Page 382 - Advanced Organic Chemistry Part B - Reactions & Synthesis

354 reaction with CH AlH has a closer resemblance to reaction of CH 3 − with ethene and
3
2
the strongest interaction is with the ethene LUMO. This interpretation is consistent with
CHAPTER 4 relative reactivity trends in which the reactivity of alkenes decreases with increasing
Electrophilic Additions alkyl substitution and alkynes are more reactive than alkenes.
to Carbon-Carbon
Multiple Bonds Effective catalysts have recently been developed for the addition of trialkyl-
aluminum reagents to alkenes (carboalumination). bis-(Pentamethylcyclopentadienyl)
zirconium dimethylide activated by tris-(pentafluorophenyl)boron promotes the
addition of trimethylaluminum to terminal alkenes. 221

(Cp*) Zr(CH ) O 2 CH 3
2
3 2
CH 3 (CH 2 ) 3 CH CH 2 + (CH 3 ) 3 Al OH
(C F ) B CH 3 (CH 2 ) 3
6 5 3
Cp* = 1,2,3,4,5 – pentamethyl- 71%
cyclopentadienide
A chiral indene derivative, structure K, has been most commonly used. 222 The catalyst
interacts with the trialkylaluminum to generate a bimetallic species that is the active
catalyst.

CH 3

CH 3 CH CH(CH )
2
3 2
( Al O )n ( Al O )n
ZrCl 2
(CH ) CH
3 2
methylalumoxane isobutylalumoxane
MAO IBAO
2
K
The detailed mechanism of the catalysis is not known, but it is believed that the Lewis
acid character of the zirconium is critical. 223 The reaction is further accelerated by
inclusion of partially hydrolyzed trialkylaluminum reagents known as alumoxanes. 224

CH 3
5 mol% cat K H +
CH (CH ) Al(i Bu) 2 + CH 2 OTBDMS IBAO CH (CH ) OH
2 6
3
2 6
3
IBAO = isobutylaluminoxane 77% yield, 91% e.e.
The adducts can be protonolyzed or converted to halides or alcohols.

cat
K H +
RCH CH 2 + R′ 3 Al RCHCH AlR′ 2 RCHCH Z
2
2
or O ,
2
R′ X 2 R′
Z = H, OH, X
221 K. H. Shaugnessy and R. M. Waymouth, J. Am. Chem. Soc., 117, 5873 (1995).
222
D. Y. Kondakov and E. Negishi, J. Am. Chem. Soc., 118, 1577 (1996); K. H. Shaugnessy and
R. M. Waymouth, Organometallics, 17, 5738 (1998).
223 E. Negishi, D. Y. Kondakov, D. Choueiry, K. Kasai, and T. Takahashi, J. Am. Chem. Soc., 118, 9577
(1996); E. Negishi, Chem. Eur. J., 5, 411 (1999).
224
S. Huo, J. Shi, and E. Negishi, Angew. Chem. Int. Ed. Engl., 41, 2141 (2002).