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.