Allenes oligomerization reactions

Metal-AUene Reactions.—Allene oligomerization with metal catalysts has been intensively studied recently. The phosphorus ligand in nickel(O) catalysts has a powerful controlling effect upon oligomerization of allene. Thus a nickel-(cyclo-octa-l,5-diene)2 complex combined with 1—4 moles of t-alkyl-or aryl-phosphine oligomerizes allene mainly to tetramer (364), whereas... 

The cycloaddition reactions are subdivided into di-, tri- and oligomerization reactions, [2-1-1]-, [2-1-2]-, [3-1-2]- and [4- -2] cycloaddition reactions and other cycloaddition reactions. The insertion reactions into single bonds are also discussed. The cyclodimerization or cyclotrimerization reactions are special examples of the [2-1-2] and the [2-I-2-I-2] cycloaddition reactions, respectively. The cumulenes vary in their tendency to undergo these reactions. The highly reactive species, such as sulfines, sulfenes, thioketenes, carbon suboxide and some ketenes, are not stable in their monomeric form. Other cumulenes have an intermediate reactivity, i.e. they can be obtained in the monomeric state at room temperature and only heat or added catalysts cause di- or trimerization reactions. In this group, with decreasing order of reactivity, are allenes, phosphorus cumulenes, isocyanates, carbodiimides and isothiocyanates. 

Beyond dimerization and oligomerization, [2 + 2]- and [4+ 2]-cycloadditions with conjugated dienes and styrenes and the addition of nucleophiles are typical reactions of strained cyclic allenes. These transformations have been studied most thoroughly with 1,2-cyclohexadiene (6) and its derivatives [1, 2]. Concerning the cycloadditions, a theoretical study had the surprising result that even the [4+ 2]-cycloadditions should proceed in two steps via a diradical intermediate [9]. In the case of nucleophiles, the sites of attack at several 1,2-cyclohexadiene derivatives having an... 

Kinetic studies 123) on the three types of oligomerization indicate that the rate of reaction increased in the order Ni(0)—P(OPh)3 < Ni(0)-PPh3 < Ni(0). Interestingly, the rates for both pentamerization and tetra-merization are first order with respect to the allene concentration, whereas the trimerization rate is nearly zero order. These results are accommodated by the following reaction sequences. 

In this account, an overview of the methods employed for the synthesis of conjugated dienes and polyenes is presented. Dienes and polyenes with isolated double bonds are excluded, as they are accessed through methods usually employed for alkene synthesis. Oligomerizations and polymerization reactions leading to polyenes are also not covered. Synthesis of 1,2-dienes, i.e. allenes, is excluded from the purview as there is a volume in the present series devoted to this functional group. Synthesis of heterodienes, conjugated enol ethers, [n]-annulenes and related compounds are also not covered here. However, enynes, dienynes and enediynes syntheses have been included in a few cases in view of their emerging importance. 

Diphenyl diselenide can add to allenes very smoothly, providing (8-(phenylsele-no)allylic selenides in high yields, as depicted in Scheme 15.66 [146]. In contrast, the photoinduced reaction of (PhS)2 with allenes affords a complex mixture, because the lower capturing ability of (PhS)2 for carbon free radicals enables oligomerization of the allenes. 

Oligomerization and polymerization reactions of allenes are discussed Chapter 13. 

Cooligomerization (hetero-oligomerization) of olefins and acetylenes with butadiene in the presence of nickel complexes prevents the formation of butadiene trimers. During hetero-oligomerization chain and cyclic compounds may be formed see equations (13.87) and (13.88). Dodeca-2,6,10-triene-l,12-diylnickel reacts with allene to give various products. Reaction (13.89) is an example. 

In the case of nickel catalysts, the rate of oligomerization increases in the series Ni(0) + P(0Ph)3first order with respect to allene concentration while the trimerization reaction is almost of zero order. 

A brief kinetic study on the cyclo-oligomerization of allene with nickel(0) complexes has been reported. The rate of reaction increases in the order Ni -P(OPh)3 < Ni -PPha < Ni . Initial rates for pentamerization and tetramerization are first-order in allene concentration but the trimerization reaction is nearly zero order. The mechanisms illustrated in Scheme 4 are proposed. The complex [Ni(cod)js] produces complexes of allene trimer and tetramer below — 30 C but at higher temperatures the nickel(O) complexes catalyse the pentamerization reaction. In the presence of 1 to 2 mol of a phosphorus ligand only the trimer complex is formed. ... 

Ni-alkyne bonding consists of contributions from both the 77, 7t- and cr,diyl tautomers. This bonding picture helps visualize the insertion reactions with alkynes, alkenes, and CO that result in the formation of metallacycles. Thanks to such insertion reactions, Ni-alkyne species are active intermediates in a number of catalytic applications such as alkyne oligomerization, carbonylation, and insertion of heterocumulenes such as CS2 and GO2. For example, a recent example of a C02-fixation reaction involved the stoichiometric, alkylative or arylative carboxylation of alkynes to give a,(3- and / ,/ -unsaturated carboxylic acids. Ni(0)-alkyne complexes have also been used as pre-catalysts in the addition of hydrosilanes to alkynes. In most cases, monoalkynes react to give the products of m-addition, whereas diynes produce enynes (1,2-addition), allenes (1,4-addition), or 1,3-butadienes (1,2,3,4-addition). ... 

The complexes formed in the reaction of allene with a stoichiometric amount of nickel (o) catalysts readily add another molecule of allene to give trimer complexes, from which a quantitative yield of 1,2,4-trimethylenecyclohexane is obtained . In the oligomerization of allene, mixtures of higher oligomers are often formed, from which the oligomers listed in Table 6.3 were isolated. 

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