608 (33) The assignment is based on our observation that a mixture of 2-and E4.5dimethyloct-4-ene shows its vinyl methyl resonances at 18.03 and 18.32 ppm. (34) The ratio in the most accurately measured spectrum (see footnote 25b) was 0.19. (35) In an uncataiyzed reaction the half-life of 1-methylcyclobutene at 50' is M. Frey. Trans. Faraday Soc., 58, 957 (1982)). 1.7 X 10'h (36) Say for hydroboration: G. Zwiefel and H. C. Brown, Org. React., 13, 1 (1963).
Scheme I
R
i' H
(e.
slowest
/-
R
R
[-w II
R
CH,
It
Thomas J. Katz,* James McGinnis, Craig Altus Department of Chemistry, Columbia University New York, New York 10027 Received September 15, 1975
Reactions of (CO)sWC(Tol)2with Alkenes. A Model for Structural Selectivity in the Olefin Metathesis Reaction Sir:
The olefin metathesis reaction' is a fascinating transition metal catalyzed reaction which involves the intermolecular exchange of alkylidene units between alkenes. Recently we discovered that (C0)5WC(CaH5)2 reacts with alkenes to give diphenylethylenes and cyclopropanm2 The diphenylethylenes were suggested to be formed through interconversion of complexes bearing both an alkene and a carbene ligand via a metallocyclobutane intermediate. This process also provides a sufficient mechanism for olefin metathesis3 that differs from all previous mechanisms' in that it alone
1
fast
'3>
(CO),W
C ' 6H
R-+
R9
(CO),W=C
t -
/C&
C,,H,
R
R
R R R requires a nonpairwise exchange of' alkylidene units between alkenes. Such nonpairwise exchange has recently been reported in several elegant studies by C h a ~ v i n kat^,^ ,~ and G r u b b ~ . ~ The olefin metathesis reaction shows a pronounced structural selectivity.' The relative rates of metathesis decrease in the order: (1) the degenerate exchange of methylene units between terminal alkenes6,' > (2) cross metathesis of terminal and internal alkene^^,*.^ > (3) metathesis of inter-
C6H5Yc6H5 It
(CO),W
Table 1. Reactions of Alkenes with (CO),WC(Tol),a Alkene
Productsbld
CH,=CH(CH,),CH,
(Tol),C=CH, 35.9 f 0.3%
(Tol),C=CH(CH,),CH, 0.06 + 0.02%
(Tol),C=CH, 13.2 f 0.2%
(Tol),C=C(CH,),
