Catalysts Schrock-Osborn

Figure 9.3 represents the hydride mechanism in which H2 adds before the olefin. Sometimes, the olefin adds first (the olefin mechanism), as proposed for the Schrock-Osborn catalyst, [Rh(dpe)(MeOH)2]BF4, formed by hydrogenation in MeOH of the placeholder cyclooctadiene (cod) ligands of the catalyst precursor, [(cod)Rh(dpe)]BF4. 

The development of homogeneous olefin hydrogenation catalysts based on soluble rhodium and iridium complexes, in particular Wilkinson s catalyst (33) [27], Crabtree s catalyst (34) [41], and the Schrock-Osborn catalyst (35)... 

While the Schrock and Osborn catalysts worked well in the Rh series, the analogous iridium compounds, also studied by them, did not show any special... 

These bind to the metal via the amide carbonyl just as in directed hydrogenation. This improves the rigidity of the alkene-catalyst complex, which in turn increases the chiral discrimination of the system. As in directed hydrogenation, a 12e catalyst fragment is required, as is indeed formed by hydrogenation of the Schrock-Osborn precatalyst. 

The catalytic activity of cationic rhodium precursors of formula [Rh(diene)(di-phosphine)]+ was also explored by Schrock and Osborn [28]. Halpern and coworkers made very detailed mechanistic studies of olefin hydrogenation by [RhS2(diphos)]+ species (diphos = l,2-bis(diphenylphosphino)ethane S = solvent) [31]. Significant differences have been observed in the reaction of the catalyst precursors [Rh(NBD)(PPh3)2]+ and [Rh(NBD)(diphos)]+ in methanol, as shown in Eqs. (8) and (9) ... 

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... 

Introduction. Homogeneous catalytic hydrogenation with cationic rhodium catalysts has been extensively explored by Schrock and Osborn. Use of these complexes in stereoselective organic synthesis has been a topic of more recent interest, and has been recently reviewed. The reagent of choice for many of these directed hydrogenations has continued to be [Rh(nbd)(dppb)]BF4 (1). 

The discovery of Wilkinson s catalyst led to the development of a new class of complexes capable of promoting hydrogenation these have the general formula I. M+ (M = Rh or Ir). The Rh series was first reported by Schrock and Osborn 81 equation 9.30 demonstrates how such a complex may be prepared. The cationic Rh(I) complex (60), interacts with a solvent such as THF or acetone to give a 12-electron, unsaturated diphosphine intermediate (61), which is considered the active catalyst. The catalytic cycle begins this time with alkene binding, followed by the oxidative addition of H2. 

With experimental support for the metal-carbene-mediated mechanism of olehn metathesis, a number of groups initiated studies with isolated metal-carbene and metallacyclobutane complexes. Early work by Chauvin and Katz on the polymerization of strained olefins using Fischer-type carbenes demonstrated the success of such an approach [56], The introduction of high oxidation state alkylidene complexes led to well-defined catalyst in which the propagating species could be observed and studied, such as the tungsten-based systems developed by Osborn, Schrock, and Basset [59,60], The best-studied and useful of these have been the Schrock arylimido alkylidene complexes, and we will return to these later in this chapter. 

Schrock, R.R., and Osborn, J.A. (1970) Rhodium catalysts for the homogeneous hydrogenation of ketones, J. Chem. Soc., Chem. Comm. 667-668. 

Homogeneous enantioselective hydrogenation of ketones has been first carried out with the ionic complexes of Schrock and Osborn using chiral phosphines like BMPP, EMPP, DIOP, etc., but the optical yields were rather low. " By preparing the catalyst with the same chiral phosphines in situ, the optical selectivity (which is determined in this case by the covalent and ionic complexes present simultaneously) was improved considerably. RhClP2 and RhPa" type intermediates are regarded to be responsible for this effect. 

Asymmetric Catalysis The corresponding RhL2+ catalysts were developed by Schrock and Osborn.Their most important application is asymmetric catalysis.Eq. 9.6 shows how the achiral alkene 9.13 can give two enantiomers 9.14, and 9.15 on hydrogenation. 

The Grubbs group have reported a lot of work on this synthesis in early experiments using the Schrock, W(=CHC (CH3)3)(=NAr)(OC(CH3)(CF3)2)2, and the Osborn, W(=CHC (CH3)3)(OCH2C(CH3)3)2Br2.GaBr3, well-defined catalysts, they were able to demonstrate much better control over the... 

The earliest homogeneous hydrogenation catalyst was Wilkinson catalyst [RhCl(PPh3)3] 1 which was active at 1 atm of H2 pressure at room temperature for monosubstituted and cis-disubstituted alkenes such as cyclohexene. The second-, [Rh(COD)(PPh3)2]PFg 2, and third-, [Ir(COD)(PCy3)(pyr)]PF6 3, generation catalysts were developed by Osborn-Schrock and Crabtree, respectively. 

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