With rhodium hydride complexes

Figure 2.68 Bond lengths in two 5-coordinate rhodium hydride complexes with bulky tertiary...

The rhodium-catalyzed reaction of pyrrolidine with iodobenzene gives 2-phenylpyrroline in a high yield (Equation (72)). This reaction involves the formation of an imine rhodium hydride complex and phenylation (Equation (73)). 

This system fulfills the four above-mentioned conditions, as the active species is a rhodium hydride which acts as efficient hydride transfer agent towards NAD+ and also NADP+. The regioselectivity of the NAD(P)+ reduction by these rhodium-hydride complexes to form almost exclusively the enzymatically active, 1,4-isomer has been explained in the case of the [Rh(III)H(terpy)2]2+ system by a complex formation with the cofactor[65]. The reduction potentials of the complexes mentioned here are less negative than - 900 mV vs SCE. The hydride transfer directly to the carbonyl compounds acting as substrates for the enzymes is always much slower than the transfer to the oxidized cofactors. Therefore, by proper selection of the concentrations of the mediator, the cofactor, the substrate, and the enzyme it is usually no problem to transfer the hydride to the cofactor selectively when the substrate is also present [66]. This is especially the case when the work is performed in the electrochemical enzyme membrane reactor. 

In 2004 Caporali investigated the hydroformylation of 1-hexene and cyclohexene using HRh(CO)(PPh3)3 [61]. The collected data indicated that the rate-determining step in the hydroformylation cycle depends upon the structure of the olefin. With an alpha-olefin like 1-hexene, the slowest step seems to be the hydrogenolysis of the acyl rhodium complex. In the presence of cyclohexene as a model for an internal olefin, the rate-determining step is the reaction of the olefin with the rhodium hydride complex (intermediate II in Fig. 6). 

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

Rate Constants (10 3 k/M 1 s ) for the Reactions of Chromium(IV) and Chromium(V) Complexes with Rhodium Hydrides ... 

Prior to the work described below (50), hydrogen transfer to superoxometal complexes has been proposed by some (124 126) and questioned by others (127) who introduced plausible alternative mechanistic pathways. The work with rhodium hydrides (50) sought to establish whether hydrogen abstraction by superoxo complexes is a feasible and reasonable mechanism for thermodynamically favorable cases. 

Unlike the straightforward chemistry observed with rhodium hydrides and ArOH, the reaction with pivaldehyde is quite complex (143,144). The observed rate constants, and even the shape of kinetic curves, change with reaction conditions and the presence of scavengers for various intermediates. Figure 10 shows some examples. 

Under mild conditions, hydroformylation of olefins with rhodium carbonyl complexes selectively produces aldehydes. A one-step synthesis of oxo alcohols is possible using monomeric or polymeric amines, such as dimethylbenzylamine or anion exchange resin analog to hydrogenate the aldehyde. The rate of aldehyde hydrogenation passes through a maximum as amine basicity and concentration increase. IR data of the reaction reveal that anionic rhodium carbonyl clusters, normally absent, are formed on addition of amine. Aldehyde hydrogenation is attributed to enhanced hydridic character of a Rh-H intermediate via amine coordination to rhodium. 

Polar apiotic solvents, such as HMPA, and long reaction times were effective. The reaction of rhodium hydride complexes with CO3 in the presence of II3O was found to form the dihydrido bicarbonato complexes RhH3(OjCOH)L2 which reduce CO to form the corresponding Rh(l)[Pg.203]

Enolates can also be prepared by rhodinm-catalyzed isomerization of allylic Uthinm alcoholates, such as 14 (equation 5)". Subsequent treatment of the intermediately formed rhodium hydride complexes (15 and 16) with an electrophile led to the formation of various products. For example, alkyl halides gave a-alkylated ketones (17) in good yields, as shown in Table 4. Interestingly, addition of benzaldehyde under kinetically controlled... 

Carbonyl groups are also utilized in catalytic C-C bond cleaving reactions. Under catalytic conditions, 8-quinolyl phenyl ketone 85 reacts with ethylene to give 8-quinolyl ethyl ketone 86 and styrene in quantitative yield [105]. Styrene is formed by cleavage of the phenyl-carbonyl bond, followed by ethylene insertion into the resultant phenyl-rhodium bond, and (3-hydride elimination. The accompanying formation of a rhodium-hydride complex is followed by incorporation of ethylene to furnish the ethyl ketone 86. 

Operation of the insertion-elimination mechanism has been demonstrated in the reaction of rhodium hydride complex, RhHL4 (L=PPh3), with two isomeric allyl phenyl carbonates [56]. Unbranched 2-butenyl phenyl carbonate was found to give branched allylic phenyl ether exclusively, whereas the decarboxylation of the branched l-methyl-2-propenyl phenyl carbonate afforded unbranched 2-butenyl phenyl ether. These results can be accounted for by assuming a precata-lytic and catalytic insertion-elimination process as shown in Scheme 7. 

Besides acting as a hydride relay, the Lewis acid fragment can also participate in substrate activation. This possibility was first illustrated by Owen and co-workers while investigating the reactivity of the Rh complex 44b with sterically demanding phosphines (Scheme 14). No reaction is observed in the absence of Hj, but under 2.5bar of Hj, rhodium hydride complexes 43c,d are formed along with tricy-clo[2.2.1.0 ]heptane. This transformation involves the addition of Hj, the cleavage of the Rh B interaction and the formation of a C—H bond (presumably via reductive... 

Discovered more than 70 years ago, hydroformylation is nowadays one of the most important reactions in the chemical industry because aldehydes can be transformed to many other products. In the enantioselective version, rhodium/ diphosphorus ligand complexes are the most important catalytic precursors, although cobalt and platinum complexes have also been widely used. For these systems, the active species are pentacoordinated trigonal-bipyramidal rhodium hydride complexes, [HRh(P-P)(CO)2]. In those complexes, the coordination mode of the bidentate ligand (equatorial-equatorial or equatorial-apical) is an important parameter to explain the outcome of the process. The most common substrates of enantioselective hydroformylation are styrenes followed by vinyl acetate and allyl cyanide. With these substrates, mixtures of the branched (b, chiral) and linear (1, not chiral) aldehydes are usually obtained. In addition, some hydrogenation of the double bond is often observed. Therefore, chemo- and regioselectivity are prerequisites to enan-tioselectivity and all of them must be controlled. An additional eomplieation is that chiral aldehydes are prone to racemise in the presenee of rhodium spe-... 

Cp = pentamethylcyclopentadienyl, bpy = 2,2 -bipyridine) as the catalyst (43). The reaction takes place at ambient temperature, and no CO was detected in the H2-CO2 mixture. A rhodium hydride complex was identified in the catalytic cycle, which undergoes a rapid H/D exchange in presence of D2O. This result is consistent with the proposed rate-determining step, which is y3-hydrogen elimination from the formate complex (Fig. 1). 

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