Rhodium catalysts neutral

Unconjugated olefins and acetylenes are rapidly hydrogenated in a ds manner in the presence of V fiUdnson s catalyst. These reactions typically occur at ambient hydrogen pressure and temperature in benzene when the reaction is conducted with a polar co-solvent, such as ethanol. These polar co-solvents might facilitate migratory insertion, which is the turnover-limiting step of the catalytic cycle presented later in this chapter. 

Relative reactivities of alkenes for hydrogenation by Wilkinson s catalyst. 

Wilkinson s catalyst undergoes the hydrogenation of alkenes without isotopic scrambling between Hj and and without isotopic scrambling of Dj and protons in the solvent or on the olefin substrate. Under carefully controlled conditions, remarkably clean cis addition of hydrogen (or deuterium) is achieved. This lack of scrambling implies that the mechanism involves a dihydride intermediate in which both hydrides are transferred to the same alkene and that the mechanism involves a migratory insertion in which the metal and the hydride add in a cis fashion across the alkene, as discussed in Chapter 9. 

Wilkinson s catalyst is frequently used to hydrogenate olefins in the presence of additional functional groups. The selectivity for olefin reduction in the presence of esters, ketones, and nitroarenes, and for reduction in the absence of olefin isomerization make this catalyst superior in many cases to heterogeneous catalysts. A few of the early applications of this catalyst are presented here. 

Addition of deuterium to ergosterol acetate was shown to form the 5a,6a-dideuterio reduction product under mild conditions (Equation 15.4). This result illustrates a number of features of the reactivity of Wilkinson s catalyst. First, the conjugated diene is reduced in preference to the C21-C22 double bond. Second, the diene is reduced from the less-crowded a-face of the steroid. Neither the remaining trisubstituted double bond nor the trans disubstituted A double bond are affected. Finally, no isotopic exchange takes place during this reaction. This selectivity has been used to specifically label prostaglandins with tritium (Equation 15.5). The sensitive 3-keto alcohol is unperturbed. 

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The differences in the steric effect between catecholborane and pinacolborane, and the valence effect between a cationic or neutral rhodium complex reverse the re-gioselechvity for fluoroalkenes (Scheme 1-4) [26]. The reaction affords one of two possible isomers with excellent regioselectivity by selecting borane and the catalyst appropriately, whereas the uncatalyzed reaction of 9-BBN or SiaiBH failed to yield the hydroboration products because of the low nucleophilicity of fluoroalkenes. The regiochemical preference is consistent with the selectivity that is observed in the hydroboration of styrene. Thus, the internal products are selectively obtained when using a cationic rhodium and small catecholborane while bulky pinacolborane yields terminal products in the presence of a neutral rhodium catalyst. 

Some neutral rhodium catalysts with chiral ligands, such as MCCPM 9 (see Scheme 33.3) [20c], Cy,Cy-oxoProNOP 15, and Cp,Cp-IndoNOP 18, demonstrate excellent enantioselectivities and reactivities in the hydrogenation of a-ketoesters and ketoamides indeed, they compare well with ruthenium-based catalysts (Table 33.2). Togni et al. have successfully used the Josiphos 47 ligand for the hydrogenation of ethyl acetoacetate [27], while the use of MannOPs has led to somewhat high enantioselectivities [18]. 

For rhodium-catalyzed conjugate addition using organosilanes, several other conditions have been reported [34]. Cationic rhodium catalysts such as [Rh(COD)2]BF4 and ]Rh(COD)(MeCN)2]BF4 are more active than neutral rhodium catalysts such as ]Rh(OH)(COD)]2. 

The complexes may be applied to some very demanding and complex transformations as illustrated by a key step in the synthesis of Bafilomycin A1 (Scheme 9), in which a neutral rhodium catalyst was employed69. In contrast, a cationic iridium-based complex was the catalyst of choice in an exacting selective homoallylic alcohol reduction to achieve a vital inversion of configuration in the synthesis of Brevetoxin B70. Functionalized alkenes may also be effectively reduced, significantly trialkylstannanes which... 

Charged rhodium catalysts particularly in methanol were less active than neutral rhodium catalysts in toluene. 

