The reactions of internal and terminal alkynes with organoalanes
containing Et, n-Pr, and i-Bu groups
in
the presence of Cp2ZrCl2 and
MeZrCp2Cl were investigated with the goal of
clarifying mechanistic details of some
representative cases. Three fundamentally different processes,
i.e., (i) C−M bond addition without C−H
activation
in the alkyl group, (ii) cyclic C−M bond addition via C−H
activation, and (iii) hydrometalation, have been observed,
and the courses of these reactions significantly depend on (i) the
nature and number of alkyl groups in organoalanes,
(ii) their amounts, and (iii) solvents. The reaction of alkynes
with Et3Al in the presence of 0.1 equiv of
Cp2ZrCl2
in nonpolar solvents, e.g., hexanes, proceeds via C−H
activation to give the corresponding aluminacyclopentenes.
Investigation of the reaction of 5-decyne with 1−3 equiv of
Et3Al and 1 equiv of
Cp2ZrCl2, which gave mono-,
di-,
or trideuterated (Z)-5-ethyl-5-decene as shown in Scheme ,
together with the previously reported structural study
on the reaction of Et3Al with
Cp2ZrCl2 leading to the formation of
well-characterized bimetallic species 9, 10,
and
11 (Scheme ), supports a catalytic cycle involving
bimetallic species 10 and 18 (Scheme ). In
summary, this
process requires a zirconocene derivative containing one Zr-bound Et
group which is linked to Et3Al (but not
to
Et2AlCl) through a Cl bridge, i.e.,
18, to produce 10 via β C−H activation.
In sharp contrast, the reaction of
Et2AlCl−Cp2ZrCl2 as well as of
(n-Pr)2AlCl−Cp2ZrCl2
does not involve any C−H activation processes. It
proceeds
well in chlorinated hydrocarbon solvents, e.g.,
(CH2Cl)2, but it is extremely sluggish in
nonpolar solvents, e.g.,
hexanes. The reaction may well involve direct C−Al bond addition
to alkynes, as suggested earlier for Zr-catalyzed
Me−Al bond addition to alkynes, but a few other alternatives cannot
be ruled out on the basis of the currently
available data. The reaction of alkynes with
(n-Pr)3Al−Cp2ZrCl2
in nonpolar solvents proceeds partially via C−H
activation and partially via hydrometalation. In contrast with the
C−H activation process observed with Et3Al,
that
with (n-Pr)3Al is totally dominated by
dimerization of alkynes to give aluminacyclopentadienes rather
than
aluminacyclopentenes, reflecting a previously established
generalization that propene can be much more readily
displaced from Zr by alkynes than ethylene. Hydrometalation is the
exclusive process with
(i-Bu)3Al−Cp2ZrCl2.
This hydrometalation reaction, however, reveals a few interesting
complications. Alkyl-substituted internal alkynes
give double bond migrated products in addition to the expected
hydrometalation products. With terminal alkynes
the reaction produces the expected hydrometalation products and the
1,1-dimetalloalkanes in comparable yields.
Various other related reactions involving other alkynes,
e.g., PhC⋮CPh, n-OctC⋮CH, and PhC⋮CH, and
other
reagents, e.g.,
Et3Al−MeZrCp2Cl,
Et2AlCl−MeZrCp2Cl, and
(n-Pr)3Al−MeZrCp2Cl, were
also studied.
