Olefins transfer hydrogenation

Precursor of Useful Chiral Ligands. OPEN is widely used for the preparation of chiral ligands. Organometallic compounds with these ligands act as useful reagents or catalysts in asymmetric induction reactions such as dihydroxylation of olefins, transfer hydrogenation of ketones and imines, Diels-Alder and aldol reactions, desymmetrization of meso-diols to produce chiral oxazolidinones, epoxidation of simple olefins, benzylic hydroxylation, and borohydride reduction of ketones, imines, and a,p-unsaturated carboxylates. ... 

Jiang Y, Berke H (2007) Dehydrocoupling of dimethylamine-b[Pg.225]

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

In hydrogen-transfer hydrogenations, various olefinic hydrogen donors are not necessarily equivalent, neither in selectivity nor in rate. The point is illustrated by selected data of Tabor et al. 97) on the transfer hydrogenation of dimethyl bicyclo[2.2,l]heptane-2.5-diene-2,3-dicarboxylate. 

In the reaction of two olefins, both olefins must be adsorbed on active sites that are close together. One of these olefins becomes a paraffin and the other becomes a cyclo-olefin as hydrogen is moved from one to the other. Cyclo-olefin is now hydrogen transferred with another olefin to yield a paraffin and a cyclodi-olefin. Cyclodi-olefin will then rearrange to form an aromatic. The chain ends because aromatics are extremely stable. Hydrogen transfer of olefins converts them to paraffins and aromatics (Equation 4-11). 

Apart from the Meerwin-Ponndorf-Verley (MPV) reaction,16 18catalytic asymmetric transfer hydrogenation has remained quite primitive,111,112 with successful examples of reduction of activated olefins, using alcohols or formic acid as hydrogen source, being reported only recently.113,114... 

Besides the mirror and addition reactions already discussed, gas phase radicals dimerize, disproportionate, transfer hydrogen, and polymerize olefins. Similar reactions in the liquid phase are an indication (but not proof) of free radical intermediates. 

Directed intramolecular transfer hydrogenations are catalyzed by rhodium complexes with the pendant alkene acting as an internal sacrificial olefin (Equation (39)). 

Scheme 15 Iridium-catalyzed couplings of 1,3-cyclohexadiene under transfer hydrogenation conditions (ratio refers to 1,4-oIefinic versus 1,5-olefinic alcohols)...

A primary alcohol and amines can be used as an aldehyde precursor, because it can be oxidized by transfer hydrogenation. For example, the reaction of benzyl alcohol with excess olefin afforded the corresponding ketone in good yield in the presence of Rh complex and 2-amino-4-picoline [18]. Similarly, primary amines, which were transformed into imines by dehydrogenation, were also employed as a substrate instead of aldehydes [19]. Although various terminal olefins, alkynes [20], and even dienes [21] have been commonly used as a reaction partner in hydroiminoacylation reactions, internal olefins were ineffective. Recently, methyl sulfide-substituted aldehydes were successfully applied to the intermolecu-lar hydroacylation reaction [22], Also in the intramolecular hydroacylation, extension of substrates such as cyclopropane-substituted 4-enal [23], 4-alkynal [24], and 4,6-dienal [25] has been developed (Table 1). 

A Diels-Alder approach to varenicline was recently published by Dr. Reddy s Laboratories. Entry to a key bicyclic intermediate is achieved by an iodide-catalyzed Diels-Alder reaction of tetrabromo dimethyl pyrazine (47) with excess norbomadiene. Dihydroxylation of 48, oxidative cleavage, and reductive amination prepares N-p-methoxybenzyl varenicline (50), which is deprotected under transfer hydrogenation conditions to give varenicline (1) in 10% yield for the sequence.47 This approach continues the theme of building the piperidine of 1 through olefin oxidative cleavage and reductive amination, but by doing so late in the sequence however, the approach... 

In a first model reaction, Danopoulos et al. [472] reacted a free pincer carbene ligand with [RuCPPhjljCl ] and obtained the corresponding octahedral pincer carbene adduct (see Figure 3.156). The complex lacks the yhdene functionality necessary for activity of the complex in olefin metathesis. Instead, the compound was successfully employed in the transfer hydrogenation of cyclohexanone, acetophenone and benzylidene anihne. Reaction temperatures were mostly low to moderate (25-55 °C) and catalyst loadings in the range of 0.015 to 0.1% with TONs of only 150 to 8800. 

Grubbs reported tandem olefin metathesis/hydrogenation as a means to make satnrated cyclic prodncts and a one-pot RCM/transfer dehydrogenation/hydrogenation ronte to (R)-(-)-Mnscone utilizing (4a) (Scheme 19). Snapper and coworkers took advantage of the isomerization side reaction... 

Equilibrium protonation to give the carbonium ion can be observed in 80 % (v/v) methanol—water containing buffers. Proton removal from the carbonium ion (pX ca. 2.7 in this solvent) by acetate and chloroacetate is thermodynamically favourable but occurs with rate coefficients of 19.1 and 4.7 1 mole-1 sec-1, respectively, which are well below the values which would be found for normal proton transfer. Protonation of the olefin by hydrogen ion is thermodynamically favourable but occurs slowly with rate coefficient 23 1 mole-1 sec-1. These results clearly show that protonation of olefinic carbon belongs to the category of slow proton transfers. 

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