Homologation of alkenes

Thiols may be converted into alkenes with one extra carbon atom by successive chloromethylation, oxidation at sulfur, and Ramberg-BScklund rearrangement. If the original thiol is prepared by free radical addition of H2S to an alkene, the sequence constitutes a one-carbon homologation of alkenes. Although these transformations were at one time important applications of the Ramberg-Backlund reaction, they have hardly been used in recent years. 

Homologation of alkenes. Under photochemical conditions the radical addition... 

Several other methods for the functionalizing homologation of alkenes have been pul forward. Chul-Ho lun of Yonsei University assembled (J. Org. Chem. 2008, 73, 5598) a Rh catalyst that effected the oxidative acylation of a terminal alkene 13 with a primary benzylic alcohol, to give the ketone 14. For now, this approach is Umiled to less expensive alkenes, as the alkene, used in excess, is the reductant in the reaction. The other procedures outlined here require only stoichiometric alkene. Yasuhiro Shiraishi of Osaka University devised Organic Lett. 2008, 10, 3117) a simple photoprocess for adding acetone to a... 

The Homer - Emmons reagent (52) is effective in the one carbon homologation of ketones possessing acidic a-hydrogen atoms <96SL875> and electron-deficient alkenes add to 2-phenylseleno-l,3-dithiane in a photo-initiated heteroatom stabilised radical atom transfer process, giving products of considerable synthetic potential <96TL2743>. 

Fig. 2.15. Effect of decreasing on KER. The peaks from MIKE measurements belong to the m/z 58 ion product ion of alkene loss (ethene to hexene) from homologous iminium ions. The KER is determined from peak width at half height.

Aminophenols from anilines, 35, 2 Anhydrides of aliphatic dibasic acids, Friedel-Crafts reaction with, 5, 5 Anion-assisted sigmatropic rearrangements, 43, 2 Anthracene homologs, synthesis of, 1, 6 Anti-Markownikoff hydration of alkenes, 13, 1... 

Among the R2C(=C) =Ru homologs promoting alkene metathesis the most recent discoveries deal vhth the allenylidene-ruthenium and related pre-catalysts. This chapter is devoted to the class of ruthenium-allenylidene metathesis precatalysts, their intramolecularly rearranged indenylidene catalysts, and their use in... 

Cyclization of nitro-stabilized radicals provides another method for the generation of cyclic nitronates (221). Oxidation of the aci-foim of nitroalkanes with ceric ammonium nitrate generates the ot-carbon centered radical, which in the presence of an alkene, leads to the homologation of the a-radical. In the case of a tethered alkene of appropriate length, radical addition leads to a cyclic nitronate (Scheme 2.20). 

The most straightforward application of the Grubbs reaction is to effect homologation of a terminal vinyl group. One concern with the use of the second generation Grubbs catalyst 3 is the cost, about 100/mmol. In conjunction with a total synthesis of the macrolide RK-397, Frank McDonald of Emory reported (J. Am. Chem. Soc. 126 2495, 2004) that the conversion of 1 to 2 required 10 mol % of 3, but that it proceeded efficiently with just 2 mol % of the Hoveyda Ru catalyst 4 (J. Am. Chem. Soc. 122 8168,2002). The alkene so prepared was cleanly trans. An advantage of 4 is that it avoids the use of the expensive PCy,. 

A simple yet powerful application of the Grubbs reaction is specific homologation of a terminal vinyl group. When there is more than one alkene in the molecule, suitably disposed, one would worry about competing cyclizalion. In studies directed toward the cryptophycins, Mark Lautcns of the University of Toronto has reported (Organic Lett. 2004,6, 1883) that the second generation Grubbs catalyst 2 smoothly converts 1 to 3. The alternative cyclization product 4 is produced only in trace amounts. 

A unique example of alkane-alkene reaction is the homologation of olefins with methane in a stepwise manner over transition-metal catalysts.269 First methane is adsorbed dissociatively on rhodium or cobalt at 327-527°C then an alkene... 

An unanticipated catalytic reaction of olefinic hydrocarbons was described in 1964 by Banks and Bailey.1 2 They discovered that C3-C8 alkenes disproportionate to homologs of higher and lower molecular weight in the presence of alumina-supported molybdenum oxide [Eq. (12.1)], cobalt oxide-molybdenum oxide, molybdenum hexacarbonyl, or tungsten hexacarbonyl at 100-200°C, under about 30 atm pressure ... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

The silylcarbonylation of alkenes catalyzed by Co2(CO)s giving homologous silyl enol ethers was reported by Murai and coworkers in 1977 as a silicon version of hydro-formylation of alkenes, and developed as a unique synthetic method302-304. Although... 

In the remainder of this section we identify members of several homologous series of alkenes and apply equation 4 to their enthalpies of formation. In this way we can assess... 

TABLE 2. Regression analyses of equations 4a and 7 for homologous series of alkenes and corresponding alkanes (kJ mol 1 )a... 

We deduced from equations 4 and 7 that only when the slope, m, is approximately equal to 1 will the enthalpies of hydrogenation for a homologous series of alkenes be reasonably constant. Inspecting the slope and intercept data from equation 7 in Table 2, together with the actual experimental enthalpies of hydrogenation, shows our conclusion to be valid. We define for each alkene homologous series... 

Because the enthalpies of formation of all homologous series of alkenes correlate with the number of carbons according to equation 4, any two series must correlate with each other. We can recast equation 10 in linear form as equation 11. 

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