Cationic Co(I)-Intermediates for Hydrofunctionalization Reactions: Regio- and Enantioselective Cobalt-Catalyzed 1,2-Hydroboration of 1,3-Dienes

Much of the recent
work on catalytic hydroboration of alkenes has
focused on simple alkenes and styrene derivatives with few examples
of reactions of 1,3-dienes, which have been reported to undergo mostly
1,4-additions to give allylic boronates. We find that reduced cobalt
catalysts generated from 1,n-bis-diphenyl­phosphino­alkane
complexes [Ph2P-(CH2)n-PPh2]­CoX2; n = 1–5)
or from (2-oxazolinyl)­phenyl­diaryl­phosphine complexes
[(G-PHOX)­CoX2] (G = 4-substituent on oxazoline ring) effect
selective 1,2-, 1,4-, or 4,3-additions of pinacolborane (HBPin) to
a variety of 1,3-dienes depending on the ligands chosen. Conditions
have been found to optimize the 1,2-additions. The reactive catalysts
can be generated from the cobalt­(II)-complexes using trimethylaluminum,
methyl aluminoxane, or activated zinc in the presence of sodium tetrakis[(3,5-trifluoro­methyl)­phenyl]­borate
(NaBARF). The complex, (dppp)­CoCl2, gives the best results
(ratio of 1,2- to 1,4-addition >95:5) for a variety of linear terminal
1,3-dienes and 2-substituted 1,3-dienes. The [(PHOX)­CoX2] (X = Cl, Br) complexes give mostly 1,4-addition with linear unsubstituted
1,3-dienes, but, surprisingly, selective 1,2-additions with 2-substituted
or 2,3-disubstituted 1,3-dienes. Isolated and fully characterized
(X-ray crystallography) Co­(I)-complexes, (dppp)3Co2Cl2 and [(S,S)-BDPP]3Co2Cl2, do not catalyze the
reaction unless activated by a Lewis acid or NaBARF, suggesting a
key role for a cationic Co­(I) species in the catalytic cycle. Regio-
and enantioselective 1,2-hydroborations of 2-substituted 1,3-dienes
are best accomplished using a catalyst prepared via activation of
a chiral phosphinooxazoline-cobalt­(II) complex with zinc and NaBARF.
A number of common functional groups, among them, -OBn, -OTBS, -OTs, N-phthalimido- groups, are tolerated, and er’s >
95:5 are obtained for several dienes including 1-alkenylcycloalk-1-enes.
This operationally simple reaction expands the realm of asymmetric
hydroboration to provide direct access to a number of nearly enantiopure
homoallylic boronates, which are not readily accessible by current
methods. The resulting boronates have been converted into the corresponding
alcohols, potassium trifluororoborate salts, N-BOC
amines, and aryl derivatives by C-BPin to C-aryl transformation.