Alkene metathesis/hydrogenation

Hess GD, Hampel F, Gladysz JA (2007) Octahedral gyroscope-like molecules with m(CO)3(X) rotaUns encased in three-spoked diphosphine stators syntheses by alkene metathesis/hydrogenation sequences, structures, dynamic properties, and reactivities. Oiganometallics 26 5129-5131... 

Carbenes are both reactive intermediates and ligands in catalysis. They occur as intermediates in the alkene metathesis reaction (Chapter 16) and the cyclopropanation of alkenes. As intermediates they carry hydrogen and carbon substituents and belong therefore to the class of Schrock carbenes. As ligands they contain nitrogen substituents and are clearly Fischer carbenes. They have received a great deal of attention in the last decade as ligands in catalytic metal complexes [58], but the structural motive was already explored in the early seventies [59],... 

Two tandem alkene metathesis-oxidation procedures using Grubb s second-generation ruthenium catalyst resulted in unique functional group transformations. Use of sodium periodate and cerium(III) chloride, in acetonitrile-water, furnished cis-diols. Oxidation with Oxone, in the presence of sodium hydrogencarbonate, yielded a-hydroxy ketones.296 Secondary alcohols are oxidized to ketones by a hydrogen... 

With the advances in pro-catalyst design that have been witnessed over the last decade or so, the transition-metal-catalysed alkene metathesis reaction has now become a practical procedure that can be utilised by the chemist at the bench. Undeniably, this has added a new dimension to the repertoire of synthetic organic chemistry as it facilitates disconnections that, pre-metathesis, simply would not have been considered. Take, for example, a macro-cyclic amide where the normal disconnection would be at the amide. Now, with the ready reduction of alkenes to alkanes, a ring-closing diene metathesis (RCM), followed by hydrogenation, becomes an alternative disconnection. And, when one considers that any of the C—C linkages could be established in such a manner, the power of the RCM disconnection becomes obvious. 

The elimination of a-hydrogen is not general and observed only with limited numbers of metal complexes. The elimination of a-hydrogen from the methyl group in the dimethylmetal complex 68 generates the metal hydride 69 and a carbene that coordinates to the metal. Liberation of methane by the reductive elimination generates the carbene complex 70. Formation of carbene complexes of Mo and Wis a key step in alkene metathesis. The a-elimination is similar to the 1,2-hydride shift observed in organic reactions. 

At present, Mo, W, Re and Ru complexes are known to catalyse alkene metathesis [7]. This unique reaction, catalysed by transition metal complexes, is impossible to achieve by other means. Later, based on studies of the reactivities of Fischer-type carbene complexes, it was discovered that carbene complexes are the intermediates in alkene metatheses. WClg reacts with EtAlCl2 to afford the diethyltungsten complex 3 by transmetallation, and subsequent elimination of a-hydrogen generates ethane and the carbene complex 4 which is the active catalyst. 

A major drawback of alkene metathesis is lack of control over the stereochemistry of the newly formed double bond. For unstrained systems, E/Z ratios are virtually unpredictable. Alkyne metathesis, on the other hand, can always be combined with subsequent Lindlar hydrogenation, thereby giving access to stereochemically pure 2-olefins. In 1998, Ftirstner and Seidel were the first to report a ring-closing alkyne metathesis [7]. Under high-dilution conditions (0.02 m) and reduced pressure (20 mbar, removal of 2-butyne, solvent 1,2,4-trichlorobenzene (b.p. 214 °C)) the Schrock catalyst was applied to assemble macrocydic... 

Sophorolipid lactone (34) was obtained from an open-chain bisalkyne precursor (concomitant formation of 2-butyne) in a macrocyclization yield of 78% [29], The epothilones have been obtained through non-stereoselective alkene metathesis [26d], making them worthwhile targets for the combination alkyne metathesis/Lindlar hydrogenation to assemble the C12-C13 Z double bond. Furthermore, epothilone C (35) shows the most complex array of functional groups among the substrates of alkyne metathesis. The macrocyclization yield, starting from the expected OTBS-protected precursor (not shown) was 81% [23, 30]. Neither... 

The first attempt seems to have been promoted by binuclear ruthenium alkene metathesis catalyst for the ROMP of cyclooctene followed by hydrogenation (50 °C, 30 psi H2) in the presence of 10 equiv. of triethylamine [72] (Scheme 31). It was motivated by the observed capability of [RuCl2(arene)]2... 

A similar approach was performed using the alkene metathesis catalyst RuCl2(= CHPh)(PCy3)2 [76]. It was known that this complex reacts with hydrogen in THF to give ruthenium hydride complexes capable of catalytic alkene hydrogenation [77] (Scheme 32). 

The ruthenium catalyst RuCl2(= CHPh)(PCy3)2 is able to promote both alkene metathesis polymerization (ROMP) and atom transfer polymerization (ATRP) [80,81]. The bifunctional catalyst A was designed to promote both ROMP of cyclooctadiene (COD) and ATRP of methyl methacrylate (MMA). Thus, catalyst A was employed to perform both polymerizations in one pot leading to diblock polybutadiene/polymethylmethacrylate copolymer (58-82% yield, PDI = 1.5). After polymerization the reaction vessel was exposed to hydrogen (150 psi, 65 °C, 8h), under conditions for Ru(H2)(H)Cl(PCy3)2 to be produced, and the hydrogenation of diblock copolymer could attain 95% [82] (Scheme 36). 

A better hydrogenation catalyst was generated by reaction of alkene metathesis ruthenium catalyst, with sodium hydride, after the RCM reaction was performed. In that case, hydrogenation can be performed under 1 bar of H2 at 20 °C [86]. Thus, cyclopentanols can be selectively prepared in one pot by RCM of the parents dienes, followed by addition of NaH and hydrogenation [86] (Scheme 40). 

Figure 7 Chen s [4]catenane 32-153 in which the guests were converted to a macrocycle via alkene metathesis through-ring and top views. Hydrogen atoms and counterions are omitted.

The photochemical reactions of arenecarboxylic acid esters with alkenes has received recent attention by Cantrell. - For example, irradiation of 2,3-dimethyl-2-butene and methyl benzoate gave a mixture of alkoxyoxetane (56), carbonyl-alkene metathesis product (57) and ketone (58), resulting from alkoxy radical allylic hydrogen abstraction and radical recombination. Such alkoxyoxetane photoproducts are... 

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