COUPLING REACTIONS OF A [1-(DIALKYLBORYL)BUTADIENE]HAFNOCENE COMPLEX WITH CARBONYL-COMPOUNDS

[Eta4-S-cis-1-(9-BBN)butadiene]hafnocene (s-cis-7) is readily available by the C-H activation reaction of (butadiene)hafnocene with 9-borabicyclo[3.3.1]nonane. Photolysis generates s-trans-7, which regioselectively adds ketones with formation of oxametallacycloheptene ring systems (9) bearing the bulky 9-BBN substituent at the carbon atom adjacent to hafnium. The structure of the benzophenone addition product (9b) to [1-(9-BBN) butadiene] hafnocene was determined by X-ray diffraction. Hexacarbonyltungsten also adds regioselectively to 7 with C-C coupling at the less substituted (9-BBN)butadiene terminus to yield a 4:1 mixture of the isomeric (pi-allylmetaloxy)- and seven-membered (sigma-allylmetaloxy)carbene complexes 10a and 10b. These react with acetone chemo- and stereoselectively to give a single nine-membered metallacyclic delta-boryl-substituted (metaloxy)carbene tungsten complex (11) under kinetic control which contains a trans-C = C double bond inside the medium-sized ring system. Subsequent thermally induced equilibration leads to cis-/trans-isomer formation (12:11 = 3:1 under thermodynamic control). The allylborane functionality which is part of the product 11 adds 1 equiv of acetone with allyl inversion and oxygen-boron bond formation. Thus, the use of a boryl-substituted butadiene ligand allows for the formation of selectively substituted and functionalized products in the sequential C-C coupling reaction between carbonyl building blocks and a conjugated diene at the group 4 bent metallocene template.