T. Straub et al., Intermolecular hydroamination of terminal alkynes catalyzed by organoactinide complexes. Scope and mechanistic studies, ORGANOMETAL, 20(24), 2001, pp. 5017-5035
Organoactinide complexes of the type CP*(2)AnMe(2) (An = Th, U) have been f
ound to be efficient catalysts for the hydroamination of terminal alkynes w
ith aliphatic primary amines. The chemoselectivity and regioselectivity of
the reactions depend strongly on the nature of the catalyst and the nature
of the amine and show no major dependence on the nature of the alkyne. The
hydroamination reaction of the terminal alkynes with aliphatic primary amin
es catalyzed by the organouranium complexes produces the corresponding imin
es where the amine and the alkyne are regioselectively disposed in a syn-re
giochemistry, whereas for similar reactions with the organothorium complex
besides the methyl alkylated imine, dimeric and trimeric alkyne oligomers a
re also produced. For (TMS)C drop CH, and EtNH2 both organoactinides produc
ed the same imine compounds when the reaction is carried out in THF or tolu
ene. In benzene, both imines E and Z (TMS)CH2CHN=Et are obtained, the earli
er undergo a 1,3-silyl Brook sigmatropic rearrangement toward the enamine,
whereas the latter remains unchanged. Mechanistic studies on the hydroamina
tion of (TMS)C drop CH and EtNH2 promoted by the organouranium complex show
that the first step in the catalytic reaction is the formation of the bis(
amido) complex, found in equilibrium with the corresponding bisamido-amine
complex, which loses an amine, yielding a uranium-imido complex. Insertion
of the alkyne into the imido bond with subsequent amine protonolysis, isome
rization, and product release comprise the primary steps in the catalytic c
ycle. The kinetic rate law was found to follow an inverse kinetic order in
amine, a first order in complex, and a zero order in alkyne, with DeltaH(do
uble dagger) = 11.7(3) kcal mol(-1), DeltaS(double dagger) = -44.5(8) eu. T
he turnover-limiting step is the release of an amine from the bisamido comp
lex yielding the imido complex. The key organoactinide intermediate for the
intermolecular hydroamination reaction was found to be the corresponding a
ctinide-imido complexes. For both actinides the complexes have been charact
erized, and for thorium the single-crystal X-ray diffraction was studied. A
plausible mechanistic scenario is proposed for the hydroamination of termi
nal alkynes and aliphatic primary amines.