Reactions of co-ordinated ligands. Part 50. The synthesis of functionalised η3-allyl complexes of molybdenum and their conversion into η4-s-trans-1,3-dienes
Abstract
Reaction of the labile bis(acetonitrile) complexes [Mo(NCMe)2(CO)2(η5-L)][BF4](L = C9H71 or C5Me52) with the 1-trimethylsilyloxybuta-1,3-dienes Me3SiOCHCR1CHCHR2(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(η3-CR2R3CHCR1CHO)(CO)2(η5-L)]4 and 6–9. Complexes 4 and 6–9 are generated in the form of isomeric mixtures, arising through syn-antiisomerism 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 Ph3PCH2 to 6(R1= R2= R3= H) and 7(R1= Me, R2= R3= H) effects smooth conversion into the η3-pentadienyl complexes [Mo(η3-CR2R3CHCR1CHCH2)(CO)2(η5-C5Me5)]11(R1= R2= R3= H) and 12(R1= Me, R2= R3= H). The low-temperature (–78 °C) protonation of compounds syn-6–8 using CF3SO3H results in a quantitative transformation into the s-trans-diene cations [Mo(η4-s-trans-trans-HOCHCR1CHCR2R3)(CO)2(η-5C5Me5)][CF3SO3]13(R1= R2= R3= H), 14(R1= Me, R2= R3= H) and 15(R1= R3= H, R2= Me), characterised in situ by 1H NMR spectroscopy. Above –30 °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 η3-pentadienyl complexes 11 and 12 undergo analogous transformations upon addition of CF3SO3H producing the thermally unstable s-trans-diene cations [Mo(η4-s-trans-trans-MeCHCRCHCH2)(CO)2(η5-C5Me5)][CF3SO3]16(R = H) and 17(R = Me), which are likewise converted into their s-cis isomers at temperatures in excess of –20 °C.