Page 887 - Advanced Organic Chemistry Part B - Reactions & Synthesis

+ + 863
C + C C C C C C C C
Y SECTION 10.1
Y
Reactions and
+ C Y Y Rearrangement
C + C C C C Involving Carbocation
C C
C C C C Intermediates
+
Y = Si or Sn

Silyl enol ethers and silyl ketene acetals also offer both enhanced reactivity
and a favorable termination step. Electrophilic attack is followed by desilylation to
give an -substituted carbonyl compound. The carbocations can be generated from
tertiary chlorides and a Lewis acid, such as TiCl . This reaction provides a method
4
for introducing tertiary alkyl groups to a carbonyl, a transformation that cannot
be achieved by base-catalyzed alkylation because of the strong tendency for tertiary
halides to undergo elimination.

O
CH 3
TiCl 4
OSi(CH ) + (CH ) CCH CH 3 C CH CH 3
2
3 2
3 3
2
–50°C
Cl CH 3 62%
Ref. 1
Secondary benzylic bromides, allylic bromides, and -chloro ethers can undergo
2
analogous reactions using ZnBr as the catalyst. Primary iodides react with silyl
2
ketene acetals in the presence of AgO CCF . 3
2
3
O O
O OSi(CH ) AgO CCF 3
2
3 3
+ CH CH CH CH I
2
2
2
3
CH CH CH CH 3
2
2
2
54%
Alkylations via an allylic cation have been observed using LiClO to promote
4
ionization. 4
O CCH 3 OC H LiClO CH 2 CO C H
2
2 2 5
+ 2 5 4
CH 2
OTBDMS
Ph Ph 92%
These reactions provide examples of intermolecular carbocation alkylations. Despite
the feasibility of this type of reaction, the requirements for good yields are stringent
and the number of its synthetic applications is limited.
1 M. T. Reetz, I. Chatziiosifidis, U. Loewe, and W. F. Maier, Tetrahedron Lett., 1427 (1979); M. T. Reetz,
I. Chatziiosifidis, F. Huebner, and H. Heimbach, Org. Synth., 62, 95 (1984).
2
I. Paterson, Tetrahedron Lett., 1519 (1979).
3 C. W. Jefford, A. W. Sledeski, P. Lelandais, and J. Boukouvalas, Tetrahedron Lett., 33, 1855 (1992).
4
W. H. Pearson and J. M. Schkeryantz, J. Org. Chem., 57, 2986 (1992).