With Wilkinson’s catalyst

More recently. Baker, Tumas, and co-workers published catalytic hydrogenation reactions in a biphasic reaction mixture consisting of the ionic liquid [BMIM][PFg] and SCCO2 [10]. In the hydrogenation of 1-decene with Wilkinson s catalyst [RhCl(PPh3)3] at 50 °C and 48 bar H2 (total pressure 207 bar), conversion of 98 %... 

The reaction of aldehydes with Wilkinson s catalyst goes through complexes of the form 26 and 27, which have been trapped. The reaction has been shown to give retention of configuration at a chiral and deuterium labeling demonstrates that the reaction is intramolecular RCOD give RD. 

High stereoselectivity was noted with Wilkinson s catalyst in the reaction of arylalkynes with diethoxymethylsilane. Interestingly, the stereoselectivity was dependent on the order of mixing of the reagents and the catalyst. When the alkyne was added to a mixture of catalyst and silane, the Z-isomer was formed. Reversing the order and adding the silane to an alkyne-catalyst mixture led to formation of the -product.78... 

Florner noted inhibition by ethers, MeN02, malonate esters and DMF in the hydrogenation of cyclohexene with Wilkinson s catalyst. Almost complete inhibi-... 

Methyl deoxypodocarpate 127 (Scheme 1) 129) represents a simple problem since the ketone 132 is well-known and readily available from Hagemann s ester in three steps. The problem of geminal alkylation of this ketone stems from its existence as an EjZ mixture of ring fusion isomers. Recognizing that decarbonylation of aldehydes occurs readily with Wilkinson s catalyst creates a structural equivalence of an acetaldehyde chain and a methyl group as in 128. This simple relationship immediately establishes several options, a simple one uses a thioacetal such as 129 as a synthon for the aldehyde. The presence of a carbonyl group three carbons away... 

The mechanism for the addition of 50 to anthraquinone (Scheme 52), 1,4-diacetylbenzene (Scheme 53), and 1,2-diacetylbenzene (Scheme 54) has been proposed.326 The addition of 50 to benzophenone followed by reaction with Wilkinson s catalyst formally results in the hydrogenation of a double bond (Equation (262)).325 This species also undergoes stereospecific inertion into the vinyl chloride bond of various halogenated alkenes with high yields in most cases (Equations (263)-(267)).329... 

In efforts to decarbonylate the allenylaldehyde 87 with Wilkinson s catalyst, Marshall and Robinson observed the formation of the furan 88 rather than the desired hydrocarbon, the allene 89 (Scheme 15.21) [46]. 

The alkynylphosphine (56) reacts with Wilkinson s catalyst to give an intermediate rhodium complex, which, when treated with diphenylacetylene followed by cyanide ion, yields the diphosphine (57), of interest as a rigid chelating ligand of fixed geometry.47... 

Initial attempts at carbocydization of the enyne 82 with Wilkinson s catalyst under an atmosphere of 1,3-butadiene in refluxing toluene furnished only trace amounts of product, which was attributed to the preference for the enyne to undergo homodimerization in a highly stereoselective manner (Tab. 12.8, entry 1 ds 19 1) [38]. Nonetheless, the coordinatively unsaturated (AgOTf-modified) catalyst furnished the [4-i-2-1-2] product in excellent yield (entry 2). The ability to alter the product distribution in this manner prompted further examination of various other silver salts. 

Substitution of the VCP is tolerated both on and adjacent to the cyclopropane ring. Diester-substituted and heteroatom (O, NTs) tethers are well tolerated. Reactions were conducted with 2-10 mol% catalyst at up to 0.20 M, as illustrated. Most importantly, reactions with the naphthalene catalyst were found to be more rapid than those with other catalysts. For example substrate 54 is readily converted in >99% yield to cycloadduct 55 in only 15 min at room temperature (entry 1). Complex 93 efficiently catalyzes the reactions of both alkynes and alkenes with VCPs, offering greater generahty than thus far observed with non-rhodium catalysts. This catalyst is particularly advantageous in the cases of substrates 100 and 102, for which the desired product is not formed cleanly with Wilkinson s catalyst due to product isomerization. 

With an eye toward increasing efficiency and eliminating the atom-uneconomical solvent waste stream involved in most organic reactions, a reusable, water-soluble catalyst, using bidentate phosphine 107 as a ligand, has been developed [36]. The catalyst is prepared by treatment of [RhCl(nbd)]2 (nbd=norbornadiene) with AgSbFs in acetone, followed by introduction of the phosphine ligand [37]. In the presence of 10 mol% catalyst in water/methanol (1 1) at a catalyst concentration of 2.0 mM, 108 reacted efficiently at 70°C to provide cycloadduct 109 after 12 h in 91% yield (GC analysis Tab. 13.8). Notably, the yield and rate compare favorably to results obtained with Wilkinson s catalyst... 

Ethylene is commonly chosen to illustrate homogeneous hydrogenation with Wilkinson s catalyst, but the process is actually very slow with this aJkene. The explanation lies with the formation of a stable rhodium ethylene complex, which does not readily undergo reaction with H,. Ethylene competes effectively with the solvent for the vacant coordination site created when triphcnylphosphinc dissociates from Wilkinson s catalyst and thus serves as an inhibitor to hydrogenation. 

