Science of Synthesis

Matteson, D. S., Science of Synthesis, (2005) 6, 71.

Catalytic hydroboration/oxidation of vinylarenes yields the same alcohols that are often more easily obtained via ketone reductions (for asymmetric reductions with hydroboranes, see Sections ‌6.1.1.7.2‌ and ‌6.1.1.8‌; other types of asymmetric reducing agents are also known). The highly successful stoichiometric asymmetric hydroborations of cyclic alkenes with boranes derived from α-pinene or other terpenes (see Section ‌6.1.1.16.2‌) have not been paralleled by catalytic methods. The catalytic hydroboration of styrenes [R1C6H4CH=CH2; R1 = H, 4-Me, 3-Cl, 4-Cl, 2-OMe, 4-OMe] can be made asymmetric by the use of 1 mol% bis(cyclooctadiene)rhodium(I) tetrafluoroborate[‌348‌] with a suitable chiral phosphine, (+)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl, commonly known as (R)-BINAP.[‌349‌,‌350‌] The enantiomeric excesses of the resulting (R)-1-arylethanols, obtained on oxidation of the boronic esters, are in the range 82–96%, and chemical yields are 74–99%. Although 1-arylethanols are more easily made by other routes, catalytic asymmetric hydroboration of vinylarenes initially produces synthetically useful boronic esters in 95–99% yields. A sequence of asymmetric hydroboration, followed by homologation provides a new route to 2-arylpropanols, 2-arylpropanals, and 2-arylpropanoic acids in 90–97% ee. The route is illustrated by the hydroboration of styrene to pinacol (R)-(1-phenylethyl)boronate ‌115‌, followed by the further transformations of this intermediate (‌Scheme 75‌).[‌351‌]

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References

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• 8.Meeeee-Meee, (8888) M 88e, 8888.