Regioselective addition of protic acids to tricarbonyliron complexes of 2,3,5,6-tetrakis(methylene)-7-oxabicyclo[2.2.1]heptane. Crystal structure of (C 10H 10O)Fe 2(CO) 6

Abstract

The main product of the thermal reaction between the title oxatetraene (I) and Fe 2(CO) 9 in ether/pentane is the bimetallic complex (C 10H 10O)Fe 2(CO) 6- diexo (II), which has C 2υ symmetry both in the solid state (X-ray analysis) and in solution. Whereas the protonation of the free ligand leads usually to polymerisation, the addition of a protic acid such as CF 3CO 2H to II proceeds cleanly at 0°C giving first a (η 3-allyl)Fe(CO) 3O 2CCF 3 complex (III). The intermediate III adds a second equivalent of acid in a slower step ( k 2/ k 1 = 0.1, CF 3CO 2D/CHCl 3, 0°C) giving the trans-bis(η 3-allyl) isomer IV with high regioselectivity. The addition of CF 3CO 2D yields the corresponding deuteriomethylallyliron tricarbonyl trifluoroacetates III′ and IV′. No further deuterium incorporation is observed at 0°C, thus confirming the kinetic control of the regioselective double addition of protic acid to II.