Solvate dihydrides

Fig. 31.4 Solvate dihydrides characterized at low temperature [S = CH3OH or CD3OD]. Dihydrides formed in situ react rapidly with the common substrates of enantioselective hydrogenation, and the reduced product is formed with high enantios-electivity. At ambient pressure, the proportion of dihydrides is 45% at -100°C for (a) with a 10 1 diastereomer ratio, and 40% for (b) at —40°C, with a 2 1 diastereomer ratio.

The accepted mechanism for hydrogenation of alkenes by Wilkinson s catalyst involves the addition of dihydrogen prior to coordination of the alkene, followed by migratory insertion [31]. The new demonstrations of the existence of solvate dihydride complexes inevitably raise the question as to whether the same mechanism can apply in rhodium enantioselective hydrogenation. The evidence in support of this possibility is analyzed in more detail later. 

A key feature of the mechanism of Wilkinson s catalyst is that catalysis begins with reaction of the solvated catalyst, RhCl(PPh3)2S (S=solvent), and H2 to form a solvated dihydride Rh(H)2Cl(PPh3)2S [1], In a subsequent step the alkene binds to the catalyst and then is transformed into product via migratory insertion and reductive elimination steps. Schrock and Osborn investigated solvated cationic complexes [M(PR3)2S2]+ (M=Rh, Ir and S= solvent) that are closely related to Wilkinson s catalyst. Similarly to Wilkinson s catalyst, the mechanistic sequence proposed by Schrock and Osborn features initial reaction of the catalyst with H2 followed by reaction of the dihydride with alkene for the case of monophosphine-ligated rhodium and iridium catalysts [12-17]. Such mechanisms commonly are characterized... 

Several detailed NMR studies with new, electron-richer ligands were undertaken around the year 2000. With these new ligands, Rh(III) solvate dihydrides, [Rh(P-P)(H)2(S)2], could be easily prepared and characterised for several diphosphines. Bargon, Brown and co-workers used chiral (but not P-stereogenic) diphosphine PHANEPHOS and special NMR techniques... 

Table 1.1 Gibbs free energies (298 K, kcal/mol) characterizing equilibria between solvate complexes, molecular hydrogen complexes, and solvate dihydride complexes for various Rh-diphosphine catyalysts computed at WB97XD/ SDD(Rh)/6-31G(d, p)(aU others)/SMD(methanol) level of theory...

Formation of solvate dihydrides Experimentally formation of solvate dihydrides has been observed for several Rh-diphosphine catalysts. Thus, two diastereomeric dihydrides 28a, b were obtain via the reversible oxidahve addition of H2 to the solvate complex [Rh(f-Bu-BisP )(CD30D)2]+ (27) and have been characterized by NMR (Scheme 1.9). 

Scheme 1.9 Experimental detection of solvate dihydrides 28. (Reprinted with permission from Gridnev, I. D. et al., /. Am. Chem. Soc., 122, 7183-7194. Copyright 2000 American Chemical Society.)...

When HD was used for the generation of the solvate dihydrides 28, the complexes with axial position of the deuterium atom were formed preferentially with the factor 1.3 1 (Scheme 1.11). These data were later used for tracking the position of the deuterium label in the hydrogenation product vide infra). 

Similar observations of reversible formation of solvate dihydrides were made for other BisP -Rh complexes, for the TangPhos-Rh com-plex, and for the PHANEPhos-Rh complex. ... 

The examples of direct observation of solvate dihydrides together with the computational data collected in the Table 1.1 illustrate the fact that these species are kinetically competent intermediates in the catalytic cycle of Rh-catalyzed asymmetric hydrogenation. 

Scheme 1.12 Irreversible formation of solvate dihydrides. (Gridnev, I. D. and Imamoto, T., Chem. Commun., 7447-7464, 2009. Reproduced by permission of The Royal Society of Chemistry.)...