The reactions involved in hydrogenation with Wilkinson s catalyst thus can be represented as follows (L = Ph,P. S - solvent molecule). ... 

The dispersions were obtained by emulsification via ultrasonication of a toluene solution of the unsaturated homopolymer in an aqueous surfactant solution. This was followed by exhaustive hydrogenation with Wilkinson s catalyst at 60°C and 80 bar H2 to produce a dispersion with an average particle size of 35 nm (dynamic light scattering and transmission electron microscopy analyses). The same a,co-diene was used as comonomer in the ADMET polymerization of a phosphorus-based monomer, also containing two 10-undecenoic acid moieties... 

The chemistry of the 2-vinyl-l,3-thiazetidines 98 is interesting because these compounds undergo rearrangement to the thiazolidines 99 on catalytic hydrogenation and rearrangement to the thiazines 100 on treatment with Wilkinson s catalyst (Scheme 29) <1999J(P1)3569>. 

An orf/io-directed lithiation allows the conversion of 25 to aryl iodide 40. Reductive ether formation of aldehyde 40 with crotyl alcohol yields compound 41. Intramolecular Heck reaction of 41 affords a mixture of the olefins 42 and 43. The undesired alkene 42 can be isomer-ized quantitatively to the desired enol ether 43 with Wilkinson s catalyst. Sharpless dihydroxylation ee 94 %) of the enol ether 43 provides lactol 44, which is oxidized directly to lactone 45. Finally, the pyridone-O-methyl ester is cleaved under acid conditions (45 — 7). 

Treatment of 3,3 -bis(phenylethynyl)-2,2 -bithiophenes 508 with Wilkinson s catalyst yields tbe cyclic rhodium complexes 509 wbicb react with elemental sulfur to give benzotritbiopbenes 510 (Scheme 82) <199781027,1997HCA111>. 

This is even better seen, when the exocyclic thione sulfur atoms of the hgand are replaced by oxygen. Subsequent reaction with Wilkinson s catalyst [RhCPPhjljCl] reveals the metaUophobic character of the carbonyl oxygen and the metallophihcity of the sulfur. This culminates in the failure to substitute the endocychc sulfur and have it coordinate to rhodiumflU) instead (see Figure 4.94) [282]. The reaction proceeds with loss of phenyl isocyanide, the reverse reaction to the synthesis of this lO-S-3 compound [276]. 

Example 8.9. Olefin hydrogenation with Wilkinson s catalyst. Wilkinson s catalyst is a dihydrido-chloro-phosphino complex of rhodium, H2RhClPh3, where Ph is an organic phosphine such as triphenyl phosphine [48-52]. The dominant mechanism of olefin hydrogenation with this catalyst, established chiefly by Halpem [53-55] in detailed studies that included measurements of equilibria in the absence of reactants and of reaction rates of isolated participants, backed by independent NMR studies [56] and ab initio molecular orbital calculations [57], is shown as 8.69 on the facing page (without minor parallel pathways and side reactions). 

Examples include acetal hydrolysis, base-catalyzed aldol condensation, olefin hydroformylation catalyzed by phosphine-substituted cobalt hydrocarbonyls, phosphate transfer in biological systems, enzymatic transamination, adiponitrile synthesis via hydrocyanation, olefin hydrogenation with Wilkinson s catalyst, and osmium tetroxide-catalyzed asymmetric dihydroxylation of olefins. 

Enantiomerically pure carboxylic acids are routinely obtained from N-acylsultams by Hydrogen Peroxide assisted saponification with Lithium Hydroxide in aqueous THF. 4 Alternatively, transesterification can be effected under neutral conditions in allyl alcohol containing Titanium Tetraisopropoxide, giving the corresponding allyl esters which can be isomerized/hydrolyzed with Wilkinson s catalyst (Chlorotris(triphenylphosphine)rhodium(I)) in Et0H-H20. This provides a convenient route to carboxylic acids containing base-sensitive functionality. Primary alcohols are obtained by treatment with L-Selectride (Lithium Tri-s-butylborohydride) in THF at ambient temperature. ... 

The reaction of aldehydes with Wilkinson s catalyst goes through complexes... 

Scheme 9.9 depicts what most consider to be the mechanism for hydrogenation with Wilkinson s catalyst. The inner cycle of reactions (surrounded by the diamond-shaped dotted-line box) represents the key catalytic steps based on the work of Halpem69 and Tolman.70 They showed that compounds 52 to 55, while detectable and isolable, were not responsible for the actual catalytic process and, moreover, the buildup of these intermediates during hydrogenation may even slow down the overall reaction.71 This work provided a valuable lesson for chemists trying to determine a catalytic mechanism—compounds that are readily isolable are probably not true intermediates. Through careful kinetic and spectroscopic... 

This reaction competes with intramolecular hydroacylation of pent-4-enals to form cyclopen-tanones. In the case of exo- and ent/o-norborn-5-ene-2-carboxaldehyde (4) if treated with Wilkinson s catalyst [tris(triphenylphosphane)rhodium(I) chloride] only decarbonylation occurs. While the exo-aldehyde exo-4 leads to norbornadiene (5), the e fi o-aldehyde endo-4 reacts to form nortricyclene (6 tricyclo[2.2.1.0 ]heptane). These results support organometallic pathways and exclude radical intermediates, since here identical products should be formed. 

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