2-Rhodaoxetanes: Their formation of oxidation of [Rh-I(ethene)](+) and their reactivity upon protonation
de Bruin B, Boerakker MJ, Verhagen JAW, de Gelder R, Smits JMM, Gal AW
CHEMISTRY-A EUROPEAN JOURNAL 6 (2): 298-312 JAN 2000

Abstract:
New cationic, pentacoordinate complexes [(TPA)Rh-1(ethene)](+), [1a](+). and [(MeTPA)Rh-1(ethene)](+) [1b](+), have been prepared (TPA = N.N.N-tri(2-pyridylmethyl)amine, MeTPA = N-[(6-methyl-2-pyridyl)-methyl]-N.N-di(2-pyridylmethyl)amine) Complex [1a](+) is selectively converted by aqueous HCl to [(TPA)Rh-III- (ethyl)Cl](+). [2a](+), The same reaction with [1b](+) results in the [(MeTPA)Rh-III- (ethyl)Cl](+) isomers [2b](+) and [2c](+). Treatment of [1a](+) and [1b](+) with aqueous H2O2 results in a selective oxygenation to the unsubstituted 2-rhoda(III)oxetanes (1-oxa-2-rhoda(III)cyclobutanes) [(TPA)Rh-III( kappa(2)-C,O-2-oxyethyl)](+), [3a](+), and [(MeTPA)Rh-III(kappa(2)C,O-2-oxyethyl)](+) [3b](+). The reactivity of 2-rhodaoxetanes [3a](+) and [3b](+) is dominated by the nucleophilic character of their 2-oxyethyl oxygen. Reaction of [3a](+) and [3b](+) with the non-coordinating acid HBA(4)(i) results in the dicationic protonated 2-rhodaoxetanes [(TPA)Rh-III(kappa(2)-2-hydroxyethyl)](2+) [4a](2+), and [(MeTPA)Rh-III(kappa(2)-2-hydroxyethyl)](2+), [4b](2+). These eliminate acetaldehyde at room temperature, probably via a coordinatively unsaturated kappa(1)- 2-hydroxyethyl complex, In acetonitrile. complex [4a](2+) is stabilised as [(TPA)-Rh-III(kappa(1)-2-hydroxyethyl)(MeCN)](2+) [5a](,2+) whereas the MeTPA analogue [4b](2+) continues to eliminate acetaldehyde. Reaction of [3a](+) with NH4Cl and MeI results in the coordinatively saturated complexes [(TPA)Rh-III(kappa(1)-2-hydroxyethyl)(Cl)](+), [6a](+), and [(TPA)Rh-III(kappa(1)-2-methoxyethyl)(I)](+), [7a](+), respectively. Reaction of [3a](+) with NH4+ in MeCN results in formation of the dicationic metallacyclic amide: [(TPA)-Rh-III(kappa(2) -O,C-2-(acetylamino)ethyl)](2+), [9](2+), via the intermediates [4a](2+), [5a](2+) and the metallacyclic iminoester [(TPA)Rh-III(kappa(2)-N,C-2-(acetimidoyloxy)- ethyl)](2+), [8](2+). The observed overall conversion of the [Rh-I(ethene)] complex [1a](+) to the metallacyclic amide [9](2+) via 2-rhodaoxetane [3a](+), provides a new route for the amidation of a [Rh-I(ethene)] fragment.