Thus, Brown et al. studied the low-temperature reaction of the solvate dihydride 47 with MAC (4) and the hydrogenation of an equilibrium mixture of the solvate 48 and the catalyst-substrate complex 49 (Scheme l.M). , Both experimental approaches afforded the same intermediate, agostic complex 50 (apparently through intermediates 51 and 52). The spectroscopic evidence clearly showed that one of the hydrides in 50 is caught on its way from Rh to carbon in this intermediate. 

Importantly, the formation of 50 was stereoselective. On the other hand, 50 was shown to exist in an equilibrium with the catalyst-substrate complex 49, the solvate dihydride 47 along with the mixture of catalyst 48 and MAC. Stereochemistry of the hydrogenation product is completely defined in 50, but the reverse reaction can bring it back to numerous interconverting intermediates with flexible stereochemistry. Nevertheless, when 50 is formed again, it results in exactly the same structure that clearly defines the absolute configuration of the hydrogenation product. 

Reactions of solvate dihydrides with prochiral substrates The accessibility of the solvate dihydrides for some catalysts (Schemes 1.10 and 1.12) makes possible the study of their direct reactions with prochiral substrates. These experiments can provide information on the late intermediates in the catalytic cycle. Besides, since the substrate is introduced in the system at very low temperature after the hydrogen activation already took place, the input from unsaturated mechanism is effectively excluded, and checking the ee of the recovered product gives direct information about the effectiveness of the enantioselection by coordination of a substrate to the octahedral solvate dihydride. 

Figure 1.4 Hydride region of NMR spectra (600 MHz, CD3OD, —95°C) of the sample initially containing solvate dihydrides 28a, b in equilibrium with the solvate complex 27 at -100°C obtained by placing the sample to the probe precooled to -100°C. Immediate stereoselective reaction takes place yielding the corresponding monohydride intermediate. The fact that the equatorial hydride trans to phosphorus (5 = -7.7, = 186 Hz) was transferred in this reaction is well illustrated.

The rapidness of the low-temperature reaction between 28 and 4 was further illustrated by the experiments using monodeuterated solvate dihydrides (Scheme 1.21). These experiments showed that the initial imbalance in the deuterium distribution in favor of the axial position is roughly conserved within low-temperature reactions of the HD solvate dihydrides with the substrate and is equal to that observed xmder the catalytic conditions. This is reflected in the preferential formation of a-deuterated product. Therefore, the formation of the monohydride 69 via 71 must have t en place much faster than the scrambling of the positions of the hydride and the deuteride ligands (the latter has been measured as 1.4 s"i at -80 C for the H2 case). ... 

Scheme 1.28 Detection of P-monohydride intermediates in low-temperature reaction of solvate dihydride 27 and ester of P-dehydroamino acid 18, and in low-temperature hydrogenation of the equilibrium mixture of 3,18, and 103.

The convergence of different pathways is further illustrated in Figure 1.7. Thus, if the catalyst can be hydrogenated itself yielding the solvate dihydride G (violet line), then a simple nonchelating coordination of the substrate will produce the key intermediate T. This situation was reproduced in numerous experiments in which the separately prepared solvate dihydrides were reacted with various prochiral substrates. 

In the early studies of nonenantioselective hydrogenation it has been found that b/s-monophosphine Ir complexes give stable trans-solvate dihydrides 1 upon removal of the coordinated diene from a precatalyst. These solvate dihydrides were found to be capable of exchanging one or two of their solvent molecules for olefins yielding dihydride olefin complexes 2 or 3, respectively which were characterized at -80°C, Equation 1.1.354,355... 

NMR Studies showed that removal of cod ligand leads to formation of a dimer existing in dynamic equilibrium with monomeric solvate dihydrides. Computational study has been carried out assuming monomeric catalyst (since the substrate could not be accommodated on the dimer) and Ir /Ir " mechanism (since stable molecular H2 complexes were not located). 

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