Page 810 - Advanced Organic Chemistry Part B - Reactions & Synthesis

786 The cyclic five- and six-membered boronate esters are used frequently. Their systematic
names are 1,3,2-dioxaborolane and 1,3,2-dioxaborinanes, respectively.
CHAPTER 9
Carbon-Carbon O O
Bond-Forming Reactions
of Compounds of Boron, R B R B
Silicon, and Tin O O

1,3,2-dioxaborolane 1,3,2-dioxaborinane

9.1.2. Carbonylation and Other One-Carbon Homologation Reactions
The reactions of organoboranes that we discussed in Chapter 4 are valuable
methods for introducing functional groups such as hydroxy, amino, and halogen into
alkenes. In this section we consider carbon-carbon bond-forming reactions of boron
6
compounds. Trivalent organoboranes are not very nucleophilic but they are moderately
reactive Lewis acids. Most reactions in which carbon-carbon bonds are formed involve
a tetracoordinate intermediate that has a negative charge on boron. Adduct formation
weakens the boron-carbon bonds and permits a transfer of a carbon substituent with
its electrons. The general mechanistic pattern is shown below.

R B + :Nu – R B Nu –
3
3
–
B +
2
R 3 Nu + E R B Nu + R E
The electrophilic center is sometimes generated from the Lewis base by formation
of the adduct, and the reaction proceeds by migration of a boron substituent.

R R
+
+ B Nu X –
R B + :Nu X R B – Nu X
3
R R
R
A significant group of reactions of this type involves the reactions of organoboranes
with carbon monoxide, which forms Lewis acid-base complexes with the organo-
boranes. In these adducts the boron bears a formal negative charge and carbon is
electrophilic because the triply bound oxygen bears a formal positive charge. The
adducts undergo boron to carbon migration of the alkyl groups. The reaction can be
controlled so that it results in the migration of one, two, or all three of the boron
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substituents. If the organoborane is heated with carbon monoxide to 100 –125 C, all

of the groups migrate and a tertiary alcohol is obtained after workup by oxidation.
The presence of water causes the reaction to cease after migration of two groups
from boron to carbon. Oxidation of the reaction mixture at this stage gives a ketone. 8
Primary alcohols are obtained when the carbonylation is carried out in the presence of

6
For a review of this topic, see E. Negishi and M. Idacavage, Org. React., 33, 1 (1985).
7 H. C. Brown and M. W. Rathke, J. Am. Chem. Soc., 89, 2737 (1967).
8
H. C. Brown and M. W. Rathke, J. Am. Chem. Soc., 89, 2738 (1967).