Intermolecular Alkene and Alkyne Hydroacylation with β-S-Substituted Aldehydes: Mechanistic Insight into the Role of a Hemilabile P-O-P Ligand

A straightforward to assemble catalytic system for the intermolecular hydroacylation reaction of beta-S-substituted aldehydes with activated and unactivated alkenes and alkynes is reported. These catalysts promote the hydroacylation reaction between beta-S-substituted aldehydes and challenging substrates, such as internal alkynes and I-octene. The catalysts are based upon [Rh(cod)(DPEphos)][ClO4] (DPE-phos= bis(2-diphenylphosphinophenyl)-ether, cod=cyclooctadiene) and were designed to make use of the hemilabile capabilities of the DPEphos ligand to stabilise key acyl-hydrido intermediates against reductive decarbonylation, which results in catalyst death. Studies on the stoichiometric addition of aldehyde (either ortho-HCOCH2CH2SMe or ortho-HCOC6H4SMe) and methylacrylate to precursor acetone complexes [Rh(acetone)2(DPEphos)][X] [X=closo-CB11H6Cl6 or [BAr4F] (Ar-F = 3,5-(CF3)(2)C6H3)] reveal the role of the hemilabile DPEphos ligand. The crystal structure of [Rh(acetone)(2)- (DPEphos)][X] shows a cis-coordinated diphosphine ligand with the oxygen atom of the DPEphos distal from the rhodium. Addition of aldehyde forms the acyl hydride complexes [Rh(DPEphos)(COCH2CH2SMe)H][X] or [Rh(DPEphos)(COC6H4SMe)H][X], which have a trans-spanning DPEphos ligand and a coordinated ether group. Compared to analogous complexes prepared with dppe (dppe=1,2-bis(diphenylphosphino)ethane), these DPEphos complexes show significantly increased resistance towards reductive decarbonylation. The crystal structure of the reductive decarbonylation product [Rh(CO)(DPEphos)(EtSMe)][closo-CB11H6I6] is reported. Addition of alkene (methylacrylate) to the acyl-hydrido complexes forms the final complexes [Rh(DPEphos)(eta(1)-MeSC2H4-eta(1)-COC2H4CO2Me)][X] and [Rh(DPEphos)(eta(1)-MeSC6H4-eta(1)-COC2H4- CO2Me)][X], which have been identified spectroscopically and by ESIMS/MS. Intermediate species in this transformation have been observed and tentatively characterised as the alkyl-acyl complexes [Rh(CH2CH2CO2Me)-(COC2H4SMe)(DPEphos][X] and [Rh(CH2CH2CO2Me)(COC6H4SMe)-(DPEphos)][X]. In these complexes, the DPEphos ligand is now cis chelating. A model for the (unobserved) transient alkene complex that would result from addition of alkene to the acyl-hydrido complexes comes from formation of the MeCN adducts [Rh(DPEphos)(MeSC2H4CO)H(MeCN)] [X] and [Rh(DPEphos)(MeSC6- H4CO)H(MeCN)][X]. Changing the ligand from DPEphos to one with a CH2 linkage, [Ph2P(C6H4)](2)CH2, gave only decomposition on addition of aldehyde to the acetone precursor, which demonstrated the importance of the hemiabile ether group in DPEphos. With [Ph2P(C6H4)](2)S, the sulfur atom has the opposite effect and binds too strongly to the metal centre to allow access to productive acetone intermediates.