Switching between Hydrogenation and Olefin Transposition Catalysis via Silencing NH Cooperativity in Mn(I) Pincer Complexes

While Mn-catalyzed (de)hydrogenation of carbonyl derivatives
has
been well established, the reactivity of Mn hydrides with olefins
remains very rare. Herein, we report a Mn(I) pincer complex that effectively
promotes site-controlled transposition of olefins. This reactivity
is shown to emerge once the N–H functionality within the Mn/NH
bifunctional complex is suppressed by alkylation. While detrimental
for carbonyl (de)hydrogenation, such masking of the cooperative N–H
functionality allows for the highly efficient conversion of a wide
range of allylarenes to higher-value 1-propenybenzenes in near-quantitative
yield with excellent stereoselectivities. The reactivity toward a
single positional isomerization was also retained for long-chain alkenes,
resulting in the highly regioselective formation of 2-alkenes, which
are less thermodynamically stable compared to other possible isomerization
products. The detailed mechanistic analysis of the reaction between
the activated Mn catalyst and olefins points to catalysis operating
via a metal–alkyl mechanismone of the three conventional
transposition mechanisms previously unknown in Mn complexes.