Page 375 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Scheme 4.9 gives some examples of the use of boranes in syntheses of alcohols, 347
aldehydes, ketones, amines, and halides. Entry 1 demonstrates both the regioselec-
tivity and stereospecificity of hydroboration, resulting in the formation of trans-2- SECTION 4.5
methylcyclohexanol. Entry 2 illustrates the facial selectivity, with the borane adding Addition at Double
Bonds via Organoborane
anti to the endo methyl group. Intermediates

H B
CH 2
CH 3
CH 3

Entry 3 illustrates all aspects of the regio- and stereoselectivity, with syn addition
occurring anti to the dimethyl bridge in the pinene structure. The stereoselectivity in
Entry 4 is the result of the preferred conformation of the alkene and approach syn to
the smaller methyl group, rather than the 2-furyl group.
OH
H CH OCH Ph
CH OCH Ph O 2 2
2
2
O CH 3 Ph
CH OH H O OCH 2
CH 3 H 3 CH 3 H CH CH 3
3
Entries 5 to 7 are examples of oxidation of boranes to the carbonyl level. In Entry
5, chromic acid was used to obtain a ketone. Entry 6 shows 5 mol % tetrapropylam-
monium perruthenate with N-methylmorpholine-N-oxide as the stoichiometric oxidant
converting the borane directly to a ketone. Aldehydes were obtained from terminal
alkenes using this reagent combination. Pyridinium chlorochromate (Entry 7) can also
be used to obtain aldehydes. Entries 8 and 9 illustrate methods for amination of alkenes
via boranes. Entries 10 and 11 illustrate the preparation of halides.

4.5.3. Enantioselective Hydroboration

Several alkylboranes are available in enantiomerically enriched or pure form
and can be used to prepare enantiomerically enriched alcohols and other compounds
available via organoborane intermediates. 196 One route to enantiopure boranes is
by hydroboration of readily available terpenes that occur naturally in enantiomer-
ically enriched or pure form. The most thoroughly investigated of these is bis-
(isopinocampheyl)borane; Ipc BH , which can be prepared in 100% enantiomeric
2
purity from the readily available terpene -pinene. 197 Both enantiomers are available.

BH
+ BH 3
2
196 H. C. Brown and B. Singaram, Acc. Chem. Res., 21, 287 (1988); D. S. Matteson, Acc. Chem. Res., 21,
294 (1988).
197
H. C. Brown, P. K. Jadhav, and A. K. Mandal, Tetrahedron, 37, 3547 (1981); H. C. Brown and
P. K. Jadhav, in Asymmetric Synthesis, Vol. 2, J. D. Morrison, ed., Academic Press, New York, 1983,
Chap. 1.