Cinnamic acid transfer hydrogenation

Simandi and Nagy studied the kinetics of the catalyzed hydrogenation of cinnamic acid (S) to dihydrocinnamic acid (SHj) under steady-state conditions 166). They concluded that the kinetically important reactions were the two successive transfers of hydrogen atoms, viz.. 

The transfer of hydrogen to the alkene also occurs by a radical process,55,50 and not by the insertion reaction which is much more common in homogeneous hydrogenation. These kinetic studies were carried out using cinnamic acid as alkene. The observation of alkenyl and allyl species in diene hydrogenation suggests that a different mechanism of hydrogen transfer may... 

In addition, a Rh-(R,R)-NORPHOS catalyst has been used to promote enantioselective transfer hydrogenation of the C=C double bond in (Z)-a-(acetylamino)cinnamic acid and in (Z)-a- and ( )-a-(benzoylamino)-2-butenoate by using 80% aqueous formic acid as the source of H2. Optical yields were improved by the addition of sodium formate representative results are presented in Table 2. Comparable, but generally somewhat lower, optical yields were obtained by using other Rh-(biphosphine ligand) catalysts, e.g., biphosphine ligand = (R,S)-(+)-BPPFA (2), (R)-(+)-PROPHOS(3), ot(R,R)-... 

High catalytic activities, with turnovers of up to 9(X) cycles min , is displayed in the transfer hydrogenation of a,p-unsaturated ketones, such as benzylideneacetone and chalcone, using 2-propanol and catalytic amounts of [Ir(3,4,7,8-Me4-phen)COD]Cl (phen = 1,10-phenanthroline COD = 1,5-cyclo-octadiene) in a weakly alkaline medium. Other Ir-chelated complexes are also active catalysts in this reaction, with over 95% selectivity for the 1,4-reduction mode. Divalent lanthanide derivatives, such as Sml2 or Ybh in stoichiometric quantities, in THF and t-butyl alcohol or methanol reduce ethyl cinnamate and cinnamic acid to give the saturated derivatives. " Similarly, 3-methylcyclohexenone is reduced to 3-methylcyclohexen-l-ol in 67% yield, but a,p-unsaturated aldehydes are nonselectively reduced with these systems. 

Hydrogenation. The double bond of cinnamic acid derivatives is saturated by subjecting it to PdCl2 in HCOOH in an alkaline medium. This transfer hydrogenation also reduces benzoylformic acid to mandelic acid in 80% yield. 

Yong, H. Photochemistry and proton transfer reaction chemistry of selected cinnamic acid derivatives in hydrogen bonded environments. Int. J. Mass Spectrom. Ion Proc. 1998, 175, 187-204. 

The benzylic free radical produced by the addition of the carbamoyl radical to the ethyl cinnamate molecule is more stable than the alternative radical alpha to the ester group. With such an orientation of addition to the a,p-unsaturated ester, this reaction should lead to derivatives of malonic acid. However, it has been found that the intermediate radical, being a stable benzylic free radical, fails to perform the subsequent abstraction of a hydrogen atom from formamide, and thus no chain-transfer step takes place. Instead of performing this step it favours the combination with a semi-pinacol radical, which is present in solution, to yield the hydroxy ester which subsequently lactonizes to give the major product of the reaction (67). 

A first example that illustrated this concept of directing a photochemical [2 + 2] dimerisation in the solid state was reported by Ito and Sheffer/ in which diamines were used to form salts from a,p unsaturated carboxylic acids. The transference of the acidic proton in carbo g lic acids by strong base leads to the molecular salts where photodimerisation occurs between two anionic moieties. For example, two cinnamate anions are positioned in parallel form suitable for a [2 -I- 2] photocycloaddition using ethylenediamine via charge-assisted hydrogen bonds 0 ). 

---