Reactions of Neutral Scandium/Phosphorus Lewis Pairs with Small Molecules

Treating a scandium mixed alkyl/diaryloxide complex with phosphino-substituted alcohols resulted in the formation of scandium mixed alkoxyl/diaryloxide complexes Sc­[OC­(CH₃)₂CH₂PR₂]­[O-2,6-tBu₂-C₆H₃]₂[THF] (6, R = Ph; 8, R = tBu). The reactivity of Sc-based Lewis pair complexes 6 and 8 toward a wide range of substrates was investigated. A ligand substitution reaction occurred when complex 6 was treated with benzophenone. The reaction of complex 6 with sulfur afforded a phosphane oxidative product. Complexes 6 and 8 also exhibited typical Sc/P frustrated Lewis pair (FLP) reactivity. They added to the carbonyl group of phenyl isocyanate to form seven-membered metallacycles. Complex 6 underwent selective Sc/P 1,4-addition reactions with α,β-unsaturated carbonyl compounds to give the corresponding nine-membered metallacyclic products. The reaction of 6 with 0.5 mol of dimethyl acetylenedicarboxylate led to the formation of bicyclo[7.7.0]­cetane-derived metallacycle 16 in a distinctive double 1,4-addition pattern. With benzil complex 6 underwent a 1,4-addition reaction to afford a cis-enediolate which was coordinated to the Sc/P FLP through Sc–O and P–O bonds. Treatment of complex 6 with cyclopropyl phenyl ketone afforded the 10-membered metallaheterocycle 18, in which a scandium Lewis acid added to the oxygen atom and the phosphine Lewis base attacked the cyclopropyl group with ring opening. A ring-opening reaction also took place when epoxycyclohexane was employed as a substrate. Complex 6 successfully cleaved the C–Br bond of benzyl bromide to give zwitterionic complex 20 with newly formed Sc–Br and P–C bonds. Scandium phosphazine complex 21 was generated by a Sc/P 1,1-addition to the terminal nitrogen atom of (trimethylsilyl)­diazomethane. Aided by the phosphorus Lewis base, scandium Lewis acid was able to abstract chloride from (Ph₃P)­AuCl to produce heterobimetallic Sc/Au complexes 22 and 23. Many complexes in this study were characterized by single-crystal X-ray diffraction.