Mechanistic with sulfamates

Several lines of inquiry have been explored to address key mechanistic issues in the rhodium-catalyzed C-H insertion of carbamates and sulfamates (Scheme 17.32) [99]. A pathway involving initial condensation between substrate 96 and PhI(OAc)2 to form iminoiodinane 97 was envisioned in the original design of this chemistry. Coordination of 97 to an axial site on the rhodium dimer would promote nitrene formation and the ensuing C-H insertion event Surprisingly, control experiments with PhI(OAc)2 and sulfamate 96 (or analogous carbamates) give no indication for a reaction between these two components. 

A recent DFT study has shown that retention of configuration observed for the above reactions is in agreement with the insertion of singlet nitrene and a concerted product formation [150 is the transition state for the reaction shown in Equation (6.135)]. Du Bois and coworkers have performed detailed mechanistic investigations of the intramolecular sulfamate ester C-H amination reaction catalyzed by a dirhodium complex. Reactivity patterns, Hammett analysis, and kinetic isotope effect studies have provided support for the concerted, asynchronous transition structure 151. A similar conclusion was arrived at for an analogous intermolecular process. ... 

Investigations by both Katsuki and Che have highlighted the application of chiral Mn(III) salen complexes for asymmetric N-atom transfer reactions (Fig. 17) [79-81]. In the former case, the use of [Mn(salen)PF6] together with PhI= Ts affords modest product yields and enantiomeric ratios for amination of simple benzylic and allylic starting materials. A high-valent Mn imido species is presumed to be the active oxidant. Methodological and mechanistic studies conducted by Che have found that the same types of Mn(III) salen complexes will induce oxidative cyclization of sulfamate esters. Although these catalysts afford... 

The results uncovered by Muller have been corroborated by the subsequent studies from the Du Bois group, particularly with respect to the intramolecular reaction. The stereospecific C(sp )—H insertion of the carbamate-derived nitrene depicted in Scheme 6A was the first relevant reaction in favor of the asynchronous concerted addition.More significantly, the extensive analysis of the rhodium-catalyzed intramolecular C(sp )-H amination of sulfamates has led to the same conclusion. The Hammett analysis (/0= —0.55 vs. and the kinetic isotope effect observed from the monodeuterated phenylpropyl sulfamate 9 (KIE = 2.6 0.2) clearly argue in favor of this mechanistic scenario (Scheme 17). Additionally, the reaction from the cyclopropyl radical clock 12 supports this hypothesis by furnishing a single product isolated with an excellent yield of 91%. 

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