H. Braunschweig et al. / Journal of Organometallic Chemistry 706-707 (2012) 144e145
145
Table 1
2. Results and discussion
Catalytic dehydrocoupling of HBCat.
Pre-catalyst (loading [mol%])a
Turnover number (GC)b
As mentioned above, the common procedure for generating the
reactive [Cp2M] species involves an in situ reduction with n-
butyllithium. Although diluted solutions of HBCat do not react with
the pre-catalysts [Cp2MCl2] (M ¼ Ti, Zr, Hf) under stoichiometric
conditions, the reductive nature of the neat borane is sufficient to
generate a catalytically active species without additional activation
of the group 4 complexes. In a typical reaction, the pre-catalyst
(0.05 mol%) and neat HBCat were heated to 110 ꢀC. Upon heating,
the solution turned black and a distinct evolution of gas could be
observed (Scheme 1). After 20 h, the turnover numbers (TON) of the
reactions were determined by GC. In the case of [Cp2TiCl2], the
formation of B2Cat2 was observed, albeit with a very low TON of 3.
However, the two heavier homologs of titanium, that is zirconium
and hafnium, turned out to be more efficient pre-catalysts for the
dehydrocoupling, with [Cp2ZrCl2] affording a TON of 40. With
[Cp2HfCl2] the TON could be further raised to 90, a value identical to
the results achieved with [(dppm)PtCl2], which turned out to be the
most efficient homogeneous late transition metal catalyst under
these reaction conditions [27,28]. In the reaction of HBCat with
[Cp2HfCl2], B2Cat2 could be isolated by removing excess borane in
vacuo and sublimation of the resulting crude product at 100 ꢀC and
4.10ꢁ2 mbar. After washing the crude product with acetonitrile and
hexane, pure 1 was yielded with a TON of 46.
[Cp2TiCl2] (0.05%)
[Cp2ZrCl2] (0.05%)
[Cp2HfCl2] (0.05%)
Ti Powder (0.4%)
Zr Powder (0.1%)
Hf Powder (0.1%)
3
40
90
5
10
13
a
Reaction conditions: neat borane was heated with the pre-catalyst to 110 ꢀC for
20 h.
b
Turnover numbers were determined by GC with docosane as internal standard.
The numbers given are the average of two or more runs.
Furthermore we observed that group 4 metal powders can also
catalyze this dehydrocoupling, albeit with lower turnover numbers.
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Scheme 1. Catalytic dehydrocoupling of HBCat.