Hydrogenation, phenyl-substituted

Rl and R2 hydrogen, phenyl, substituted pheiqrl, all l, cycloalkyl, ethylenlc, oleflnlc, thlexQrl, and... 

Pfaltz has recently reported a new class of chiral phosphanodihydrooxazole-iridium catalysts 34 for the enantioselective hydrogenation of imines [38]. Based on Crabtree s success using similar achiral catalysts for the hydrogenation of normally unreactive tri- and tetrasubstituted alkenes [39], Pfaltz has now found that chiral phosphanodihydrooxazole-iridium complexes 34a-g will hydrogenate phenyl-substituted alkenes with high enantioselectivity [40]. As shown in Table 7, the trisubstituted alkene 35 can be hydrogenated under mild conditions (50 bar H2 at 23 C) with superior results using complex 34f (333 1 substrate to catalyst, >99% conversion and 98% ee, entry 6 ). 

Phenyl or mixed alkyl/phenyl substituted silicon hydrides show similar reactivities to trialkylsilanes. Indeed, by replacing one alkyl by a phenyl group the effect on the hydrogen donating ability of SiH moiety increases only slightly. ... 

The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols. The reaction mechanism is reported in Scheme 5 and shows that alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical abstracts hydrogen from the silane. This procedure, which was coined polarity-reversal catalysis, has been applied to dehalogenation, deoxygenation, and desulfurization reactions.For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of triethylsilane in the presence of a catalytic amount of ferf-dodecanethiol. 

Hydrogenation of ( )-l,2-diphenylpropene was chosen as a model reaction to test the potential of these catalysts [Scheme 46]. The best results (99% yield and 98% ee) were obtained with the complex 72a in which R = 1-Ad and R = 2,6-(i-Pr)2C6H3. The corresponding t-butyl-, diphenyhnethyl- and phenyl-substituted oxazohne complexes (72b, 72c and 72d) were foimd to be less effective. The complex 72f obtained with R = 1-Ad and by changing the R group into a cyclohexyl group was inactive. In the case of R = 1-Ad and just... 

The diminished stereospecificity is similar to that noted for hydrogen halide addition to phenyl-substituted alkenes. 

To understand the interdependence of the creation of the two chiral centers relative to each other and to the sulfoxide, monosubstituted vinyl sulfoxides (S)-53 and (S)-54 were prepared and reduced with BH3-THF under the same conditions (Scheme 5.19). Both the 2- and 3-phenyl substituted substrates gave the chiral products 54 and 55 with complete stereo specificities dictated by the configuration of the starting sulfoxides. These results again were unexpected and indicated that both hydrogens were delivered solely directed by the chiral sulfoxide. This was not consistent with the mechanism in which the chirality of the initially formed chiral center at the 3-postion dictates the chirality of the subsequently formed chiral center at the 2-position. 

The effect of structural variation on crystal structure was studied for 12 phenyl-substituted derivatives of 6-amino-2-phenylsulfonylimino-l,2-dihydropyridine [37], The title compound contains both hydrogen bond donor and acceptor sites, and the... 

Various phenyl-substituted ketimines and aldimines react with metallocenes 1 and 2, in a manner that depends on the substituents present [41]. In all cases, elimination of the al-kyne is observed. Complex 2b reacts with PhN=CMePh to give the r 2-complex 64, which is stabilized by an additional pyridine ligand [41a], In the reactions of 1 or 2a with the ketimine HN=CPh2, hydrogen transfer generates complexes 65. Two molecules of the aldimine PhN=CHPh are coupled by 2a to give the cyclic diamido complex 66 [41b]. 

The hydrogenation of 109 over palladium on charcoal in acidic methanol produced (l/ ,7aA)-l,3-diphenyl-5,6,7,7a-tetrahydro-1 //-pyrrolo[ 1,2-r]imidazolc 110 in 61% isolated yield (Equation 14). This reaction was totally unexpected and the authors proposed a mechanism that explains this transformation <2005JOC2368>. The mechanism is shown in Scheme 13. Many of the steps are interchangeable, but the end product is the same. The partial racemization observed could occur by epimerization of the phenyl-substituted carbon adjacent to the imine produced after the cleavage of the bicyclo-adduct. 

Our syntheses of these new materials showed that both hydrogenation and epoxidation occurred exclusively from the exo face of both VN and < >VX. This is consistent with extensive literature precedent for a variety of reactions on norbornenyl double bonds (9). H1 NMR spectroscopy of these materials also revealed an interesting pattern. All phenyl substituted compounds with endo imide rings (< >VN, HYVN, VNE) evidenced signals due to one of the ten aromatic protons (five on each phenyl ring) being shifted to unusually high field (6.4 +. 16). This is consistent with a field effect on a proton of one of... 

The addition reaction of alkyl and/or phenyl substituted silicon hydrides to acetylenes has limitations mainly due to the hydrogen donation step (cf. Scheme 5.1). Reaction (5.17) shows that the replacement of Ph by MesSi group in silanes made the reaction easier, the effect being cumulative. Indeed, the reaction time decreased from 88 h for PhsSiH to 3h for (TMS)3SiH [39], together with the amelioration of yields, and a slightly better cis stereoselectivity. 

---