The reaction of the diarylallenylidene complexes
[(CO)5MCCC(C6H4R-p)2]
(M = Cr
(1), W (2); R = H (a), Me
(b), OMe (c), NMe2 (d))
with the ynamines MeC⋮CNEt2 and
PhC⋮CNEt2 affords two products: alkenylallenylidene
(3, 5) and cyclobutenylidene complexes (4, 6). The alkenylallenylidene
complexes
[(CO)5MCCC(NEt2)C(R‘)C(C6H4R-p)2] (R‘ = Me, M = Cr (3), W
(5), R = H (a), Me (b), OMe
(c), NMe2 (d); R‘ = Ph, M = Cr
(3),
R = OMe (e), NMe2 (f)) are formed
via cycloaddition of the C⋮C bond of the ynamine to
the
C2C3 bond of 1 and 2,
respectively, and subsequent cycloreversion. The
cyclobutenylidene
= Me, M = Cr (4), W (6), R
=
H (a), Me (b), OMe (c),
NMe2 (d); R‘ = Ph, M = Cr (4), R
= OMe (e), NMe2 (f)) are
formed
by cycloaddition of the ynamines to the C1C2
bond of 1 and 2. The compounds 3,
4a−c, 5,
and 6a−c are stable at room temperature.
In contrast, 4d−f and 6d
decompose on contact
with air, light, or silica. Complex 3d was
characterized by an X-ray structural analysis.
The product ratios 3/4 and
5/6 strongly depend on the solvent and the
substitution pattern
of both the allenylidene complexes 1 and 2 and
the ynamine. In general, decreasing polarity
of the solvent increasingly favors formation of cyclobutenylidene
complexes. The solvent
dependence indicates that the transition state for the formation of
4 and 6 is significantly
less polar than that for the formation of 3 and
5. The ratios 3/4 and
5/6 increase in the
series a < b < c < d.
Kinetic measurements of the reaction of 1c,d
with the ynamines
MeC⋮CNEt2 and PhC⋮CNEt2 reveal that the
complex pairs 3,4 and 5,6
are formed in
parallel pathways with an associative rate-determining step for each.
The reactions follow
second-order kinetics, first-order in the concentrations of the
allenylidene complexes 1,2
and of the ynamines. The activation enthalpies
ΔH
⧧ are small, and the activation
entropies
ΔS
⧧ are strongly negative.
ΔS
⧧ is more negative for the formation
of the alkenylallenylidene
complexes than for the formation of the cyclobutenylidene complexes.