One application of this hydrogenation with the structurally related 4,4-dimethyl-2, 3-furandione is shown in Equation 15.53. This a-ketoester undergoes hydrogenation with a neutral rhodium catalyst containing the bpm ligand shown in this equation with spectacularly high turnover numbers and good enantioselectivity. This product is used by Roche in the synthesis of pantothenic acid, a B-vitamin used for the synthesis of coenzyme... 

Takeuchi has shown that the stereoselectivity of alkyne hydrosilylation can be controlled by both the ligand and the solvent (Equation 16.31). addition of the Si-H bond across the alkyne has been observed for the hydrosilylation of hexyne by triethyl-silane in ethanol solvent in the presence of a catalyst generated from the combination of [Rh(COD),] " and PPhj. In contrast, trans addition occurs for the same reaction in the same medium with a neutral rhodium catalyst lacking added phosphine. Trans addition also occurs in the presence of Wilkinson s catalyst in toluene. (Equation 16.32). As discussed in Section 16.3.5.2.4 below, trans addition likely results from the isomerization of a vinyl intermediate. -... 

Hydroboration of styrene derivatives has been extensively studied, and perhaps these are the best substrates to consider in a discussion of the efficiency and selectivity of the catalysts (Table 1-1). A neutral rhodium-phosphine complex... 

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

Menthone and camphor undergo asymmetric hydrosilylation to give alkoxysilanes with up to 82% optical purity using neutral rhodium(I) catalysts containing DIOP or neomenthyl- or menthyl-diphenylphos-phine even triphenylphosphine gave about 65% ee (300). Hydrolysis to alcohols was not reported. The ferrocenyl ligands (28, 29) are similarly effective for asymmetric hydrosilylation (255), and could be used for production of the optically active alcohols. 

In less-coordinating solvents such as dichloromethane or benzene, most of the cationic rhodium catalysts [Rh(nbd)(PR3)n]+A (19) are less effective as alkyne hydrogenation catalysts [21, 27]. However, in such solvents, a few related cationic and neutral rhodium complexes can efficiently hydrogenate 1-alkynes to the corresponding alkene [27-29]. A kinetic study revealed that a different mechanism operates in dichloromethane, since the rate law for the hydrogenation of phenyl acetylene by [Rh(nbd)(PPh3)2]+BF4 is given by r=k[catalyst][alkyne][pH2]2 [29]. 

One last remark concerning the two catalysts we have discussed in more detail, cationic rhodium catalysts and the neutral chloride catalyst of Wilkinson. The difference of the catalytic system discussed above from that of the Wilkinson catalyst lies in the sequence of the oxidative addition and the alkene complexation. The hydrogenation of the cinnamic acid derivative involves a cationic catalyst that first forms the alkene complex the intermediate alkene (enamide) complex can be observed spectroscopically. 

Carbonyl groups are not reduced with classical Wilkinson catalysts. However, some cationic rhodium complexes show catalytic activity 52K There are only a few examples of asymmetric hydrogenation of ketones. Addition of base to a neutral rhodium complex is also a way to produce a catalyst for ketone reduction 44). Acetophenone... 

Homogeneous hydrogenation in the fluorous phase has been so far reported only for a limited set of simple olefins (Richter et al., 1999, Rutherford et al., 1998), as exemplified with the neutral rhodium phosphine complex 18 as catalyst precursor (eq. 5.7). Isomerization of the substrate 1-dodecene (17a) was observed as a competing side reaction under the reaction conditions. The catalyst formed from 18 could be recycled using a typical FBS protocol, but deactivation under formation of metal deposits limited the catalyst lifetime. 

Catalytic hydroboration of perfluoroalkenes 68 with catecholborane provides either terminal 69 or internal alcohols 70 regioselectively <19990L1399>. The regioselectivity is controlled by a judicious choice of catalyst. The anti-Markovnikov alcohol can be obtained with very high selectivity by using cationic rhodium catalysts such as Rh(COD)(DPPB)+BF4, while neutral Rh catalysts such as Wilkinson s catalyst provide the Markovnikov product (COD = cyclooctadiene Equation 3) <19990L1399>. 

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