Science of Synthesis

Negishi, E.-i.; Takahashi, T., Science of Synthesis, (2003) 2, 734.

It was observed in the late 1970s that, under the conditions of the zirconium-catalyzed monomethylalumination of alkynes with trimethylaluminum and dichlorobis(η5-cyclopentadienyl)zirconium(IV) (‌49‌),[‌184‌] terminal alkenes did react, but did not produce either the desired methylalumination product or alkene polymers in significant yields. Reinvestigation using oct-1-ene gave two major products of 2-octyloct-1-ene and 2-methyloct-1-ene in about 60 and 20% yield, respectively. Evidently, the desired (2-methyloctyl)metal must have formed, but then must have undergone hydrogen-transfer hydrometalation to produce 2-methyloct-1-ene and the octylmetal, which would then undergo a similar carbometalation and a hydrogen-transfer hydrometalation.[‌189‌] Clearly, successful development of an enantioselective single-stage carboalumination of alkenes must somehow avoid dehydrometalation leading to hydrogen-transfer hydrometalation. In sharp contrast, however, the use of bulky and chiral indene derivatives, especially 1-menthan-3-yl-1H-indene,[‌57‌] has been shown not only to avoid the side reactions due to hydrogen-transfer hydrometalation, but also to lead to reasonably high enantiomeric excess.[‌189‌] Thus, various monosubstituted alkenes ‌145‌ in the presence of a catalytic amount of ‌48‌ react with 1 molar equivalent of trimethylaluminum at 22°C to give the corresponding 2-methylalkan-1-ols ‌146‌ in 77–92% yields after oxidation with oxygen; except in the case of sty­rene (‌145‌, R1 = Ph), where the product is formed in 30% yield (‌Scheme 66‌). Only minor amounts, if any, of polymers and hydrogen-transfer hydrometalation products are pres­ent in the product mixtures. Furthermore, 1H and 13C NMR spectroscopic analysis of the esters derived from (+)- and (−)-3,3,3-trifluoro-2-methoxy-2-phenylpropanoic acid indicates that the reaction proceeds with 65–75% enantioselectivity.[‌189‌]

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