Abstract
Ruthenium( II) complexes can be used to oxidise N-Boc-hydroxylamine in the presence of tert-butyl hydroperoxide (TBHP) to the corresponding nitroso dienophile, which is trapped by cyclohexa-1,3-diene as the hetero-Diels–Alder adduct. Direct evidence has been obtained for the intervention of a triphenylphosphine oxide-stabilised ruthenium( IV) oxo-complex as the catalytically active species. Use of a chiral bidentate bis-phosphine derived ruthenium ligand (BINAP or PROPHOS) results in very low asymmetric induction (8 and 11%). Ruthenium( II) salen complexes also catalyse the oxidation of N-Boc-hydroxylamine in the presence of TBHP, to give the N-Boc-nitroso compound which can be efficiently trapped with a range of dienes. However, use of an enantiopure ruthenium salen complex does not produce asymmetric induction via the trapping of the intermediate acyl nitroso dienophile with cyclohexadiene, which strongly suggests that the intermediate dissociates readily from the chiral ruthenium complex involved in the oxidation step prior to Diels–Alder cycloaddition.
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