Intramolecular [2 + 2] Cycloaddition of Allyl CC and Allenylidene C_αC_β Bonds: Formation and Deprotonation of Cyclobutylidene Rings^†

The mixed-phosphine complex [Ru(η5-C5H5)Cl{κ1(P)-Ph2PCH2CHCH2}(PPh3)] (1) has been prepared by a phosphine exchange reaction between [Ru(η5-C5H5)Cl(PPh3)2] and Ph2PCH2CHCH2 (ADPP) (1:1 molar ratio) in refluxing THF. The treatment of complex 1 with NaPF6 in refluxing ethanol affords diastereoselectively the cationic complex [Ru(η5-C5H5){κ3(P,C,C)-Ph2PCH2CHCH2}(PPh3)][PF6] (2b). The reaction of complexes [Ru(η5-C9H7){κ3(P,C,C)-Ph2PCH2CHCH2}(PPh3)][PF6] (2a) and 2b with propargyl alcohols HC⋮CC(OH)R1R2 (R1, R2 = C12H8; R1 = Ph, R2 = Ph, H, Me) in refluxing THF yields regio- and diastereoselectively the cyclobutylidene complexes (CnHm = C9H7, R1, R2 = C12H8 (3a), R1 = Ph, R2 = Ph (3b), H (3c), Me (3d); CnHm = C5H5, R1, R2 = C12H8 (4a), R1 = Ph, R2 = Ph (4b), H (4c)). The formation of complexes 3a−d and 4a−c proceeds through an intramolecular cycloaddition of the CC allyl and CαCβ bonds in the intermediate allenylidene complexes [Ru(η5-CnHm)(CCCR1R2){κ1(P)-Ph2PCH2CHCH2}(PPh3)][PF6]. The allenylidene complex [Ru(η5-C9H7)(CCCPh2){κ1(P)-Ph2PCH2CHCH2}(PPh3)][PF6] (5) has been isolated from the reaction of 2a with 1,1-diphenyl-2-propyn-1-ol in CH2Cl2. The deprotonation of complexes 3a−d and 4a with potassium tert-butoxide gives rise to the neutral complexes (CnHm = C9H7, R1, R2 = C12H8 (6a), R1 = Ph, R2 = Ph (6b), H (6c), Me (6d); CnHm = C5H5, R1 = Ph, R2 = H (7c)). The structures of derivatives 3a and 6d have been determined by single-crystal X-ray diffraction analysis.