The η2(3e)-vinyl complex is oxidized by [FeCp2]+, [CPh3]+, or B(C6F5)3 to form the 17-electron cation which on warming loses H to form the cationic η2(4e)-alkyne complex [Mo(η2-PhC⋮CPh){P(OMe)3}2Cp]+. In the case of the borane there is evidence for a competing reaction between the η2-vinyl complex and the acid (H2O)B(C6F5)3, resulting in the formation of a labile trans-stilbene complex.

Reaction of cis-[Mo(NCMe)(2)(CO)(2)(eta(5)-L)][BF4] (L = C5H5 or C5Me5) with 1-acetoxybuta-1,3-diene gives the cationic complexes [Mo(eta(4)-syn-s-cis-CH2CHCHCH(OAc)}(CO)(2)(eta(5)-L)][BF4], which, on reaction with aqueous NaHCO3/CH2Cl2, afford good yields of the anti-aldehyde substituted complexes [Mo(eta(3)-exo-anti-CH2CHCH(CHO)}(CO)(2)(eta(5)-L)]2(L = C5Me5), 4 (L = C5H5)]. The corresponding eta(5)-indenyl substituted complex 5 was prepared by protonation (HBF4 . OEt2) of [Mo(eta(3)-C3H5)(CO)(2)(eta(5)-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(eta(3)-exo-anti-CH2CHCHCH2(OH)}(CO)(2)(eta(5)-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(eta(3)-exo-anti-CH2CHCHCH2(OMe)}(CO)(2)(eta(5)- 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(eta(4)-exo-buta-1-3-diene)(CO)(2)(eta(5)-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(eta(4)-exo-s-cis-syn-CH2CHCHCH(OH)}(CO)(2)(eta(5)-C5Me5)][BF4] with Na[BH3CN]. Reaction of 4 with the Grignard reagents MeMgI, EtMgBr or PhMgCl afforded moderate yields of the alcohols [Mo(eta(3)-exo-anti-CH2CHCHCH(OH)R}(CO)(2)(eta(5)-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 eta(3)-s-pentadienyl complex [Mo(eta(3)-exo-anti-CH2CHCH(CHCH2)}(CO)(2)(eta(5)-C5H5)], which was structurally identified by X-ray crystallography. Interestingly, oxidation of 6 with [(Bu4N)-N-n][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(eta(2)-CH=CHCH2OC(H)Me)(CO)(NO)(eta(5)-C5H5)]. (C) 1998 Elsevier Science S.A.

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 Na[BH3CN]. 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-Download : Download full-size image(H)Me}(CO)(NO)(η5-C5H5)].

Barriers to metal-arene ring rotation in [1,4-C6H4(SiMe3)2)Cr(CO)2PPh3 and (1,3,5-C6H3tBu2)Cr(CO)2PPh3 are lowered relative to a previously reported barrier of 32 J mol−1 for (1,4-C6H4tBu2)Cr(CO)2PPh3. Reasons for the decreased barriers are discussed in the light of crystal structure determinations of these complexes.