Comparative Nitrene-Transfer Chemistry to Olefinic
Substrates Mediated by a Library of Anionic Mn(II) Triphenylamido-Amine
Reagents and M(II) Congeners (M = Fe, Co, Ni) Favoring Aromatic over
Aliphatic Alkenes
posted on 2018-08-22, 00:00authored byVivek Bagchi, Anshika Kalra, Purak Das, Patrina Paraskevopoulou, Saidulu Gorla, Lin Ai, Qiuwen Wang, Sudip Mohapatra, Amitava Choudhury, Zhicheng Sun, Thomas R. Cundari, Pericles Stavropoulos
Selective
amination of σ and π entities such as C–H
and CC bonds of substrates remains a challenging endeavor
for current catalytic methodologies devoted to the synthesis of abundant
nitrogen-containing chemicals. The present work addresses an approach
toward discriminating aromatic over aliphatic alkenes in aziridination
reactions, relying on the use of anionic metal reagents (M = Mn, Fe,
Co, Ni) to attenuate reactivity in a metal-dependent manner. A family
of MnII reagents bearing a triphenylamido-amine scaffold
and various pendant arms has been synthesized and characterized by
various techniques, including cyclic voltammetry. Aziridination of
styrene by PhINTs in the presence of each MnII catalyst
establishes a trend of increasing yield with increasing MnII/III anodic potential. The FeII, CoII, and NiII congeners of the highest-yielding MnII catalyst
have been synthesized and explored in the aziridination of aromatic
and aliphatic alkenes, exhibiting good to high yields with para-substituted
styrenes, low to modest yields with sterically congested styrenes,
and invariably low yields with aliphatic olefins. CoII mediates
faster styrene aziridination in comparison to MnII but
is less selective than MnII in competitive aziridinations
of conjugated versus nonconjugated olefins. Indeed, MnII proved to be highly selective even versus well-established copper
and rhodium aziridination reagents. Mechanistic investigations and
computational studies indicate that all metals follow a two-step styrene
aziridination pathway (successive formation of two N–C bonds),
featuring a turnover-limiting metal–nitrene addition to an
olefinic carbon, followed by product-determining ring closure. Both
steps exhibit activation barriers in the order Fe > Mn > Co,
most
likely stemming from relevant metal–nitrene electrophilicities
and MII/III redox potentials. The aziridination of aliphatic
olefins follows the same stepwise path, albeit with a considerably
higher activation barrier and a weaker driving force for the formation
of the initial N–C bond, succeeded by ring closure with a miniscule
barrier.