TY - JOUR
TI - 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
AU - Vivek Bagchi
AU - Anshika Kalra
AU - Purak Das
AU - Patrina Paraskevopoulou
AU - Saidulu Gorla
AU - Lin Ai
AU - Qiuwen Wang
AU - Sudip Mohapatra
AU - Amitava Choudhury
AU - Zhicheng Sun
AU - Thomas R. Cundari
AU - Pericles Stavropoulos
JO - ACS Catalysis
PY - 2018
VL - 8
TODO - 10
SP - 9183--9206
PB - American Chemical Society (ACS)
SN - 2155-5435
TODO - 10.1021/acscatal.8b01941
TODO - manganese; iron; cobalt; trisamido-amine ligands; nitrene-transfer
catalysis; electrochemistry; mechanistic studies; computational studies
TODO - Selective amination of sigma and pi entities such as C-H and C=C 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 Mn-II 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 PhI=NTs in the presence
of each Mn-II catalyst establishes a trend of increasing yield with
increasing Mn-II/III anodic potential. The Fe-II, Co-II, and Ni-II
congeners of the highest-yielding Mn-II 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. Co-II aziridination in comparison to
Mn-II but is less selective than Mn-II in competitive aziridinations of
nonconjugated olefins. Indeed, Mn-II 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 M-II/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.
ER -