Selective Hydrogenation of Cinnamaldehyde and Other α,β-Unsaturated Substrates Catalyzed by Rhodium and Ruthenium Complexes

Abstract

The complexes [Rh(PhBP 3 )(cod)] ( 1) and [{Ru(PhBP 3 )(µ-Cl)} 2 ] (8), containing the tripodal phosphanoborate ligand [PhB(CH 2 PPh 2 ) 3 ] -, are efficient catalysts for the selective hydrogenation of cinnamaldehyde to the corresponding allyl alcohol. Complex 8 is one of the most efficient catalysts reported to date for this reaction, in terms of activity (TOF 527 h -1 ) and selectivity (g97%) under mild reaction conditions (6.8 atm H 2 , 75 °C). The rhodium system also displays good catalytic features in the hydrogenation of cinnamaldehyde (TOF 219 h -1 ), particularly a high selectivity (81%) for this metal in the reduction of the CdO bond. Crotonaldehyde can also be reduced, although the selectivities are not as high as for cinnamaldehyde; 2-cyclohexenone is rapidly and specifically reduced to cyclohexanone by both catalysts. The ruthenium catalyst 8 operates via heterolytic activation of hydrogen, involving monohydride intermediates and possibly ionic hydrogen transfer, while the rhodium complex 1 involves oxidative addition of dihydrogen to form dihydride intermediates and follows a substrate route. Indeed, complex 1 reacts with hydrogen in acetonitrile to give the dihydride complex [Rh(PhBP 3 )(H) 2 (NCMe)] (3), while protonation of one of the phosphane arms of the ligand occurs on treatment of complex 1 with HBF 4 to give the cationic species [Rh{PhB(PH)P 2 }(cod)]BF 4 . The hydride ligands in 3 are easily removed as molecular hydrogen by reaction with CO under atmospheric pressure to give the rhodium(I) complex [Rh(PhBP 3 )(CO) 2 ]. In this reaction, the replacement of acetonitrile by CO takes place previously to the elimination of hydrogen, which indicates that substrates can coordinate to the metal in 3 by replacement of the labile acetonitrile ligand. Under an atmosphere of argon, complex 3 reacts with chloroform to give an equimolecular mixture of the cis and trans isomers of [{Rh(PhBP 3 )(H)(µ-Cl)} 2 ] and, eventually, complex [Rh(PhBP 3 )Cl 2 ] in one day.

Zwitterionic Relatives of Cationic Platinum Group Metal Complexes: Applications in Stoichiometric and Catalytic σ‐Bond Activation

“…[82] In contrast to zwitterions featuring BPh 4 À or Bcat 2 À that rely on h 6 -arene interactions to bind a formally cationic PGM fragment, the three MÀP linkages featured in k 3 -[RBP 3 ]ML n complexes serve to anchor the borate ligand to the metal, thereby affording more robust zwitterionic structures. The crystallographically determined M···B distances in ruthenium, [83,84] rhodium and iridium, [82,83,[85][86][87] and platinum [88] [82] Subsequent studies confirmed the ability of 20 to participate in analogous double geminal SiÀH and GeÀ H bond activation processes with other secondary silanes (for example with R = Me, Et, or Ph), primary silanes (R = Mes or 2,4,6-triisopropylphenyl), and Mes 2 GeH 2 . [86] However, the course of such E À H bond activation chemistry was found to be sensitive to the nature of the substituents at phosphorus within the [RBP 3 ] ligand; analogous reactions of the di(isopropyl)phosphino complex 22 with Et 2 SiH 2 or Ph 2 SiH 2 each afforded the corresponding silyl-capped trihydrido species 23 with loss of 1,3-cyclooctadiene (Scheme 11).…”

“…[87] The utility of [RBP 3 ]-ligated PGM species in catalysis remained undocumented until the use of zwitterions 24 and 25 as pre-catalysts for the selective hydrogenation of a,bunsaturated substrates was reported in 2009. [83] Both 24 and 25 proved to be particularly efficient catalysts for the selective hydrogenation of cinnamaldehyde (26) to the corresponding allyl alcohol 27, with the ruthenium zwitterion 25 being one of the most efficient catalysts known for this transformation (Scheme 12).…”

Beyond Triphos – New hinges for a classical chelating ligand

“…It was hoped that by implementing 2 as the ligand, the high electron-donating ability of the di-isopropylphosphino groups would facilitate the loss of anthracene with concomitant nitride transfer to generate the desired Ni N subunit, but this did not occur. More recent work has focused on second row transition metals such as rhodium [31,32] and ruthenium [32,33]. Rhodium derivatives have displayed a wide range of chemistry, with many reported complexes (64-76, Scheme 12) [32,34,35].…”

“…More recent work has focused on second row transition metals such as rhodium [31,32] and ruthenium [32,33]. Rhodium derivatives have displayed a wide range of chemistry, with many reported complexes (64-76, Scheme 12) [32,34,35]. As these complexes have been studied as hydrogenation and hydrogen transfer catalysts, several complexes feature Rh-H fragments (67, 68, 72 and 74).…”

“…Complexes featuring R'B(CH 2 PR 2 ) 3 ligands have been widely used across many applications, ranging from hydrogenation [32] reactions to their use as precursors for metallic silver aerosol-assisted chemical vapour deposition [21]. These complexes have displayed a rich chemistry for the stabilization of reactive species via activation of small molecules, as well as mediating their subsequent reactivity.…”