posted on 2012-02-01, 00:00authored byChristine Brinkmann, Anthony G. M. Barrett, Michael S. Hill, Panayiotis A. Procopiou
The heavier group 2 complexes [M{N(SiMe3)2}2]2 (1, M = Ca; 2, M = Sr) and [M{CH(SiMe3)2}2(THF)2] (3, M = Ca; 4, M = Sr)
are shown
to be effective precatalysts for the intermolecular hydroamination
of vinyl arenes and dienes under mild conditions. Initial studies
revealed that the amide precatalysts, 1 and 2, while compromised in terms of absolute activity by a tendency toward
transaminative behavior, offer greater stability toward polymerization/oligomerization
side reactions. In every case the strontium species, 2 and 4, were found to outperform their calcium congeners.
Reactions of piperidine with para-substituted styrenes
are indicative of rate-determining alkene insertion in the catalytic
cycle while the ease of addition of secondary cyclic amines was found
to be dependent on ring size and reasoned to be a consequence of varying
amine nucleophilicity. Hydroamination of conjugated dienes yielded
isomeric products via η3-allyl intermediates and
their relative distributions were explained through stereoelectronic
considerations. The ability to carry out the hydroamination of internal
alkynes was found to be dramatically dependent upon the identity of
the alkyne substituents while reactions employing terminal alkynes
resulted in the precipitation of insoluble and unreactive group 2
acetylides. The rate law for styrene hydroamination with piperidine
catalyzed by [Sr{N(SiMe3)2}2]2 was deduced to be first order in [amine] and [alkene] and
second order in [catalyst], while large kinetic isotope effects and
group 2 element-dependent ΔS⧧ values implicated the formation of an amine-assisted rate-determining
alkene insertion transition state in which there is a considerable
entropic advantage associated with use of the larger strontium center.