While the development of Rh2(esp)2 has allowed for the efficient inter- and intramolecular catalytic amination of saturated C–H bonds, further attempts to design more efficient dirhodium catalysts for intermolecular amination of 3° C-H bonds have met with little success. Efforts to identify pathways for catalyst degradation and/or arrest revealed a single-electron oxidation event that gives rise to a red-colored, mixed-valence dimer, [Rh2(esp)2]+. This species is fortuitously reduced by pivalic acid, a byproduct generated in the reaction cycle with each turnover of the diacyloxyiodine oxidant. These findings have led to the conclusion that the high performance of Rh2(esp) 2 is due in part to the superior kinetic stability of its one-electron oxidized form relative to other dimeric Rh complexes. Use of the more reducing carboxylic acid, PhMe2CCO2H, has facilitated the development of the first efficient and general method for the intermolecular amination of 3° C-H bonds.
Rh2(S-nap)4, a chiral dirhodium tetracarboxamidate complex, has been developed and shown to be an effective catalyst for the asymmetric, intramolecular C–H amination of sulfamate esters. Enantiomeric excesses range from 60–99% for a collection of disparately substituted 3-arylpropylsulfamates. In addition, Rh2( S-nap)4 is found to promote chemoselective allylic C–H oxidation of unsaturated sulfamates, a property not observed with other dirhodium complexes tested to date. It is postulated that the exceptional performance of Rh2(S-nap)4 is due in part to the flexibility of the carboxamidate ligands.
While iodine(III) oxidants have proven invaluable for C-H amination reactions, insights gained through mechanistic studies of these processes suggest that other terminal oxidants, including common halogenating agents such as NaOCl, could function to promote the reaction. We have found that the combination of NaOCl and 3 mol% Rh2(oct)4 is effective in select cases for converting sulfamate substrates to the corresponding [1,2,3]-oxathiazinane-2,2dioxide heterocycles. The mechanism for C–H functionalization, however, is distinct from that induced by hypervalent iodine reagents and likely involves the intermediacy of an N-centered radical.
|School Location:||United States -- California|
|Source:||DAI-B 70/10, Dissertation Abstracts International|
|Keywords:||Amination, Carbon-hydrogen bond, Diacyloxyiodine, Electron oxidized, Rhodium|
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