Monoenes, alkenes

Consequently, it is I ICo(CN)s3 that functions as a catalyst in hydrogenation processes. In the first step of the process shown in Figure 22.9, the alkene coordinates to HCo(CN)s3 as one hydrogen atom is added to the molecule so that only one double bond remains. The monoene is bonded to the cobalt in rf fashion. In the second step, another HCo(CN)53- transfers hydrogen to the alkene, which undergoes reductive elimination and leaves, having been converted to 1-butene. 

Ruthenium complexes are active hydrogenation catalysts for the reduction of dienes to monoenes. Both zerovalent and divalent ruthenium complexes containing various (alkene, diene and phosphine) ligands have been employed as catalysts that have met with different degrees of success. 

Conjugated dienes (such as 1,3-cyclohexadiene, cyclopentadiene, 2,4-hexadienoic-sorbic-acid) and polyenes can be selectively hydrogenated to monoenes unactivated alkenes are totally unreactive [20]. Unfortunately, the possibilities for modification of the catalyst by ligand alteration or by the use of additives are very limited [50, 51]. 

Cyclic monoenes and alkenes with chelating heteroatoms readily form complexes with platinum(II). The structure of Pt2Cl2(/ -Cl)2(cyclopentene)2 shows only a slight elongation of the C=C double bond on coordination.674... 

A different class of catalyst is exemplified by HCo(CN)3-, formed by homolytic activation of H2 by Co(CN)5 ion. These catalysts are particularly effective for reducing activated alkenes, for example XCH=CH2 to XEt (where X = C02H, C02R, Ph, CN) and 1,3-dienes to monoenes. Radical intermediates have been implicated in these reactions.156... 

The pentacyanocobaltate(II) ion has long been known to catalyze alkene hydrogenation, mainly of conjugated dienes. A review of the early work is available.45 The catalyst system shows negligible activity for the hydrogenation of non-activated monoenes. A major disadvantage is that the system is inhibited by excess substrate, and the turnover numbers obtained are generally less than 2. 

In DMF as solvent, PdCl2 or [PdCl2(DMF)2] will catalyze the hydrogenation of conjugated dienes and alkynes to alkenes.194 The above systems quite generally give selective reduction of dienes to monoenes. 

Morpholine, piperidine and diethylamine were used as bases. The catalyst precursor was [Pd(OAc)2] (106) and P(o-tol)3 or a similar phosphine. The mechanism is thought to involve vinylation of the alkene as in Scheme 74 as the initial process. This leads to the formation of an i73-allylpalladium derivative, which is then trapped by the amine to give the final product. If dienes are used instead of monoenes, 2,5-dienylamines are produced (equation 193).651... 

Chatancin, via ring-closing metathesis, 11, 252 Chelate coordination, monoenes, in Pd(II) complexes, 8, 331 Chelated alkenes, with iron, 6, 133 Chelated allyl complexes, with iron, 6, 140 Chelating aryloxides, with Zr(IV), 4, 784 Chelating bis(amido) complexes, with Zr(IV), 4, 767 Chelating bis(amido) phosphine-donor complexes, with Zr(IV), 4, 816... 

It has been shown that toxic carbon tetrachloride can be replaced by ethyl acetate in the ruthenium-catalysed oxidation of alkenes and monoenic fatty acids. Oxidative... 

Most epoxides that have been characterized are epoxy derivatives of n-alkanes, mono-alkenes and di-alkenes that function as pheromone and sex attractant components, and the occurrence of many of them is reviewed above in this chapter. Epoxides are usually bio-synthetically derived from monoene and polyene long-chain hydrocarbons and many are used as pheromone components and sex attractants by four macrolepidopteran families the Geometridae, Noctuidae, Arctiidae, and Lymantriidae (Millar, 2000 Millar, Chapter 18, this book). As of 2009, sex pheromones or attractants have been identified for more than 120 geometrids (El-Sayed, 2009). 

Pyridinium-3-olates and pyrylium-3-olates react with a variety of monoenes, dienes, and trienes. Each of these systems react readily with dienophiles to yield cycloadducts of type 433. For Z = NMe, an electron-withdrawing X group is required in the dienophile, but with Z = 0 or A-(2-pyridyl) even unactivated alkenes react. 1-Phenylpyridi-nium-3-olate and benzyne give 434 dienes give adducts of type 435. Fulvenes behave as trienes to give adducts across the 2,6-positions 436. 2-Benzothiopyrylium-4-olate 437 gives a thermal dimer across the 1,3-positions. 

A variety of synthetic routes to monoene and polyene tri-fluorophosphine-transition metal complexes have been devised. Direct photochemically induced reaction of a metal-PF3 complex with an activated alkene or diene (method A) has proved useful only for iron, the products being either [Fe(PF3)4(alkene)J or [Fe(PF3)3(diene)] (194). Mixed carbonyl-trifluorophosphine complexes of the type [Fe(PF3)x(CO)3 x(diene)] result from either thermal or photochemical reactions of dieneiron carbonyl complexes and PF3 (52, 53) (method B). The compounds are fluxional. 

Simple 1,3-dienes are reduced mainly to monoenes. Some alkynes are reduced satisfactorily to c(s-alkenes. 

The hydrogenation of conjugated dienes to monoalkenes poses questions of both regio- and stereoselectivity and is a function of the metal and the conditions.The distribution of products depends not only on the inherent selectivity of the initial addition process but also on the competition between the diene and the first-formed alkene(s) for the reactive sites. The latter may add hydrogen or be isomerized. If the structure of the diene allows the change, the diene itself may be isomerized. As for alkynes, palladium and nickel catalysts tend to be the most selective to the monoene. 

The complex [OsHBr(CO)(PPh3)3] is effective for the selective hydrogenation of conjugated and non-conjugated dienes to monoenes, and this complex also catalyzes the hydrogenation of linear and cyclic alkenes. (-)-Carvone (9) is selectively reduced to 2-methyl-5-(l-methylethyl)cyclohex-2-en-l-one (10) or 2-methyl-5-(l-methylethyl)cyclohexanone (11), depending on the reaction conditions. ... 

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