... 

It was suggested by Lepley and Guimanini 96> that the intimate immonium ion pairs 19 and 20 are the precursors to the rearrangement products rather than ylids or ylid anions because of the isolation of traces... 

Ester- and nitrile-stabilized ylid anions are more reactive than the corresponding neutral ylids and react with ketones (even enolizable ones) as well as aldehydes. E a,p- thylenecarboxylic acid esters. 2 eqs. BuLi (in hexane) added to a stirred suspension of startg. diphenylphosphonium bromide in THF, 1 eq. acetone added, and stirring continued at room temp, for 30 min product. Y 43% (Y 10% using 1 eq. BuLi and 0% with PhjP = CHC02Me). F.e. incl. a,p-ethylenenitriles, and trans stereo-selectivity with aldehydes, s. E.G. McKenna, B.J. Walker, J. Chem. Soc. Chem. Commun. 1989, 568-9 (E)-stilbene s. Tetrahedron Letters 29, 485-8 (1988). 

The obvious choice for a reagent is again a sulphur ylid, but how are we to control the regioselectivity of the addition The more reactive sulphur ylids, notably (26) and (27), add directly to the carbonyl group (kinetic control, cf p T 117 ) giving epoxides (29) while the more stable ylid (28), which combines the anion-stabilisations of (26) and (27), adds reversibly and gives the thermodynamic product (25). 

Under favourable circumstances, the initially formed /V-ylid reacts further through C-N cleavage. Thus, in the presence of a strong nucleophile, such as a phenoxide anion, the quaternary dichloromethylammonium cation forms an ion-pair with the phenoxide anion (Scheme 7.27), which decomposes to yield the alkyl aryl ether and the /V-formyl derivative of the secondary amine [22, 23]. Although no sound rationale is available, the reaction appears to be favoured by the presence of bulky groups at the 4-position of the aryl ring. In the absence of the bulky substituents, the Reimer-Tiemann reaction products are formed, either through the breakdown of the ion-pair, or by the more direct attack of dichlorocarbene upon the phenoxide anion [22,23],... 

The formation of carbanions, according to Scheme 6, has been much studied but has proved to be of little preparative use. The benzyl anion, generated from benzyl-dimethylsulphonium tosylate, reacted with acrylonitrile but the addition product was formed in only low yield Similarly the reactive ylid formed by deprotonation of trimethylsulphonium salts has been cathodically generated and trapped by several aldehydes and ketones as well as ethyl maleate and fumarate examples are given in Scheme 7. For the best case (benzophenone), the epoxide was formed in 40%... 

A summary of the in situ use of the azobenzene probases is given in Table 2. Apart from the generation of ylid, referred to above, the main applications have been for N- and C-alkylation of weak nitrogen and carbon acids, for the promotion of condensation and substitution reactions involving carbanions such as the cyano-methyl anion, for an interesting carboxylation reaction (entries 4 and 17), and for base-promoted cyclisations (entries 5 and 6). 

Derivatives of the general formula (7) in Table 6 have been successfully used as probases and their properties in this context are being further explored. In common with the azobenzenes and ethenetetracarboxylate esters, the fluoren-9-ylidene derivatives usually display two reversible one-electron peaks in cyclic voltammetric experiments. Although disproportionation is possible (cf. Scheme 12) it is the dianions which are the effective bases. It was shown early on that the radical-anions of such derivatives are long-lived in relatively acidic conditions (e.g. in DMF solution the first reduction peak of Ph C -.QCN) remains reversible in the presence of a 570-fold molar excess of acetic acid, at 0.1 V s ). Even the dianions are relatively weak bases, useful mainly for ylid formation from phosphonium and sulphonium salts (pKj s 11-15) they are not sufficiently basic to effect the Wittig-Homer reaction which involves deprotonation of phosphonate esters... 

Aryl-5-triphenylphosphonium-1,2,3-triazole ylids (216) with MP give high yields161 of adducts 217, which were also obtainable from the anion 218. 

The n orbitals of the ylids 2.98 and 2.100 are isoelectronic with those of an allyl anion, in which produces Ti-bonding on both sides of the central atom, restricting rotation. 

When the ylid derived from the oxide 7 was treated with benzaldehyde, a 2-methoxy-l,3-diene was formed. This led us to attempt a synthesis of (i) ar-turmerone by a sequence (Scheme 3) in which 7 effectively acts as an acyl anion equivalent. The required aldehyde 8 was synthesized in good yield using initially diphenyl(methoxymethyl)phosphine oxide (3), and then chlorotri-methylsilane-sodium iodide (12) to cleave the enol ether. 

The unstable aspect of the ylid is the carbanion phosphonium salts are stable compounds so any substituent that stabilises the anion also stabilises the ylid and this reverses the stereoselectivity to favour the -alkene. Even benzylic ylids give -alkenes as in the reaction9 with the anthracene 37 that gives a good yield of crystalline 38 having a coupling constant between the two marked Hs of 17 Hz. One possible explanation is that the formation of the betaine or oxaphosphetane is reversible if the ylid is stabilised and only the faster of the two eliminations occurs to give the E -alkene. 

When the substituent becomes very anion-stabilising, as in 42, the ylid may not react with ketones and anions of phosphonate esters are usually preferred in the Homer-Wadsworth-Emmons (HWE) variant.11 The reagent triethyl phosphonoacetate 46 is made by combining a phosphite (EtO)3P instead of a phosphine, with ethyl bromoacetate. Displacement of bromide 44 gives a phosphonium ion that is dealkylated by bromide 45. 

As the ylid 51 is stabilised by the nitrile as well as the benzene ring, the phosphonate ester 54 is preferred in the manufacture and the reaction is strongly trans selective.13 The by-product is the anion of dimethyl phosphate 55 which is water-soluble and very easy to separate from the product 50. By contrast, triphenylphosphine oxide is insoluble in water and can be difficult to separate from the alkene. 

Silyl triflates may also be used as the catalyst to the same goal. In both cases, conjugated base induces desilylation to create the ylid (compare with the action of tetrabutylammonium fluoride on the same imine which led to the anion see Section VI.B.6.a).333,391... 

Similarly to the action of acids, HF-pyridine has been shown able to promote formation of azomethine ylids from SMA imines.399 This is an interesting result compared with the action of fluoride anion which led to the formation of the corresponding anion (vide supra). 

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