Allenyl enolate

Scheme 2.27 Regioselectivity of the protonation of the allenyl enolate obtained by 1,6-cuprate addition to enynoate 74.

The Diels-Alder reaction outlined above is a typical example of the utilization of axially chiral allenes, accessible through 1,6-addition or other methods, to generate selectively new stereogenic centers. This transfer of chirality is also possible via in-termolecular Diels-Alder reactions of vinylallenes [57], aldol reactions of allenyl eno-lates [19f] and Ireland-Claisen rearrangements of silyl allenylketene acetals [58]. Furthermore, it has been utilized recently in the diastereoselective oxidation of titanium allenyl enolates (formed by deprotonation of /3-allenecarboxylates of type 65 and transmetalation with titanocene dichloride) with dimethyl dioxirane (DMDO) [25, 59] and in subsequent acid- or gold-catalyzed cycloisomerization reactions of a-hydroxyallenes into 2,5-dihydrofurans (cf. Chapter 15) [25, 59, 60],... 

As is implicit in the fact that the products of the (stoichiometric) 1,6-cuprate addition - the lithium allenyl enolate and the organocopper compound - are formed as independent species, it is also possible to conduct the reaction catalytically through in situ regeneration of the cuprate. The reaction can thus be run in a continuous mode, with only catalytic amounts of the preformed cuprate being necessary (with simultaneous addition of the substrate and the organolithium compound) enabling the desired allenes to be prepared even on larger scales (Eq. 4.17) [3oj. 

As previously mentioned, allenes can only be obtained by 1,6-addition to acceptor-substituted enynes when the intermediate allenyl enolate reacts regioselectively with an electrophile at C-2 (or at the enolate oxygen atom to give an allenyl ketene acetal see Scheme 4.2). The regioselectivity of the simplest trapping reaction, the protonation, depends on the steric and electronic properties of the substrate, as well as the proton source. Whereas the allenyl enolates obtained from alkynyl enones 22 always provide conjugated dienones 23 by protonation at G-4 (possibly... 

Scheme 4.3. Regioselectivity of trapping reactions of acyclic allenyl enolates with different electrophiles.

Other magnesium allenyl enolates, such as 22, obtained by transmetallation of the lithium species have been used successfully in the preparation of a,-unsaturated acyl silanes (equation 28). ... 

An interesting class of chiral enolates are allenyl enolates. These ambident nucleophiles bear an axis of chirality. Krause and coworkers have found that an axis to center chirality transfer takes place in the aldol reaction of chiral magnesium allenyl enolate with pivalic aldehyde . The aldol reaction proceeds with good diastereofacial selectivity if... 

Addition to an enynone (1). Cuprates, particularly higher-order ones, undergo 1,6-addition to 1 to provide the less stable (Z)-dienone 2 as the major product. The (Z)-stereoselectivity increases with the bulk of the transferred R group. The addition may involve an allenyl enol. Addition of QHjSLi to 1, however, provides (E)-dienones, possibly via an intermediate 1,2-adduct. 

Similar to the addition reactions of acceptor-substituted dienes (Scheme 16), the outcome of the transformation depends on the regioselectivity of the nucleophilic attack of the organocopper reagent (1,4- vs. 1,6-addition) and of the electrophilic capture of the enolate formed. The allenyl enolate obtained by 1,6-addition can afford either a conjugated diene or an allene upon reaction with a soft electrophile, and thus opens up the possibility to create axial chirality. The first copper-mediated addition reactions to Michael acceptors of this type, for example, 3-alkynyl-2-cyclopentenone 75,... 

In contrast to the protonation, the regioselectivity of reactions between other electrophiles and allenyl enolates derived from 2-en-4-ynoates is independent of the steric and electronic properties of the reaction partners.7,7a... 

As expected according to the HSAB principle, hard electrophiles such as silyl halides and triflates react at the enolate oxygen atom to form allenyl ketene acetals, while soft electrophiles such as carbonyl compounds attack at C2. Only allylic and propargylic halides react regioselectively at G4 of the allenyl enolate to give substituted conjugated dienes. 

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