Rhodium‐Catalyzed Branched‐Selective Alkyne Hydroacylation: A Ligand‐Controlled Regioselectivity Switch

Abstract

It's all in the ligand: By choice of the appropriate diphosphine ligand a previously linear‐selective alkyne hydroacylation process can be “switched” to be highly branched‐selective (see scheme, l=linear, b=branched). Structural data for the ortho‐iPr‐dppe–rhodium catalyst suggest restricted rotation of the phosphine aryl units may be responsible for the observed selectivity.

Well‐Defined and Robust Rhodium Catalysts for the Hydroacylation of Terminal and Internal Alkenes

“…An alternative approach is to increase the rate of reductive elimination of ketone product, a step that is often (although not exclusively7) the turnover‐limiting process in hydroacylation 3a. 8 Wide‐bite‐angle, or sterically bulky, ligands have this effect, but also change the ratio of alkene versus aldehyde hydroacylation9 or the linear/branched selectivity 10. Additionally, these systems in general still require high loadings, and activated alkenes or terminal alkynes as substrates.…”

“…In [D 6 ]acetone the C 6 H 5 F ligand in 3 a is replaced by acetone to form [Rh( 2 a )(acetone) 2 ][BAr F 4 ] 13. 16 Subsequent addition of 4 a afforded a complex mixture of products, in which the intermediate acyl hydride [Rh( 2 a )(H)(κ,σ‐1,2‐SMe(CO)C 6 H 4 )(acetone)][BAr F 4 ] ( 8 ) was identified [ δ (H) −20.4, br] 10. 13, 16 Over 2.5 h this reaction proceeded to give the corresponding decarbonylation product [Rh( 2 a )(SMePh)(CO)][BAr F 4 ] ( 9 ).…”

“…[3a, 8] Wide-bite-angle, or sterically bulky, ligands have this effect, but also change the ratio of alkene versus aldehyde hydroacylation[9] or the linear/branched selectivity. [10] Additionally, these systems in general still require high loadings, and activated alkenes or terminal alkynes as substrates. Small-bite-angle phosphine ligands, R 2 PCH 2 PR 2 (e.g., R= t Bu, Cy), initially developed by Hofmann et al,[11] have been shown to favor the products of reductive elimination[12] and we recently demonstrated that catalyst systems exemplified by [Rh(R 2 PCH 2 PR 2 )(η 6 -C 6 H 5 F)][BAr F 4 ] [R= t Bu, 1 ; Ar F =3,5-C 6 H 3 (CF 3 ) 2 ] can be used at low catalyst loadings (e.g., 0.1 mol %) to couple terminal and activated internal alkenes with β-substituted aldehydes.…”

“…[13, 16] Subsequent addition of 4 a afforded a complex mixture of products, in which the intermediate acyl hydride [Rh( 2 a )(H)(κ,σ-1,2-SMe(CO)C 6 H 4 )(acetone)][BAr F 4 ] ( 8 ) was identified [ δ (H) −20.4, br]. [10, 13, 16] Over 2.5 h this reaction proceeded to give the corresponding decarbonylation product [Rh( 2 a )(SMePh)(CO)][BAr F 4 ] ( 9 ). This timescale for decarbonylation is similar to that for 1 ,[13b] suggesting that the OMe group does not offer strong stabilization to the acyl hydride in the absence of alkene.…”

Effect of paste processing on residue levels of imidacloprid, pyraclostrobin, azoxystrobin and fipronil in winter jujube

“…Pfs are used to evaluate the risk associated with the intake of pesticide residues, especially for food products (Christensen et al 2003). They are also important in recommending MRLs for processed products (González-Rodríguez et al 2011). …”