REACTIONS OF COORDINATED LIGANDS .50. THE SYNTHESIS OF FUNCTIONALIZED ETA-3-ALLYL COMPLEXES OF MOLYBDENUM AND THEIR CONVERSION INTO ETA-4-S-TRANS-1,3-DIENES

Reaction of the labile bis(acetonitrile) complexes [Mo(NCMe)2(CO)2(eta-5-L)][BF4] (L = C9H7 1 or C5Me5 2) with the 1-trimethylsilyloxybuta-1,3-dienes Me3SiOCH = CR1CH = CHR2 (R1 = R2 = H; R1 = Me, R2 = H; R1 = H, R2 = Me) results in initial co-ordination of the diene followed by a rapid desilylation affording the 4-oxo-functionalised allyls [Mo(eta-3-CR2R3CHCR1CHO)(CO)2(eta-5-L)] 4 and 6-9. Complexes 4 and 6-9 are generated in the form of isomeric mixtures, arising through syn-anti isomerism at C2 of the allyl ligand together with the presence of non-interconvertible exo and endo orientations for the latter with respect to the metal fragment. Addition of Ph3P = CH2 to 6 (R1 = R2 = R3 = H) and 7 (R1 = Me, R2 = R3 = H) effects smooth conversion into the eta-3-pentadienyl complexes [Mo(eta-3-CR2R3CHCR1CHCH2)(CO)2(eta-5-C5Me5)] 11 (R1 = R2 = R3 = H) and 12 (R1 = Me, R2 = R3 = H). The low-temperature (-78-degrees-C) protonation of compounds syn-6-8 using CF3SO3H results in a quantitative transformation into the s-trans-diene cations [Mo(eta-4-s-trans-trans-HOCHCR1CHCR2R3)-(CO)2(eta-5-C5Me5)][CF3SO3] 13 (R1 = R2 = R3 = H), 14 (R1 = Me, R2 = R3 H) and 15 (R1 = R3 = H, R2 = Me), characterised in situ by H-1 NMR spectroscopy. Above -30-degrees-C, 13-15 undergo an irreversible rearrangement in solution to the corresponding s-cis diene cations. Deprotonation of 13-15 by treatment with NEt3 regenerates the syn isomers of 6-8, whilst the s-cis diene cations produce only the anti species under the same conditions. These results are interpreted in terms of an overall retention of stereochemistry of the C4 ligand throughout these manipulations. The eta-3-pentadienyl complexes 11 and 12 undergo analogous transformations upon addition of CF3SO3H producing the thermally unstable s-trans-diene cations [Mo(eta-4-s-trans-trans-MeCHCRCHCH2)(CO)2(eta-5-C5Me5)][CF3SO3] 16 (R = H) and 17 (R = Me), which are likewise converted into their s-cis isomers at temperatures in excess of -20-degrees-C.