Catalytic reaction, olefin reductions

The C-C coupling reaction between RMgX and R X is considered to proceed though an Ni(R)(R )Lm intermediate, and acceleration of the reductive elimination of R-R by coordination with olefinic or aromatic R X to Ni(R)(R )Lm is necessitated for a smooth catalytic reaction [15,16]. On these bases Ni-pro-moted dehalogenative polycondensation of dihalo organic compounds is suited to the preparation of 7i-conjugated aromatic and olefinic polymers. 

The CD fragment 1s synthesized starting with resolved bicyclic acid 129. Sequential catalytic hydrogenation and reduction with sodium borohydride leads to the reduced hydroxy acid 1. The carboxylic acid function is then converted to the methyl ketone by treatment with methyl-lithium and the alcohol is converted to the mesylate. Elimination of the latter group with base leads to the conjugated olefin 133. Catalytic reduction followed by equilibration of the ketone in base leads to the saturated methyl ketone 134. Treatment of that intermediate with peracid leads to scission of the ketone by Bayer Villiger reaction to afford acetate 135. The t-butyl protecting... 

Several reaction pathways for reaction 1 are possible. A clear reaction mechanism has not been elucidated. Although it is premature to discuss the details of the reaction pathway for this silylation reaction, one possible pathway for the chelation-assisted silylation of C-H bonds is shown in Scheme 2. The catalytic reaction is initiated by oxidative addition of hydrosilane to A. Intermediate B reacts with an olefin to give C. Then, addition of a C-H bond to C leads to intermediate D. Dissociation of alkane from D provides Ru(silyl)(aryl) intermediate E. Reductive elimination making a C-Si bond gives the silylation product and the active catalyst species A is regenerated. Another pathway, addition of a C-H bond to A before addition of hydrosilane to A is also possible. At present, these two pathways cannot be distinguished. 

These catalytic reactions provide a unique pathway for addition of aromatic C-H bonds across C=C bonds. In contrast with Friedel-Crafts catalysts for olefin hydroarylation, the Ru-catalyzed hydrophenylation reactions of a-olefins selectively produce linear alkyl arenes rather than branched products. Although the selectivity is mild, the formation of anti-Markovnikov products is a unique feature of the Ru(II) and Ir(III) catalysts discussed herein. Typically, the preferred route for incorporation of long-chain linear alkyl groups into aromatic substrates is Friedel-Crafts acylation then Clemmensen reduction, and the catalysts described herein provide a more direct route to linear alkyl arenes. 

The discovery by the recent Nobel-laureate, Ryoji Noyori, of asymmetric hydrogenation of simple ketones to alcohols catalyzed by raras-RuCl2[(S)-binap][(S,S)-dpen] (binap = [l,l -binaphthalene-2,2/-diyl-bis(diphenylphosphane)] dpen = diphenylethylenediamine) is remarkable in several respects (91). The reaction is quantitative within hours, gives enantiomeric excesses (ee) up to 99%, shows high chemoselecti-vity for carbonyl over olefin reduction, and the substrate-to-catalyst ratio is >100,000. Moreover, the non-classical metal-ligand bifunctional catalytic cycle is mechanistically novel and involves heterolytic... 

The evidence is in accord with an addition-elimination mechanism (addition of ArPdX followed by elimination of HPdX) in most cases." In the conventionally accepted reaction mechanism," a four-coordinate aryl-Pd(II) intermediate is formed by oxidative addition of the aryl halide to a Pd(0) complex prior to olefin addition. This suggests that cleavage of the dimeric precursor complex, reduction of Pd , and ligand dissociation combine to give a viable catalytic species." If these processes occur on a time scale comparable to that of the catalytic reaction, nonsteady-state catalysis could occur while the active catalyst is forming, and an... 

Complementary to catalytic hydrogenations, the use of diborane provides convenient routes towards the elimination of olefins. However, this reaction has the added advantage of introducing new functionality at the site of olefin reduction. Because of regiochemical considerations, this reaction is particularly useful when applied to exocyclic olefins. Scheme 6.76 shows that a sugar derivative was treated with diborane with an oxidative workup to yield the illustrated product bearing a new hydroxyl group exclusively at the primary center [117]. 

An alternative method of amine activation is opened via the oxidative addition of the N-H bond to an appropriate transition metal in a lower oxidation state. After formation of the /ff-aminoalkyl compound by insertion of the olefin into the transition-metal-nitrogen bond, the alkylamine can be generated by reductive elimination (Scheme 2), and with the reformed reduced transition metal complex the catalytic reaction can run again. 

The Pauson-Khand reaction is a powerful tool for the synthesis of cyclopentanones, 246, from a>-alkenylacetylenes, 245, and carbon monoxide.176 Enyne cyclization has been catalyzed with nitriles using catalytic (77S-CsH5)2Ti(PMe3)2 95177-179 and other variants have since been discovered where the desired cyclopentenones can be directly prepared from the enyne and CO using (77S-CsHs)2Ti(CO)2 68 (Scheme 33) 176,180-184 Addition of PMe3 to the latter reaction mixture has proved to be beneficial. Stoichiometric reactions established that the initial step in the catalytic cycle is reductive coupling of the alkyne and the olefin to form the titanacycle. Carbon monoxide insertion followed by reductive elimination generates the observed product. 

Nevertheless, it must be pointed out that the formation of such transient species has never been spectroscopically observed. Native CDs are effective inverse phase-transfer catalysts for the deoxygenation of allylic alcohols, epoxydation,or oxidation " of olefins, reduction of a,/ -unsaturated acids,a-keto ester,conjugated dienes,or aryl alkyl ketones.Interestingly, chemically modified CDs like the partially 0-methylated CDs show a better catalytic activity than native CDs in numerous reactions such as the Wacker oxidation,hydrogenation of aldehydes,Suzuki cross-coupling reaction, hydroformylation, " or hydrocarboxylation of olefins. Methylated /3-CDs were also used successfully to perform substrate-selective reactions in a two-phase system. 

However, the mechanism of this practically important catalytic reaction remains unclear. The mechanism proposed in early papers [70, 71] for the Wilkinson catalyst involves oxidative addition of a B-H bond to the metal center, followed by olefin coordination to the metal center accompanied with dissociation of one of the two PPh3 s, further followed by migratory insertion of olefin into the M-H bond and subsequent reductive elimination of the B-C bond. However, still unresolved are several important questions (1) whether the reaction occurs with phosphine dissociation or not, (2) which of M-B and M-H bond insertions of olefin is energetically more favorable, and (3) how competitive is the a-bond metathesis pathway involving coordination of the HBcat and olefin to the complex followed by simulta-... 

Another catalytic cycle studied by Matsubara, Morokuma, and coworkers [77] is the hydroformylation of olefin by an Rh(I) complex. Hydroformylation of olefin by the rhodium complex [78-80] is one of the most well known homogeneous catalytic reactions. Despite extensive studies made for this industrially worthwhile reaction [81, 82], the mechanism is still a point of issue. The active catalyst is considered to be RhH(CO)(PPh3)2, 47, as presented in Fig. 25. The most probable reaction cycle undergoes CO addition and phosphine dissociation to generate an active intermediate 41. The intramolecular ethylene insertion, CO insertion, H2 oxidative addition, and aldehyde reductive elimination are followed as shown with the surrounding dashed line. Authors have optimized the structures of nearly all the relevant transition states as well as the intermediates to determine the full potential-... 

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