Page 368 - Advanced Organic Chemistry Part B - Reactions & Synthesis

340 The haloboranes BH Cl, BH Br, BHCl , and BHBr are also useful hydroborating
2
2
2
2
reagents. 159 These compounds are somewhat more regioselective than borane itself, but
CHAPTER 4 otherwise show similar reactivity. A useful aspect of the chemistry of the haloboranes
Electrophilic Additions is the potential for sequential introduction of substituents at boron. The halogens can
to Carbon-Carbon
Multiple Bonds be replaced by alkoxide or by hydride. When halogen is replaced by hydride, a second
hydroboration step can be carried out.
R BX + NaOR′ R 2 BOR′
2
R BX + LiAlH 4 R BH
2
2
RBX + LiAlH 4 RBH 2
2
X = Cl, Br
Examples of these transformations are discussed in Chapter 9, where carbon-carbon
bond-forming reactions of organoboranes are covered.
Amine-borane complexes are not very reactive toward hydroboration, but the
pyridine complex of borane can be activated by reaction with iodine. 160 The active
reagent is thought to be the pyridine complex of iodoborane.

–
+
N –B H 3 N –B H I + 0.5 H 2
+
–
2
The resulting boranes can be subjected to oxidation or isolated as potassium trifluo-
roborates.
1) pyrBH I O CH
2
PhC CCH 3 CH 3 + Ph 3
2 ) NaO H Ph O
2
15:1
1) pyrBH I
2
C H CH CH 2 – +
4 9
2
2 4
3
CH 3 (CH ) CH BF K
2) KHF 2
Catecholborane and pinacolborane, in which the boron has two oxygen
substituents, are much less reactive hydroborating reagents than alkyl or haloboranes
because the boron electron deficiency is attenuated by the oxygen atoms. Never-
theless, they are useful reagents for certain applications. 161 The reactivity of catechol-
borane has been found to be substantially enhanced by addition of 10–20% of N,N-
dimethylacetamide to CH Cl . 162
2
2
CH 3 CH 3
CH
CH 3 3
O O O O
B B
H H
catecholborane pinacolborane

159
H. C. Brown and S. U. Kulkarni, J. Organomet. Chem., 239, 23 (1982).
160 J. M. Clay and E. Vedejs, J. Am. Chem. Soc., 127, 5766 (2005).
161 C. E. Tucker, J. Davidson, and P. Knockel, J. Org. Chem., 57, 3482 (1992).
162
C. E. Garrett and G. C. Fu, J. Org. Chem., 61, 3224 (1996).