Page 817 - Advanced Organic Chemistry Part B - Reactions & Synthesis

The reactions can be made enantioselective by using enantiomerically pure IpcBH 793
2
for hydroboration of alkenes and then transforming the products to enantiomerically
pure derivatives of 9-BBN by reaction with 1,5-cyclooctadiene. 22 SECTION 9.1
Organoboron
Compounds
BH 1) HO(CH ) OH
1) (Ipc) 2 2 3
CH CH CH CH 3
3 3 CH O 3 2) LiAlH
2) CH 3 4 ) CH
(CH ) CH B(OH) (CH 3 2 BBN
3 2
CH 3 3) NaOH H 2 3) 1,5-cyclooctadiene H
CH
3 NaOt Bu CH 3
) CH
(CH ) CH BBN + BrCH CO C H 5 (CH 3 2 CH CO C H 5
2
2
3 2
2
2
2
2
H H 55 %
The mechanism of these alkylations involves a tetracoordinate boron intermediate
formed by addition of the enolate of the -bromo ester to the organoborane. The
migration then occurs with displacement of bromide ion. In agreement with this
mechanism, retention of configuration of the migrating group is observed. 23
R R
_ – RO –
R B + CHCO C H 5 R B CHCO 2 C H 5 R B CHCO 2 C H 5 RCH CO C H 5
2
2
2
2
2
2
3
2
Br R Br R
-Halo ketones and -halo nitriles undergo similar reactions. 24
25
A closely related reaction employs -diazo esters or -diazo ketones. With these
compounds, molecular nitrogen acts as the leaving group in the migration step. The
best results are achieved using dialkylchloroboranes or monoalkyldichloroboranes.
RBCl + N CHCO CH 3 RCH CO CH 3
2
2
2
2
2
A number of these alkylation reactions are illustrated in Scheme 9.2. Entries 1 and
2 are typical examples of -halo ester reactions. Entry 3 is a modification in which
the highly hindered base potassium 2,6-di-t-butylphenoxide is used. Similar reaction
conditions can be used with -halo ketones (Entries 4 and 5) and nitriles (Entry 6).
Entries 7 to 9 illustrate the use of diazo esters and diazo ketones. Entry 10 shows an
application of the reaction to the synthesis of an amide.
9.1.4. Stereoselective Alkene Synthesis
Several methods for stereoselective alkene synthesis are based on boron interme-
diates. One approach involves alkenylboranes, which can be prepared from terminal
alkynes. Procedures have been developed for the synthesis of both Z- and E-alkenes.

22
H. C. Brown, N. N. Joshi, C. Pyun, and B. Singaram, J. Am. Chem. Soc., 111, 1754 (1989).
23
H. C. Brown, M. M. Rogic, M. W. Rathke, and G. W. Kabalka, J. Am. Chem. Soc., 91, 2151 (1969).
24 H. C. Brown, M. M. Rogic, H. Nambu, and M. W. Rathke, J. Am. Chem. Soc., 91, 2147 (1969);
H. C. Brown, H. Nambu, and M. M. Rogic, J. Am. Chem. Soc., 91, 6853, 6855 (1969).
25
H. C. Brown, M. M. Midland, and A. B. Levy, J. Am. Chem. Soc., 94, 3662 (1972); J. Hooz, J. N. Bridson,
J. G. Calzada, H. C. Brown, M. M. Midland, and A. B. Levy, J. Org. Chem., 38, 2574 (1973).