Zinc Borates: Functionalized Hard Nucleophiles for Coupling Reactions with Secondary Allylic Acetates
We have succeeded in developing zinc borates of the general structure 3 for coupling reaction with allylic acetates. The advantages of using compounds 3 are their compatibility with carbonyl groups such as aldehyde, ketone, and ester groups, and their high reactivity toward secondary allylic acetates. Zinc borates 3 were prepared from boronate esters 1 [RT = p-(CHO)C6H4, p-(Ac)C6H4, p-{Ac(CH2)2}C6H4, p-(AcOCH2)C6H4, p-{AcO(CH2)3}C6H4, p-{EtO2C(CH2)2}C6H4, (E)-CH=CH(CH2)4OAc] with MeZnCl; subsequent treatment with allylic acetates 4 [R = n-C5H11, c-C6H11, (CH2)2CH(−O(CH2)2O−)] in the presence of NiCl2(PPh3)2 (10 mol-%) in THF−DMI (1,3-dimethyl-2-imidazolidinone) (10 equiv.) at 40−50 °C overnight furnished the coupling products 5 in good yields. Among the products, 5bb, possessing one free and one protected aldehyde group, is a highlight of this type of reaction. The stereochemical aspects of the reaction were also examined. Thus, the alkenyl groups of (E)- and (Z)-alkenyl borates 3b and c were transformed with retention of the olefinic geometry into acetates 4a and b (R = n-C5H11, c-C6H11), while reaction of cyclic acetate 11 proceeded with inversion at the carbon center involved in the reaction. In addition, we found that the anions generated from (EtO)2P(=O)CH2CO2Et and (MeO)2P(=O)CH2Ac under Masamune’s conditions attacked the aldehyde carbon in the boronate 1d to produce — after reduction of the double bond — the boronate esters 1i and 1j, respectively, in good yields.