ConspectusChiral alcohols
are ubiquitous in organic structures.
One efficient method to generate chiral alcohols is the catalytic
asymmetric addition of a carbon nucleophile to a carbonyl compound
since this process produces a C–C bond and a chiral center
simultaneously. In comparison with the carbon nucleophiles such as
an organolithium or a Grignard reagent, an organozinc reagent possesses
the advantages of functional group tolerance and more mild reaction
conditions. Catalytic asymmetric reactions of aldehydes with arylzincs,
vinylzincs, and alkynylzincs to generate functional chiral alcohols
are discussed in this Account.Our laboratory has developed
a series of 1,1′-bi-2-naphthol (BINOL)-based chiral catalysts
for the asymmetric organozinc addition to aldehydes. It is found that
the 3,3′-dianisyl-substituted BINOLs are not only highly enantioselective
for the alkylzinc addition to aldehydes, but also highly enantioselective
for the diphenylzinc addition to aldehydes. A one-step synthesis has
been achieved to incorporate Lewis basic amine groups into the 3,3′-positions
of the partially hydrogenated H8BINOL. These H8BINOL–amine compounds have become more generally enantioselective
and efficient catalysts for the diphenylzinc addition to aldehydes
to produce various types of chiral benzylic alcohols. The application
of the H8BINOL–amine catalysts is expanded by using in situ generated diarylzinc reagents from the reaction
of aryl iodides with ZnEt2, which still gives high enantioselectivity
and good catalytic activity. Such a H8BINOL–amine
compound is further found to catalyze the highly enantioselective
addition of vinylzincs, in situ generated from the
treatment of vinyl iodides with ZnEt2, to aldehydes to
give the synthetically very useful chiral allylic alcohols.We have discovered that the unfunctionalized BINOL in combination
with ZnEt2 and Ti(OiPr)4 can catalyze
the terminal alkyne addition to aldehydes to produce chiral propargylic
alcohols of high synthetic utility. The reaction was conducted by
first heating an alkyne with ZnEt2 in refluxing toluene
to generate an alkynylzinc reagent, which can then add to a broad
range of aldehydes at room temperature in the presence of BINOL and
Ti(OiPr)4 with high enantioselectivity. It was
then found that the addition of a catalytic amount of dicyclohexylamine
(Cy2NH) allows the entire process to be conducted at room
temperature without the need to generate the alkynylzincs at elevated
temperature. This BINOL–ZnEt2–Ti(OiPr)4–Cy2NH catalyst system can be used
to catalyze the reaction of structurally diverse alkynes with a broad
range of aldehydes at room temperature with high enantioselectivity
and good catalytic activity.The work described in this Account
demonstrates that BINOL and its derivatives can be used to develop
highly enantioselective catalysts for the asymmetric organozinc addition
to aldehydes. These processes have allowed the efficient synthesis
of many functional chiral alcohols that are useful in organic synthesis.