Oligomerization allenes

Oligomeric allene derivatives were also prepared using phenolic-formaldehyde oligomers illustrated in Eq. 1 ... 

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... 

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... 

The thermal dimerization and oligomerization of allenes are well documented (see Houben-Weyl, Vol. 4/4, p 151). The cyclodimers are the result of both head-to-head 2 and head-to-tail 3 dimerization with the head-to-head dimer 2 normally predominating.21-22... 

The second part deals with the complex chemistry and catalytic oligomerizations of allene. We emphasize the importance of the role played by auxiliary ligands of transition metals in determining the paths of catalytic oligomerizations. Recent reviews 11-12) covering most of the literature published up to 1972 lack a perspective view on catalysis. The most recent review 13) describes mainly oligomerization and cooligomerization. 

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 the co-oligomerization of methylenecyclopropane (26) with allene (63 equation 25) on a palla-dium(O) catalyst, prepared in situ from [Pd(acac)2], triisopropylphosphine and diethylaluminum ethoxide, or from [Pd(DBA)2] and triisopropylphosphine, cyclic hydrocarbons such as 2-methylenespiro[2.4]hep-tane (27), 1,3-dimethylenecyclopentane (64) and 1,3,5-trimethylenecycloheptane (65) are formed. ... 

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. 

Alkynoic esters, 493-494 Alkynyldiethyl alanes, 144 Allene cyclo-oligomerization, 41 Allenes, 161,315-316,438 Allenic adds, 149-150 Allenic amides, 255... 

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. 

Allene complexes and coordination compounds that readily bind allenes often catalyze oligomerization and polymerization of allenes. Allene, phosphine, phosphine-carbonyl, phosphite, olefin, carbonyl, etc. complexes of metals of groups 8-10 exemplify such behavior, for instance, [Ni P(OR)3 4], [Ni(PR3)4], [Ni(CO)2 (PR3)2], [Ni(COD)2], [Rh2Cl2(CO)4], [RhCl(PPh3)3], [Rh(acac)(CH2 = C = CH2)2], etc. 

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. 

The oligomerization proceeds according to the mechanism of coordination and migration (insertion) of consecutive allene molecules which is proven by isolation of intermediate products see, e.g., structure (13.111). Therefore, the mechanism of oligomerization may be represented as scheme (13.112). 

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. ... 

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