Structure and reactions of oxametallacyclobutanes and oxametallacyclobutenes of ruthenium

Article abstract of DOI:10.1021/om00055a061

Structure and reactivity studies are reported with the ruthenium metallacycles prepared as described in the previous paper. A C-C cleavage reaction by an apparent β-Me elimination pathway at 45°C is reported for the PMe₃-substituted oxametallacyclobutane complex (PMe₃)₄Ru(OC(Me)(Ph)CH₂) (1), while the analogous DMPE-substituted metallacyclobutane (DMPE)₂Ru(OC(Me)(Ph)CH₂) (2) is stable at 140°C. Similarly, compound 1 undergoes insertion of CO into the metal-carbon bond, while 2 is inert toward this substrate. Addition of protic acids and electrophiles leads to rapid extrusion of α-methylstyrene with both metallacycles. X-ray structural analysis of the acetone dianion complex (PMe₃)₄Ru((CH₂)₂CO) (17) was performed and displays a dihedral angle of 46°C in the metallacycle. In contrast, the 4,4-dimethyl-2-butanone dianion complex (PMe₃)₄Ru(CH2C(CHCMe3)O) (15) contains a flat metallacycle that is bound through the CH₂ group and the oxygen atom. Reactivity studies with 15 showed that, unlike compounds 1 and 2, the organic portion remained intact upon addition of protic acids. The addition of 4,4-dimethyl-2-butanone led to a second C-C cleavage reaction, forming the di-tert-butylacetylacetonate complex (PMe₃)₃Ru-(Me)(CH(COCH₂CMe₃) ₂) (19). Reactivity studies with 17 showed reversible formation of the isolable oxatrimethylenemethane complex 18, which was isolated and structurally characterized. Addition of acetone to 17 led to formation of mesityl oxide dianion complex (PMe₃)₄Ru(OC(Me)CHC(Me)CH) (19); mesityl oxide is presumably formed by aldol condensation at the metal center. Reactivity studies of the oxametallacyclobutene complex (PMe₃)₄Ru(OC(CMe₃)CH) showed formation of free ketone upon addition of protic acids and insertion into the metal-oxygen bond upon addition of CO₂. The metallacycle was converted to the silyl enol ether upon addition of trimethylsilane and to the free ketone following addition of H₂.