| Abstract: | Reaction of cis-Mo(NCMe)2(CO)2(η5-L)]BF4] (L=C5H5 or C5Me5) with 1-acetoxybuta-1,3-diene gives the cationic complexes Mo{η4-syn-s-cis-CH2CHCHCH(OAc)}(CO)2(η5-L)]BF4], which, on reaction with aqueous NaHCO3/CH2Cl2, afford good yields of the anti-aldehyde substituted complexes Mo{η3-exo-anti-CH2CHCH(CHO)}(CO)2(η5-L)] 2 (L=C5Me5), 4 (L=C5H5)]. The corresponding η5-indenyl substituted complex 5 was prepared by protonation (HBF4·OEt2) of Mo(η3-C3H5)(CO)2(η5-C9H7)] followed by addition of CH2=CHCH=CH(OAc) and hydrolysis (aq. NaHCO3/CH2Cl2). 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 NaBH4 afforded the alcohols Mo{η3-exo-anti-CH2CHCHCH2(OH)}(CO)2(η5-L)] 6 (L=C5H5), 8 (L=C5Me5)]; however, prolonged (30 h) reaction with NaBH4/MeOH surprisingly gave good yields of the methoxy-substituted complexes Mo{η3-exo-anti-CH2CHCHCH2(OMe)}(CO)2(η5-L)] 7 (L=C5H5), 9 (L=C5Me5)], 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 B2H6 to form the potential leaving group HOBH3−, which on ionisation affords Mo(η4-exo-buta-1-3-diene)(CO)2(η5-L)]+ which is captured by reaction with OMe−. Complex 8 is also formed in good yield on reaction of 2 with HBF4·OEt2 followed by treatment of the resulting cation Mo{η4-exo-s-cis-syn-CH2CHCHCH(OH)}(CO)2(η5-C5Me5)]BF4] with NaBH3CN]. Reaction of 4 with the Grignard reagents MeMgI, EtMgBr or PhMgCl afforded moderate yields of the alcohols Mo{η3-exo-anti-CH2CHCHCH(OH)R}(CO)2(η5-C5H5)] 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/Me2CO] of 11 resulted in an elimination reaction and the formation of the η3-s-pentadienyl complex Mo{η3-exo-anti-CH2CHCH(CHCH2)}(CO)2(η5-C5H5)], which was structurally identified by X-ray crystallography. Interestingly, oxidation of 6 with Bu4nN]RuO4]/morpholine-N-oxide affords the aldehyde complex, 4 in good yield. Finally, reaction of 11 with NO]BF4] followed by addition of Na2CO3 affords the fur-3-ene complex Mo{η2-(H)Me}(CO)(NO)(η5-C5H5)]. |
