Anion stabilisation

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). 

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. 

Many insect pheromones are derivatives of simple alkenes. Disparlure 56, an attractant for the gypsy moth, is an epoxide derived by stereospecific epoxidation from the Z-alkene 57. As neither substituent is anion-stabilising, a simple Wittig should give the right geometry. 

Any combination of two carbonyl or other electron-withdrawing groups will do this reaction. Compound 69 was needed for a barbiturate synthesis. As cyanide is very anion-stabilising, disconnection gives the ketone 70, whose synthesis will be discussed in chapter 30, and the nitrile 71. The synthesis was straightforward once the right conditions had been found.12... 

Overall, the transformations are equivalent to carbene additions to the styrenes. However, a carbene mechanism can be ruled out since the only alkenes which are successful are those carrying anion-stabilising groups. 

A silyl or aryl anion-stabilising group at the terminus of the alkyne is essential for rapid exodig cyclisations other than those of alkyllithiums 177178 aryl and vinyllithiums will cyclise onto alkynes bearing alkyl substituents only very slowly even in the presence of TMEDA.180 The allenyllithium cyclisation from 407 is a remarkable example of a cyclisation initiated by... 

Figure 224 Scheme for an aqueous, anionically stabilised polyurethane dispersion. Source Klein H.P. and Schwab, M., Aqueous polyurethane dispersions - an environment friendly alternative for high quality paint systems, Resin News, number 29 (July) published by Hoechst AG, Frankfurt, 1993... 

Anions stabilised in this way may undergo similar reactions to the ones that have been outlined above in which the anions were stabilised by one or more a-carbonyl groups. 

Most cyanoacrylate adhesives are maintained in a liquid state at room temperature by the addition of free radical and anionic stabilisers at suitable concentrations so as not to interfere with the functionality of the adhesive. These stabilisers/inhibitors are added at... 

Sultones were the earliest anionic stabilisers used in cyanoacrylates but fell from favour because of their potential carcinogenicity. Chelates of boric acid derivatives with polyhydroxy compounds also were considered as anionic inhibitors. Anionic inhibitors are normally added at concentrations between 0.001 and 0.01% depending on the application. 

Allyl anions with electron-withdrawing and anion-stabilising functional groups form anions much more easily and then react (mostly) in the a-position. There are many examples of this sort of substituent including extended enolates (chapter 11), and allyl anions stabilised by phospho-nium 159, phosphonate 160, sulfone 161, and cyanide 162 functional groups. All of these prefer to react in the a-position (at least kinetically) and we shall explore some of these. 

We shall finish this chapter by considering two ways to achieve condensation between an allyl anion and an aldehyde (or ketone) in the synthesis of dienes 163. The reagent 165 is an allyl anion stabilised by a heteroatom (Z) that can be lost with the OH group after addition to R CHO. 

Anions of allyl sulfides have been quite widely used but their regioselectivity is unpredictable and the vinyl sulfides 107 Z = SR resulting from y-addition are difficult to hydrolyse.32 Anions of allyl ethers are more difficult to prepare, but generally better behaved.33 A study of such anions led Still to propose that allyl anions with very anion stabilising substituents Z = COR 118, S02Ph 119, +PPh3120 etc, react a with all electrophiles, those with moderately anion-stabilising substituents... 

You may think that we already know how to do this by conjugate or Michael addition where Z is an anion-stabilising group. These reactions do add nucleophiles to double bonds but the... 

The tetrahydropyran ring 48 has a tertiary carbon joined to oxygen 48a and this C-O bond can be made by addition of an alcohol to an alkene. Addition of an anion-stabilising sulfone 50 allows a C-C disconnection back to the epoxide 51 that does look like an AE product except for the allylic chloride. 

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