RhCl j

Rhodium(I) and ruthenium(II) complexes containing NHCs have been applied in hydrosilylation reactions with alkenes, alkynes, and ketones. Rhodium(I) complexes with imidazolidin-2-ylidene ligands such as [RhCl( j -cod)(NHC)], [RhCl(PPh3)2(NHC)], and [RhCl(CO)(PPh3)(NHC)] have been reported to lead to highly selective anti-Markovnikov addition of silanes to terminal olefins [Eq. 

The standard preparation of tra j-[Rhpy4Cl2] is due to Delepine, who noticed that reaction of pyridine with [RhCl j leads to mixtures of [RhpyjClj] isomers, but that in the presence of primary or secondary alcohols, trflni-[Rhpy4Cl2]Cl is formed. (The role of the alcohol in this reaction is discussed in more detail in Section 48.6.2.5.iii.) Far IR spectra (pyridine ring vibrations, Rh—N and Rh—Cl stretches) of the fac and mer isomers of [Rhpy3Cl3] confirmed the earlier isomer assignment. ... 

The extent of double-bond isomerization over homogeneous catalysts is influenced by choice of solvent. Saturation of the double bond in 4-(4-me-thoxyphenyl)-3-(2 nitro-4-methoxyphenyl)-l-pentene was achieved smoothly by reduction over RhCl(Ph3P)j in benzene wiihout any hydrogenation of the nitro function. If the solvent were benzene-ethanol, isomerization of the double bond to a conjugated position also occurred ihis styryl bond was inert to reduction under these conditions (77). 

Figure 2.6 The 3IP NMR spectrum of RhCl(PPh3)3 at 30°C in C6H6 solution. (Reprinted with permission from J. Am. Chem. Soc., 1972,94, 340. Copyright (1972) American Chemical Society.)...

The Rh-Rh distance is 3.12 A, long compared with Rh-Rh single bonds (2.624A in Rh2(MeCN) J([, 2.73 A in Rh4(CO)12) there is a weaker (3.31 A) intermolecular attraction. Dipole moment and IR studies indicate that the structure is retained in solution and is, therefore, a consequence of electronic rather than solid-state packing effects. Furthermore, it is found for some other (but not all) [RhCl(alkene)2]2 and [RhCl(CO)(PR3)]2 systems. SCF MO calculations indicate that bending favours a Rh-Cl bonding interaction which also includes a contribution from Rh—Rh bonding [56b]. 

Some other enantioselective approaches have been attempted, still with moderate enantioselectivities, by making use of in situ systems containing a chiral NHC precursor. Luo and co-workers reported on the use of the bidentate chiral imidazo-lium salt 16, derived from L-proUne, in combination with [RhCia-COCcod)], leading to an enantiometic excess of around 20% [30]. The use of chiral imidazolium salt 17 in combination with [RhCl(CH2=CHj)j]j by Aoyama afforded slightly better ee (Fig. 7.3) [31 ]. So far, Bohn and co-workers have obtained the best enantioselectivities (up to 38% ee) for the catalytic addition of phenylboronic acid to aromatic aldehydes by using planar chiral imidazolium salts 18, derived from paracyclophane, in combination with [Rh(OAc)2]2 [32]. 

The hydrogenation of ketones with O or N functions in the a- or / -position is accomplished by several rhodium compounds [46 a, b, e, g, i, j, m, 56], Many of these examples have been applied in the synthesis of biologically active chiral products [59]. One of the first examples was the asymmetric synthesis of pantothenic acid, a member of the B complex vitamins and an important constituent of coenzyme A. Ojima et al. first described this synthesis in 1978, the most significant step being the enantioselective reduction of a cyclic a-keto ester, dihydro-4,4-dimethyl-2,3-furandione, to D-(-)-pantoyl lactone. A rhodium complex derived from [RhCl(COD)]2 and the chiral pyrrolidino diphosphine, (2S,4S)-N-tert-butoxy-carbonyl-4-diphenylphosphino-2-diphenylphosphinomethyl-pyrrolidine ((S, S) -... 

Direct heterolytic activation of hydrogen by RhCl(CO)(PPh3)2 has been suggested, but likely involves an intermediate dihydride D. Evans, J.A. Osborn, G. Wilkinson, /. Chem. Soc. A 1968, 3133. 

Figure 3.1. Variation of tho hydrogenation rate of maleic (a) and fumaric (b) acids in water-diglyme mixtures as a function of the solvent composition. 0.001 M [RhCl(TPPMS)3], 0.05 M substrate, 60 °C, K, I bar total pressure. Reprinted with permission from J, Chem. Sac., Chem. Commun. 1993, 1602. Copyright (1993) American Chemical Society.

Figure 3.2b, The effect of pH on the initial rate of hydrogenation of fumaric acid (FA) catalyzed by [RhCl(TPPMS>3] in aqueous solution. [Rh]=5.2 x lO" M, [FA1=0.05 M, 60 C, Hi, 1 bar total pressure. The calculated distribution (a%) of nondissociated (H A) and dissociated (HA , fumaric acid is also shown. Reprinted with permission from Chem. Eur. J. 2001, 7, 193. Copyright (2001) Wiley-VCH Verlag GmbH.

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