The synthesis, structure and reactivity of aldehyde substituted η3-allylic complexes of molybdenum

Abstract

Reaction of cis-[Mo(NCMe) 2(CO) 2( η 5-L)][BF 4] (L=C 5H 5 or C 5Me 5) with 1-acetoxybuta-1,3-diene gives the cationic complexes [Mo{ η 4- syn- s- cis-CH 2CHCHCH(OAc)}(CO) 2( η 5-L)][BF 4], which, on reaction with aqueous NaHCO 3/CH 2Cl 2, afford good yields of the anti-aldehyde substituted complexes [Mo{ η 3- exo- anti-CH 2CHCH(CHO)}(CO) 2( η 5-L)] 2 (L=C 5Me 5), 4 (L=C 5H 5)]. The corresponding η 5-indenyl substituted complex 5 was prepared by protonation (HBF 4·OEt 2) of [Mo( η 3-C 3H 5)(CO) 2( η 5-C 9H 7)] followed by addition of CH 2CHCHCH(OAc) and hydrolysis (aq. NaHCO 3/CH 2Cl 2). An X-ray crystallographic study of complex 2 confirmed the structure and showed that there is a contribution from a zwitterionic form involving donation of electron density from the molybdenum to the aldehyde carbonyl group. Treatment of 2 and 4, in methanol solution, with NaBH 4 afforded the alcohols [Mo{ η 3- exo- anti-CH 2CHCHCH 2(OH)}(CO) 2( η 5-L)] [ 6 (L=C 5H 5), 8 (L=C 5Me 5)]; however, prolonged (30 h) reaction with NaBH 4/MeOH surprisingly gave good yields of the methoxy-substituted complexes [Mo{ η 3- exo- anti-CH 2CHCHCH 2(OMe)}(CO) 2( η 5-L)] [ 7 (L=C 5H 5), 9 (L=C 5Me 5)], the structure of 7 being confirmed by single crystal X-ray crystallography. This methoxylation reaction can be explained by coordination of the hydroxyl group present in 6 and 8 onto B 2H 6 to form the potential leaving group HOBH 3 −, which on ionisation affords [Mo( η 4- exo-buta-1-3-diene)(CO) 2( η 5-L)] + which is captured by reaction with OMe −. Complex 8 is also formed in good yield on reaction of 2 with HBF 4·OEt 2 followed by treatment of the resulting cation [Mo{ η 4- exo- s- cis- syn-CH 2CHCHCH(OH)}(CO) 2( η 5-C 5Me 5)][BF 4] with Na[BH 3CN]. Reaction of 4 with the Grignard reagents MeMgI, EtMgBr or PhMgCl afforded moderate yields of the alcohols [Mo{ η 3- exo- anti-CH 2CHCHCH(OH)R}(CO) 2( η 5-C 5H 5)] [ 11 (R=Me), 12 (R=Et), 13 (R=Ph)]. Similarly, treatment of 2 with MeLi gave the corresponding alcohol 14. An attempt to carry out the Oppenauer oxidation [Al(OPr′) 3/Me 2CO] of 11 resulted in an elimination reaction and the formation of the η 3- s-pentadienyl complex [Mo{ η 3- exo- anti-CH 2CHCH(CHCH 2)}(CO) 2( η 5-C 5H 5)], which was structurally identified by X-ray crystallography. Interestingly, oxidation of 6 with [Bu 4 n N][RuO 4]/morpholine- N-oxide affords the aldehyde complex, 4 in good yield. Finally, reaction of 11 with [NO][BF 4] followed by addition of Na 2CO 3 affords the fur-3-ene complex [Mo{ η 2- (H)Me}(CO)(NO)( η 5-C 5H 5)].