Allyl anions homoenolate anion equivalent

Siloxycyclopropanes are known as homoenolate anion equivalents and have been favorably utilized in organic synthesis. Murai, Sonoda, and coworkers found that desi-lylative dimerization of the siloxycyclopropanes occurs on exposure to AgBp4 [26a], The reaction of 54 gives 1,6-diketone 55 in good yield (Sch. 13). The 8-silver ketone 56 is considered to be a key intermediate in the dimerization. The intermediary ver ketone, generated from siloxycyclopropane 57 and AgP, can be successfully trapped with allyl chloride to afford a (5,e-unsaturated ketone 58 [26b]. 

Homoenolate Anion Equivalent. The reaction of (1) with acid chlorides in the presence of titanium(IV) chloride affords the corresponding y-keto aldehydes after hydrolysis (eq 2) O-Acylation to form the 1-silyl allyl ester (17-30%) competes with the Se alkylation of the double bond. Bulkier silyloxy derivatives give y-keto aldehydes in higher yield. 

No other synthetic method Is known that achieves the equivalent transformation. Rather elaborate procedures using an allylic anion type of the homoenolate "equivalents" or homoenolate radicals have been reported, but their tolerance to the structure of the enone acceptor is much narrower. 

Alternatively, the ambident oi-hetero substituted allyl anions have been utilized as homoenolate equivalents. For example, in the presence of HMPA, allyl phenyl sulfides (251),192 allyl phenyl sulfones (252)192b c and allyl phenyl selenides (253)192d e add to a,(3-enones in a l,4(0)-mode, while allyl phenyl sulfoxides (254) and allyl phosphine oxides (255) afford 1 A j-addition exclusively, irrespective of solvent used.193 Hua has shown that additions of either chiral sulfoxide (254 R1 = R2 = R3 = R4 = H, R5 = p-tolyl) or allyl oxazaphospholidine oxide (256) occur with excellent enantioselectivity (>95% ee).194 Similarly, Ahlbrecht reports that the a-azaallyl (257) adds exclusively in a 1 A j-mode to acceptor (59) to afford 1,5-diketones (Scheme 86).195... 

Heteroatom-substituted allylic anions as homoenolate anion equivalents 862... 

In accordance with this model one finds diastereoselectively anti products on reaction of aldehydes with ( )-allyl compounds, whereas allyl systems with the (Z)-configuration give mainly syn products and it is even possible to effect asymmetric induction. As the double bond of the product can be oxidatively cleaved to a CW3 group, the reaction can be regarded as a stereoselective aldol reaction, an aspect which explains the widespread interest in this type of reaction. With heterosubstituted allylic anions it is sometimes possible to effect predominantly y-attack with different electrophiles by the choice of the heteroatom.2 For instance it is well known that with sulfur substituents like —SR, —SOR or —SOjR the a-attack dominates, but doubly lithiated allenethiol possesses high y-reactivity and can be used as a homoenolate anion equivalent in reaction with electrophiles such as alkyl halides (Scheme 7). ... 

On the other hand the inherent a-selectivity of allylsulfur carbanions can sometimes be transformed to a y-reactivity by a sigmatropic rearrangement, earning in addition the stereoselectivity typical for such reactions (see Section 4.S.2.3). Due to their significance in synthesis a lot of work has been done on reactions of heteroatom-substituted allyl anions with special emphasis on their use as homoenolate anion equivalents. The more recent developments, with the possibility of introducing diastereoselectivity, will be discussed later in Section 4.S.3.2.I. 

There are two main synthetic applications where the reaction of an allyl system with electrophiles is accompanied by an allylic rearrangement. One consists of the use of heteroatom-substituted allylic anions as homoenolate anion equivalents and the other represents a synthetically valuable alternative to the aldol reaction by addition of allyl metal compounds to aldehydes. 

Heteroatom-substituted allylic anions can serve as homoenolate anion equivalents in reaction with electrophiles, when y-attack can be realized and the formed vinyl heterocompound can be hydrolyzed to an aldehyde (Scheme 76). ... 

Just as anions of allyl derivatives can be homoenolate equivalents (chapter 13) so anions of vinyl derivatives can be acyl anion equivalents. Vinyl (or enol) ethers can be lithiated reasonably easily, especially when there is no possibility of forming an allyl derivative, as with the simplest compound 81. The most acidic proton is the one marked and the vinyl-lithium derivative 82 reacts with electrophiles to give the enol ether of the product17 84. However, tertiary butyl lithium is needed and compounds with y-CHs usually end up as the chelated allyl-lithium 85. These vinyl-lithium compounds add directly to conjugated systems but the cuprates will do conjugate addition.18... 

Heteroatom-stabilized Carbanions. Heteroatom-stabilized and allylic carbanions serve as homoenolate anions and acyl anion equivalents, e.g. a-anions of protected cyanohydrins of aldehydes and Q ,/3-unsaturated aldehydes are intermediates in general syntheses of ketones and Q ,/3-unsaturated ketones (eq 36). Allylic anions of cyanohydrin ethers may be a-alkylated (eq 37) or, if warmed to —25°C, may undergo 1,3-silyl migration to cyanoenolates which may be trapped with TMSCl. Metalated Q -aminonitriles of aldehydes are used for the synthesis of ketones and enamines (eq 38). Similarly, allylic anions from 2-morpholino-3-alkenenitriles undergo predominantly a-C-alkyl-ation to give, after hydrolysis, a,/3-unsaturated ketones (eq 39). ... 

Heteroatom-substituted allylic anions as homoenolate anion equivalents 4.5.322 Addition of ally Imetal compounds to aldehydes synthesis of homoallylic alcohols... 

Homoenolate equivalents (d reagents) are less conunon than nucleophilic acyl equivalents. The uses of 1-trimethylsilylallyl alcohols as effective homoenolate equivalents have been discussed. y-Alkylation of the trimethylsilyloxy-stabil-ized allylic anion formed by Brook rearrangement leads to the (isolable) tri-methylsilylenol ether (18) (Scheme 16). y-Alkylation of the functionalized ally-loxy-carbanion (19) has been reported. ... 

Factors influencing the regioselectivity of alkylation of allylic anions continue to be studied, a-Alkylation of the keten thioacetalide anion (13) is favoured by hard leaving groups/ whereas the a-aminonitrile anion (14) reacts with ketones in the a-position at low temperature (-78 °C), but behaves as a homoenolate equivalent (y-attack) at At the higher temperature, competing aldol... 

Ready access to the allylic anion (26) is obtained as a result of the increased kinetic acidity of the proton a- to the carbamate group. The anions react with electrophiles at the y-position (R 5 H), and thus function as homoenolate equivalents. ... 

---