Page 821 - Advanced Organic Chemistry Part B - Reactions & Synthesis

If the alkyne is hydroborated and then protonolyzed a Z-alkene is formed. This method 797
was used to prepare an insect pheromone containing a Z-double bond.
SECTION 9.1
1) LiC C(CH 2 ) 3 CH 3 Organoboron
CH 3 CO 2 (CH 2 ) 4 CH CH 2 + BH 3 [CH 3 CO 2 (CH 2 ) 6 ] 3 B CH 3 CO 2 (CH 2 ) 6 C C(CH 2 ) 3 CH 3 Compounds
2) I 2
1) 9-BBN H H
CH 3 CO 2 (CH 2 ) 6 C C(CH 2 ) 3 CH 3
2) CH 3 CO 2 H CH 3 CO 2 (CH 2 ) 6 (CH 2 ) 3 CH 3
Ref. 33
The B → C migration can also be induced by other types of electrophiles.
Trimethylsilyl chloride or trimethylsilyl triflate induces a stereospecific migration to
34
form -trimethylsilyl alkenylboranes having cis silicon and boron substituents. It has
been suggested that this stereospecificity arises from a silicon-bridged intermediate.

R
– R R′
R B R′
2
R B – C C R′ + (CH ) Si X C C R B Si(CH )
3 3
3
2
3 3
)
Si(CH 3 3
X
Tributyltin chloride also induces migration and gives the product in which the C–Sn
bond is cis to the C–B bond. Protonolysis of both the C–Sn and C–B bonds by acetic
acid gives the corresponding Z-alkene. 35
R R′ CH 3 CO 2 H R R′
–
R 3 B C C R′ + ClSnR″ 3
R B SnR″ 3 H H
2

9.1.5. Nucleophilic Addition of Allylic Groups from Boron Compounds
Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give
allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl
oxygen, which both increases the electrophilicity of the carbonyl group and weakens
the C–B bond to the allyl group. The dipolar adduct then reacts through a cyclic
TS. Bond formation takes place at the -carbon of the allyl group and the double
36
bond shifts. After the reaction is complete, the carbinol product is liberated from the
borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and
ketones were usually above 90% for 9-allyl-9-BBN.

O OH
O B ) NH
C B HO(CH 2 2 2
R 2 R C
2
C O + CH R 2 CCH 2 CH CH 2
R 2 2 CHCH B CH 2
2
C C CH CH CH
H 2 2 2
H
33
H. C. Brown and K. K. Wang, J. Org. Chem., 51, 4514 (1986).
34 P. Binger and R. Koester, Synthesis, 309 (1973); E. J. Corey and W. L. Seibel, Tetrahedron Lett., 27,
905 (1986).
35 K. K. Wang and K.-H. Chu, J. Org. Chem., 49, 5175 (1984).
36
G. W. Kramer and H. C. Brown, J. Org. Chem., 42, 2292 (1977).