Page 381 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Other disubstituted boranes have also been used for selective hydroboration of 353
alkynes. 9-BBN can be used to hydroborate internal alkynes. Protonolysis can be
carried out with methanol and this provides a convenient method for formation of a SECTION 4.6
disubstituted Z-alkene. 217 Hydroalumination,
Carboalumination,
Hydrozirconation,
R R MeOH R R and Related Reactions
R C C R + 9-BBN
H B H H

A large number of procedures that involve carbon-carbon bond formation have
been developed based on organoboranes. These reactions are considered in Chapter 9.

4.6. Hydroalumination, Carboalumination, Hydrozirconation,
and Related Reactions

Aluminum is the immediate congener of boron, and dialkyl and trialkyl aluminum
compounds, which are commercially available, have important industrial applica-
tions. They also have some similarities with organoboranes that can be exploited for
synthetic purposes. Aluminum is considerably less electronegative than boron and as a
result the reagents also share characteristics with the common organometallic reagents
such as organomagnesium and organolithium compounds. The addition reactions of
alkenes and dialkylaluminum reagents occur much less easily than hydroboration.
Only terminal or strained alkenes react readily at room temperature. 218 With internal
and branched alkenes, the addition does not go to completion. Addition of dialkyl-
alanes to alkynes occurs more readily, and the regiochemistry and stereochemistry
are analogous to hydroboration. The resulting vinylalanes react with halogens with
retention of configuration at the double bond. 219

R H I R H
RC CH + (i – Bu) AlH 2
2
H Al(i-Bu) 2 H I 74%
With trialkylaluminum compounds, the addition reaction is called carboalumination.
As discussed below, this reaction requires a catalyst to proceed.
R
catalyst
Al + CH CHR′ R AlCH CHR′
R 3 2 2 2
Computational studies of both hydroalumination and carboalumination have
indicated a four-center TS for the addition. 220 The aluminum reagents, however, have
more nucleophilic character than do boranes. Whereas the TS for hydroboration is
primarily electrophilic and resembles that for attack of CH + on a double bond, the
3
217 H. C. Brown and G. A. Molander, J. Org. Chem., 51, 4512 (1986); H. C. Brown and K. K. Wang,
J. Org. Chem., 51, 4514 (1986).
218
F. Ansinger, B. Fell, and F. Thiessen, Chem. Ber., 100, 937 (1967); R. Schimpf and P. Heimbach,
Chem. Ber., 103, 2122 (1970).
219 G. Zweifel and C. C. Whitney, J. Am. Chem. Soc., 89, 2753 (1967).
220
J. W. Bunders and M. M. Francl, Organometallics, 12, 1608 (1993); J. W. Bunders, J. Yudenfreund,
and M. M. Francl, Organometallics, 18, 3913 (1999).