An Investigation into the Formation of a Variety of C3 Organometallic Species via the Isomerisation of Alkynes by Ruthenium (II) and Rhodium (I) Complexes

A novel synthetic route for the formation of ruthenium complexes [Ru(κ2-OAc)2(PR3)2] 1 has been developed. These complexes have been used to investigate the stoichiometric formation of geminal alkenes from the reaction of triphenylphosphine complex [Ru(κ2-OAc)2(PPh3)2] 1a with propargylic alcohols. It was found that the reaction was promoted by the use of the bulky, electron-rich triisopropylphosphine. This lead to the proposal that the mechanism involves a cationic intermediate and the suggested structure of this intermediate was vinyl carbene [Ru(κ2 OAc)(OC{Me}OCC{H}=CH2)(PPh3)2][BF4] 26.
A series of analogues of 26 were synthesised and their reactivity investigated. Whilst no evidence was obtained of their being involved in the transformation of propargylic alcohols to alkenes, it was found that their reactivity can be tuned by changing the substituents on the vinyl moiety. Deprotonation leads to the formation of either the allenylidene complex [Ru(κ2-OAc)(κ1-OAc)(PPh3)2(=C=C=CPh2)] 30 or the vinyl vinylidene complexes [Ru(κ2-OAc)(κ1-OAc)(PPh3)2(=C=CH-C(R)=CH2)] 32 (R = Ph, Me). A theoretical investigation into this reaction has led to the suggestion that vinyl carbene complexes like 26 could be important reaction intermediates in the formation of allenylidenes from propargylic alcohols.
The reactivity of 1a towards the triphenylphosphine-substituted alkyne [HC≡CCH2PPh3]+ 39 has also been investigated. It has been found that this propargylic phosphonium is particularly prone to isomerisation to its allene isomer [H2C=C=CHPPh3]+ 40 and, as a result, reaction with organometallic precursors leads primarily to the formation of the allene complexes [Ru(κ2 OAc)(κ1 OAc)(η2 H2C=C=CHPPh3)(PPh3)2][BF4] 45a and [RhCl(η2 H2C=C=CHPPh3)(PiPr3)2][BPh4] 60. Experimental and theoretical investigations have concluded that the isomerisation is spontaneous (the allene is 37 kJ mol-1 more stable than the alkyne) and not metal promoted. It is however counter-ion dependant and use of the tetraphenylborate salt 39c slows the process enough for novel vinylidene complexes [Ru(κ2-OAc)(κ1-OAc)(C=C=CHCH2PPh3)(PPh3)2][BPh4] 38 and [RhCl(C=C=CHCH2PPh3)(PiPr3)2][BPh4] 63 to be synthesised.