Ketones reaction with allylboronates

Relatively few studies of the reactions of allylboronates and ketones have appeared. The reaction of (85) and ethyl pyruvate, for example, was conducted under 6 kbar pressure at 45 C for 80 h to give a 9 1 mixture of diastereomers (86a) and (86b). The stereochemistry of this reaction parallels that seen with crotyl-9 BBN (Figure 10) in that the structure of the major isomer is consistent with a transition state in which the —C02Et unit adopts an equatorial position. The same result could occur, however, via a boatlike transition state with an axial —C02Et group. 

Diastereoselective reactions of oxime (198) and phenylsulfenimines such as (201) with allyl metal reagents have been described (Scheme 33). The reaction of (198) and allylboronate (144) provides (199) with modest selectivity, while excellent diastereoselectivity was realized in the reactions of (201) and its C-2 epimer with the allylzinc reagent." The corresponding ketone derivatives, however, gave 70 30 mixtures of (204) and (205) upon reaction with diallylzinc, while with allyl Grignard, (205) is almost the exclusive product." The latter result is suggestive of a chelated transition state. 

Although most reactions of allylboronates involve aldehydes, other electrophilic partners are also possible. As exemplified in Equation 40, ketones can also react with allylboronates, yielding tertiary homoallylic alcohols [109]. Additions with simple ketones are much slower than similar reactions with aldehydes, and the stereoselectivity in these additions is quite variable, depending on the difference in size between the two substituents on the ketone. This was also noted in a recent study of binaph-thol-derived allylboronates, where high levels of enantioselection (>96% ee) were obtained in the reaction of 3,3 -(CF3)2-BINOL allylboronate 82 with several aromatic ketones (Equation 41) [110]. 

I.3.3.3.3.2.2. Simple Diastereoselection Reactions of Racemic -Substituted Allylboron Reagents with Achiral Aldehydes and Ketones... 

A copper-catalyzed reaction using a chiral diphosphine hgand, DuPHOS, with an added lanthanide salt, provides good levels of enantioselectivity (67-91% ee) in additions of the simple allylboronate 31 to both aromatic and aliphatic ketones that present a large difference of steric bulk on the two sides of the carbonyl group. One such example is shown in Eq. 81. On the basis of B NMR experiments and on the lack of diastereoselectivity in crotylation reactions, the... 

Homologation reactions. CICH2I undergoes I/Li exchange with an alkyl-lithium reagent and the chloromethyllithium thus obtained can be used to convert esters into chloromethyl ketones, Fischer carbene complexes into methyl ketones, and alkenylboronates into allylboronates. ... 

DFT has been used to probe the mechanism of allylation of ketones by allylboronates in the presence of diethylzinc. Results favour a double y-addition stepwise route, rather than concerted Lewis acid. Diethylzinc is found to be weakly catalytic, with the addition of ethanol substantially accelerating the reaction via Zn(OEt)2 catalysis, with Zn(OH)2 and ZnF2 efficiencies also being calculated. 

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