Rhodium complexes hydride transfer

Probably the first non-covalent immobilization of a chiral complex with diazaligands was the adsorption of a rhodium-diphenylethylenediamine complex on different supports [71]. These solids were used for the hydride-transfer reduction of prochiral ketones (Scheme 2) in a continuous flow reactor. The inorganic support plays a crucial role. The chiral complex was easily... 

In the case of terminal C=C (1,2 addition units), i.e. when R=R =H and R" (or R111) = polymer chain, two types of hydride migration are possible, namely (i) The Markownikoff s addition which would lead to the formation of B type repeating units and (ii) The anti Markownikoff s addition which would result in the formation of the observed repeating units C. In the case of Markownikoff s type addition the hydride transfer occurs to Ca and results in the formation of branched alkyl-rhodium intermediate complex shown by Structure 2. Whereas when anti Markownikoff s addition occurs, the resulting intermediate alkyl-rhodium complex has linear alkyl ligand as shown by Structure 3. 

Complex 9 (Scheme 43.3) can be reduced by different redox equivalents to the active rhodium(I) species 10 namely, by electrons, formate [37, 38], and hydrogen. This hydrido complex then transfers the hydride ion onto the nicotinamide. In electrochemical applications, TOFs in the range of 5 to 11 h-1 have been reported [31, 39]. It is noteworthy that this complex accepts NAD+ and NADP+ as substrates with the same efficiency and almost exclusively produces the 1,4-reduced cofactor (selectivity >99%). 

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. 

Systems that fulfill these conditions are substituted or unsubstituted (2,2 -bipyridyl)(pentamethylcyclopentadienyl)rhodium complexes. Electrochemical reduction of these complexes at potentials between -680 mV and -840 mV vs. SCE leads to the formation of rhodium hydride complexes. Strong catalytic effects observed in cyclic voltammetry and preparative electrolyses are indicating a very fast hydride transfer from the complex to NAD(P)+ under formation of the starting complex, as shown in the following reaction scheme [64] ... 

There are two systems so far which fulfill these requirements tris(2,2 -bipyridyl) rhodium complexes [109, 110] and substituted or nonsubstituted (2,2 -bipyridyl) (pentamethylcyclopentadienyl)-rhodium complexes [111]. At potentials between —680 and —840 mV vs SCE, the electrochemical reduction of these complexes leads to the formation of rhodium hydride complexes. Hydride ions are transferred from the complex to NAD(P)+ under specific formation of 1,4-NAD(P)H and the initial complex. 

Promise is held in MPV reactions carried out under catalytic conditions. Instead of, for example, stoichiometric amounts of aluminum as the metal ion activator, catalytic quantities of complexes of rhodium and iridium can sometimes be used to bring about the same reactions. Although the catalytic mechanisms have not been established, postulation of the usual six-membered transition state in the critical step of hydride transfer appears reasonable. The strongly basic conditions of the MPV reaction are avoided. Reductions of aryl ketones (69 equation 30) using (excess) isopropyl alcohol as hydrogen donor and at partial conversions have led to the formation of alcohol (70) in modest enantiomeric excesses with various chiral ligands. " ... 

A further step was taken when first Halpern [28] and then Brown [29] were able to identify a further intermediate, the rhodium alkyl hydride formed by addition of dihydrogen to the enamide complex with transfer of a single hydride to the benzylic carbon. For the simple dppe complex studied by Halpern, the interpretation of the experiment was straightforward, but the intermediate derived from DIPAMP by Brown and Chaloner provided a major surprise only the disfavored minor diastereomer of the enamide complex was reactive towards H2. The major/minor equilibrium is so strongly biased towards the former below -50 °C that reaction with H2 is undetected. Only when the solvate complex is allowed to react with the dehydroamino acid derivative under H2, well below -50 °C (under which conditions up to 35% of the minor diastereomer is initially observed) is the alkyl hydride observed, concomitant with disappearance of that minor diastereomer. This reactive intermediate was characterized by its H-NMR (hydride), the distinctive P-NMR and by both heteronuclear coupling and chemical shifts in the C-NMR spectra of alkyl hydrides derived from singly and doubly labeled dehydroamino esters. 

The reaction is not chelation-controlled and does not involve hydride transfer to the Lewis acid since BF3 OEt2, which can only be four-coordinate, also gives excellent anti stereoselectivity. Tethering the reagent and intramolecularizing the process is also important for the rate of reaction - the analogous intermolecular process was found to be sluggish, even at 0 °C. Rhodium(I) complexes also catalyze the reduction reaction. Burk... 

The mechanism proposed for the rhodium-catalyzed intramolecular insertion of carbenoids into C-H bonds adjacent to ethers [63] is more complex than previously recognized for such reactions. According to this mechanism (Scheme IV-32), the acetal product arises by oxygen-assisted hydride transfer to the electrophilic carbon of the carbenoid. 

Cationic iridium and rhodium complexes of the formula [M(COD)(PPh3)2]+ (M = Ir, Rh) are catalyst precursors for the homogeneous hydrogenation of ben-zothiophene (BT) to 2,3-dihydrobenzothiophene (DHBT). These catalyst precursors react with BT and hydrogen to produce [MH2(j (S)-BT)2(PPh3)2]+, which then forms [MH2( f(C=C)-BT)(PPh3)2], followed by hydride transfer to the C2=Cs bond. Displacement of the hydrogenated dihydrobenzothiophene by BT restarts the cycle (55). 

According to the proposed mechanism, addition of the silyl-rhodium moiety to the coordinated carbonyl group converts it into the a-siloxyalkyl-rhodium complex (III), which most likely is an equilibrium mixture of complexes Ilk and Iltt. Then, transfer of the hydride ligand in III from the metal center to the alkyl carbon affords the products, IVfl and IV, respectively. The formation of the a-siloxyalkyl-rhodium intermediate is quite probable in view of the well-documented soft-hard conception , and must be characteristic of the ketone hydrosilylation. 

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