Ketones allenylzinc reagent addition

The earliest studies on allenylzinc reagents were mainly concerned with the regioselec-tivity of addition reactions to aldehydes and ketones. Moreau and Gaudemar converted... 

When the alcohol adduct from the allenylzinc reagent and diisopropyl ketone was treated with 80 mol% of allenylzinc bromide in HMPA, a mixture containing 12% of diisopropyl ketone and 88% of recovered alcohol was obtained after 7 days at ambient temperatures (equation 1). Thus, it may be deduced that the allenylzinc additions are reversible. Presumably, the propargyl adducts are intrinsically favored, but steric interactions between the R1 and R2 substituents in the propargyl product favors an increased proportion of allenyl adducts in a reversible process (see Table 1). HMPA would expectedly facilitate reversal of the addition by decreasing the ion pairing between the alkoxide anion and ZnBr cation of the adducts. This expectation was subsequently confirmed by a study of solvent effects. 

Under carefully controlled conditions, the reaction proceeds with excellent stereocontrol. Addition of the allenylzinc reagent derived from the (i )-mesylate 101 to (i )-aldehyde 100 proceeded at -20 °C to give the anti-anti triad 102 in 70 % yield with a small amount of the anti-syn isomer [30], As an intramolecular version, the propargyl benzoate 103 attacked the methyl ketone to afford the cyclopentanol 104 using Et2Zn and an Lewis acid with high stereocontrol. The most effective Lewis acid was Yb(OTf)3. A good catalyst was Pd(OAc)2-P(n-Bu)3 [31]. 

Propargylic zinc derivatives react with aldehydes or ketones with variable selectivity affording a mixture of allenic and homopropargylic alcohols [135]. However, under appropriate reaction conditions, high enantioselectivities and diastereo-selectivities can be achieved. Marshall and coworkers have shown that chiral propargylic mesylates such as 188 are converted to allenylzinc reagents 189 through treatment with a Pd(0)-catalyst. Their addition to an aldehyde such as 190... 

Reactions of allenylzinc bromide reagents with aldehydes afford increased amounts of propargyl adducts compared to analogous additions to ketones (Table 3). Presumably, diminished steric interactions render the aldehyde propargylic adducts more stable than their ketone counterparts. Alternatively, equilibration of the kinetic propargylic adducts with the allenyl isomers is slower for the adducts of aldehydes. 

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