Formate complexes decarboxylation

The reversible complexing of carbon dioxide by bis[bis(l,2-diphe-nylphosphino)ethane]iridium(I) chloride, [Ir(dpe)2]Cl, in acetonitrile [Eq. (36)] (48) appears not to involve carboxylation of a cyanomethylir-idium(III) complex or its formation by decarboxylation of the cyanoacetate... 

Decarboxylation of carbonate complexes is usually effected by acid hydrolysis with the formation of a C02 free oxide or hydroxide complex.128 All such reactions involve a protonated (bicarbonate) intermediate but there are some useful deferences which, in many instances, may be reconciled with the three main structural types of carbonate complexes. Both unidentate and chelate carbonates readily yield C02 on acidification, while there is a greater resistance to C02 loss when the carbonate is a bridging ligand. Unidentate carbonate complexes decarboxylate with the initial formation of a bicarbonate intermediate and subsequent loss of C02 without rupture of the M—O bond, viz. structure (3). By contrast, in chelate carbonate complexes, cleavage of the M—O bond occurs (with ring opening) with the formation of a bicarbonate aqua ion before the loss of C02, viz. equation (5).29... 

Formation of Bonds between Hydrogen and Transition Metals 1.10.9 by Decarboxylation of Hydroxycarbonyl or Formate Complexes. 

The r)3-allylpalladium formate complex is considered as a model of the intermediate in a catalytic reductive cleavage of allylic formate or allylic acetate combined with formic acid to olefins. The r)3-allylpalladium formate was revealed to be decarboxylated to release olefins upon coupling of the produced palladium hydride with the r)3-allyl ligand (Eq. 7). 

Formates. The decarboxylation reaction of metal formates is a fairly general route for the synthesis of metal hydrides and it has been applied to many transition metals. As an example, allyl palladium formates, which are believed to be intermediates in the catalytic reductive cleavage of allylic acetates and carbonates with formic acid to give monoolefins (Scheme 6.32), have been synthesized. In fact the complexes undergo decarboxylation and the reductive elimination of the allyl hydrido fragments, supporting the catalytic cycle proposed [105]. 

Kinetic parameters for formation and decarboxylation of carbonato-rhod-ium(III) complexes are given in Tables 5.16 and 5.17. In acid solution, the cis-[Rh(en)2(C03)] cation loses carbon dioxide slowly. Rate constants and activation parameters (A// and A5 ) are reported for the two paths indicated by ... 

A variety of transition-metal hydroxo complexes, [M(NH3)50H] [M = Co(III), Rh(III), and Ir(III)], react with CO2 to give the corresponding monodentate carbonato complexes [M(NH3)s0C02]. The formation and decarboxylation kinetics of these complexes have now been studied as a function of pressure up to 1000 bar. The volumes of activation for CO2 uptake are -10.1 0.6 (Co(III)), -4.7 0.8 (Rh(III)), and -4.0 1.0 (Ir(III)) cm mor, whereas the corresponding values for decarboxylation are +6.8 0.3, +5.2 0.3, and +2.5 0.4 cm mol , respectively. Combined with partial molar volume measurements, these values enable the construction of overall reaction volume profiles. Bond formation during CO2 uptake and bond breakage during decarboxylation are approximately 50% completed in the transition state of these processes. 

The formation of 1-and 2-aIkenes can be understood by the following mechanism. In the presence of formate anion, the 7r-allylpalladium complex 572 is converted into the 7r-allylpalladium formate 573. The most interesting feature is the attack of the hydride from formate to the more substituted side of the (T-allylic system by the cyclic mechanism shown by 574 to form the 1-alkene 575[367]. The decarboxylation and hydride transfer should be a concerted... 

The reaction has been applied to more complex enamines 13) and to dienamines 19). The reduction may be rationalized by initial protonation at the enamine carbon and subsequent decarboxylation of formate ion and addition of the hydride ion to the iminium cation. This mechanism has been given support by the reaction of the enamine (205) with deuterated formic acid 143) to give the corresponding amines. The formation of 206 on reaction with DCOOH clearly indicates that protonation at the enamine carbon is the initial step. 

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