Coupling of Two Alkenes

A sample of 83 (0.35 g, 0.94 mmol) and benzene (10 ml) were placed in a round-bottomed flask attached to a swivel-frit assembly. The soln was stirred under N2O (1 atm) at 55 °C for 3 d, after which the soln was filtered and the filtrate was taken to diyness under vacuum. The resulting solid was washed with petroleum ether (3x5 mL), and the blue product was collected on a filter frit to give microcrystalline 84 jdeld 0.16 g (44%). 

A sample of 84 (0.12 g, 0.31 mmol) and benzene (10 ml) were placed in a round-bottomed flask, to which Ij (0.08 g, 0.31 mmol) was added under an argon counterflow. There was an immediate color change from blue to brown. The soln was stirred for 30 min at rt and filtered, and the filtrate was evaporated using a rotary evaporator. The residue was chromatographed (silica gel, hexane/EtOAc 100 3) to give 85 as a colorless oil yield 0.03 g (47%). 

---

The cross-coupling of two alkenes also takes place. Alkenes such as acrylate react regioselectively with 1,3-dimethyluracil (290) to afford 5-(l-alkenyl)ura-cils such as 291 in a high yield[260]. 

Indeed, both kinds of cycloaddition products (Type A and Type B) can be obtained in the presence of Ni(0) catalysts while Pd(0) catalysts exclusively lead to Type A codimers. The real course of these reactions however is somewhat more complicated. While distal ring-opening via Route a really leads to cycloaddition products of Type A, proximal ring-opening via Route b results only in an isomerization of methylenecyclopropane. Cycloaddition products of Type B are obtained indirectly via oxidative coupling of two alkene units with low-valent nickel followed by a cyclo-propylmethyl/3-butenyl rearrangement22,148b). Reductive elimination terminates the catalytic cycle (Eq. 78). 

A further support for the mechanism outlined in Eq. 118 is that with Ni(0) catalysts a second type of [3+2]-cycloaddition can occur which involves the oxidative coupling of two alkenes coordinated at the nickel (one must be methylenecyclopropane). The initially formed nickelacyclopentane derivative may collapse to give a spiro[2.3]cyclohexane derivate or rearrange into a 4-methylenenickelacyclohexane derivate, which at the end of this catalytic cycle gives methylenecyclopentanes with a new substitution pattern by reductive elimination (see Eq. 78 and Scheme 8). 

Formation of ketones via hydroacylation can be achieved either via transition metal catalyzed hydrocarbonylative coupling of two alkenes ... 

In the same way as oxidative addition of an olefin to a metallacyclopropane occurs only if the metal is very electron rich or the olefin very electron poor, oxidative coupling of two alkenes usually requires the same conditions. 

Unsymmetrical alkenes can be prepared from a mixture of two ketones, if one is in excess. " The mechanism consists of initial coupling of two radical species to give a 1,2-dioxygen compound (a titanium pinacolate), which is then deoxygenated. " ... 

Titanium in a low valence state, as prepared by treatment of solutions of titanium trichloride with potassium [206] or magnesium [207] in tetrahydro-furan or with lithium in dimethoxyethane [206], deoxygenates ketones and effects coupling of two molecules at the carbonyl carbon to form alkenes, usually a mixture of both stereoisomers. If a mixture of acetone with other ketones is treated with titanium trichloride and lithium, the alkene formed by combination of acetone with the other ketone predominates over the symmetrical alkene produced from the other ketone [20(5] Procedure 39, p.215). 

A reaction that is ordinarily of minor consequence in the animal body but which may be enhanced by a deficiency of sulfite oxidase is the reductive coupling of two molecules of sulfite to form thiosulfate (Eq. 24-45, step e). Several organic hydrodisulfide derivatives such as thiocysteine, thioglutathione, and thiotaurine occur in animals in small amounts. Another biosynthetic pathway, outlined in Eq. 24-47 converts sulfite and PEP into coenzyme M (Fig. 15-22)475/475a This cofactor is needed not only for methane formation (Fig. 15-2) but also for utilization of alkenes by soil bacteria.4751 ... 

Coupling of two electron-rich components like alkenes, enol acetates, enol ethers, carbanions, or carboxylates ... 

Accounts of die reductive coupling of two molecules of ketone via the McMurry alkene synthesis have been described66-68 earlier under Miscellaneous Aldols. 

The mixed coupling of two different alkenes allows the formation of new functional unsaturated products but requires high regioselectivity. A ruthenium hydride complex, generated in situ from the reaction of RuHCl(CO)(PCy3)2 with HBF4.OEt2, was found to be an effective catalyst for the hydrovinylation of alkenes [8]. The reaction of styrene with ethylene produced the hydrovinylation compound 10 in 93% yield (Eq. 5). Initial hydrometallation of the alkene and insertion of ethylene seemed to be a plausible mechanism. 

Ruthenium vinylidene intermediates have also been proposed in the mechanism of the coupling of unactivated alkenes with terminal alkynes to afford 1,3-dienes as a mixture of two isomers, linear and branched derivatives. The linear one was favored [56] (Eq. 42). The same system has allowed the ruthenium-catalyzed alkenylation of pyridine [57]. 

The catalytic cycle proposed for the enantioselective ethyhnagnesation involves the chiral zirconocene-ethylene complex (121), formed upon reaction of the dichloride with two equivalents of EtMgCl. Coupling of the alkene substrate leads to the formation of the metallacyclopentane intermediate, which gives the zirconate (122) with a... 

Other reports of kinetic studies deal with mechanisms of thermal oxidation of a variety of simple ketones monitored via gas evolution (CO, CO2, H2, etc.), alkaline oxidation of aldehydes with copper and silver tellurates, [M (H2Te06)2] , and oxidation of acetals of simple aldehydes in aqueous acetic acid with (i) N-chlorobenzamide (H2OCI+ is the oxidant inferred) and (ri) W-chlorosaccharin. Accounts of the reductive coupling of two molecules of ketone via the McMurry alkene synthesis have been described earlier under Miscellaneous Aldols. 

The radical 36 can react with cupric chloride by two pathways, one of which leads to addition and the other to substitution. Even when the addition pathway is taken, however, the substitution product may still be formed by subsequent ehmination of HCl. Note that radical reactions are presented in Chapter 14, but the coupling of an alkene with an aromatic compound containing a leaving group prompted its placement here. Note also the similarity to the Heck reaction in 13-10. 

Two major side reactions compete with the coupling reaction protonation of the radical anion followed by further reduction and protonation leading to the saturated dihydro product, and polymerization induced by the basic dianion formed by coupling of two radical anions. Other, less typical reaction pathways include reaction between a radical anion and a molecule of substrate. Scheme 2, dimerization of two radicals formed by protonation of the initial radical anion. Scheme 3, or, infrequently, cleavage of the radical anion followed by coupling. However, for radical anions derived from monoactivated alkenes, the pathway in Scheme 2 has only been unequivocally established as a major pathway in a few cases in which the final zero-electron product is a cyclobutane, that is, a cycloaddition product. 

The inter- and intramolecular coupling of two carbonyl groups of aldehydes or ketones in the presence of a low-valent titanium species produces a C-C bond with two adjacent stereocenters, a 1,2-diol (a pinacol). These may be further elaborated into ketones by the pinacol rearrangement or be deoxygenated to alkenes (McMurry reaction). 

---