Addition reactions allenylzinc reagents

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

Additions of the chiral allenylzinc reagents to enantiomeric -methyl-/ -OBn aldehyde substrates proceeded with a high degree of reagent control to afford anti,syn or anti,anti adducts (equations 23 and 24). In these additions, the preferred anti orientation of the allenyl methyl and the aldehyde substituents requires the reaction to proceed by the normally less-favored anti Felkin-Anh pathway (equation 25). 

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. 

The addition of a chiral allenyl metal to an aldehyde generating a 2-substituted butynyl structure is named Marshall-Tamaru MT) reaction (Scheme 5-13). An allenyl palladium species is generated via a formal Sn2 substitution of the mesylate, which in turn undergoes a transmetalation with diethylzinc yielding a nucleophilic species (Scheme 5-13). The reaction of the electrophile proceeds via a similar -ester enolate transition state as depicted in Scheme 5-13. Corresponding allenylindium reagents can also be used instead of allenylzinc intermediates. ... 

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