Page 374 - Advanced Organic Chemistry Part B - Reactions & Synthesis

346 The alkene can be used more efficiently if the hydroboration is done with dimethyl-
borane. 189
CHAPTER 4
Electrophilic Additions (CH ) BH NH X H 2 O
3 3
2
to Carbon-Carbon RCH CH 2 RCH CH B(CH ) (CH ) BNCH CH R RCH CH NH 2
2
2
2
3 2
3 2
2
2
2
Multiple Bonds
Secondary amines are formed by reaction of trisubstituted boranes with alkyl or
aryl azides. The most efficient borane intermediates are monoalkyldichloroboranes,
. 190
which are generated by reaction of an alkene with BHCl Et O. The entire sequence
2
2
of steps and the mechanism of the final stages are summarized by the equation below.
BHCl + RCH CH 2 RCH CH BCl 2
2
2
2
R′ R′
+ H O
2
RCH CH BCl + R′ N 3 Cl 2 B – N N N Cl 2 BNCH 2 CH 2 R R′NHCH CH R
2
2
2
2
2
RCH 2 CH 2
This reaction has been used to prepare -N-methylamino acids using CH BBr. 191
3 2
(CH ) BBr
3 2
PhCHCO H PhCHCO H
2
2
N 3 NHCH 3
Secondary amines can also be made using the N-chloro derivatives of primary
amines. 192
Cl H
(CH CH ) B + HN(CH ) CH 3 CH 3 CH N(CH ) CH 3
2
2 7
2 3
3
2 7
90%
Organoborane intermediates can also be used to synthesize alkyl halides.
Replacement of boron by iodine is rapid in the presence of base. 193 The best yields are
obtained using sodium methoxide in methanol. 194 If less basic conditions are desirable,
the use of iodine monochloride and sodium acetate gives good yields. 195 As is the
case in hydroboration-oxidation, the regioselectivity of hydroboration-halogenation is
opposite to that observed by direct ionic addition of hydrogen halides to alkenes.
Terminal alkenes give primary halides.
H
1) B 2 6
RCH CH 2 RCH CH Br
2
2
2) Br , NaOH
2
189 H. C. Brown, K.-W. Kim, M. Srebnik, and B. Singaram, Tetrahedron, 43, 4071 (1987).
190
H. C. Brown, M. M. Midland, and A. B. Levy, J. Am. Chem. Soc., 95, 2394 (1973).
191 R. L. Dorow and D. E. Gingrich, J. Org. Chem., 60, 4986 (1995).
192
G. W. Kabalka, G. W. McCollum, and S. A. Kunda, J. Org. Chem., 49, 1656 (1984).
193 H. C. Brown, M. W. Rathke, and M. M. Rogic, J. Am. Chem. Soc., 90, 5038 (1968).
194 N. R. De Lue and H. C. Brown, Synthesis, 114 (1976).
195
G. W. Kabalka and E. E. Gooch, III, J. Org. Chem., 45, 3578 (1980).