Alkenes dimerisation

Scheme 13.8 Attempted alkene dimerisation with [Nil (NHC) ] complexes...

When the same [NiI (NHC)2] complexes are employed as alkene dimerisation catalysts in ionic liquid (IL) solvent [l-butyl-3-methylimidazolium chloride, AICI3, A-methylpyrrole (0.45 0.55 0.1)] rather than toluene, the catalysts were found to be highly active, with no evidence of decomposition. Furthermore, product distributions for each of the catalyst systems studied was surprisingly similar, indicating a common active species may have been formed in each case. It was proposed that reductive elimination of the NHC-Ni did indeed occur, as outlined in Scheme 13.8, however, the IL solvent oxidatively adds to the Ni(0) thus formed to yield a new Ni-NHC complex, 15, stabilised by the IL solvent, and able to effectively catalyse the dimerisation process (Scheme 13.9) [25-27],... 

Scheme 13.9 Alkene dimerisation with (NHC)2NiIj complexes in IL solvent...

Schrock has proposed that the reverse reaction occurs during some catalytic alkene dimerisation reactions (20) but, in studies of decomposition of alkyl substituted platinacyclobutanes, no... 

The enhanced reaction rates and regioselectivities (head-to-head) in the alkene dimerisation (cyclobutane formation e.g. 18-19) via radical cation catalysed reactions have led numerous studies in this direction [5,10, 36-39]. The head-to-head stereochemistry of these dimerisations have been explained in terms of the addition of the radical cation to a neutral molecule giving the stabilised 1,4-radical cation. An interesting application of the cation-radical initiated (2 + 2)-cyclodimerisation strategy was reported by Mizuno et al. [40] for the synthesis of macrocyclic 2-m-dioxabicyclo (m-1,2,0) ring systems (21) from the PET reactions of 20 (Scheme 6). 

Randjelovic, I., Bengtson, G., Fritsch, D. (2002). Catalytic membranes alkene dimerisation by means of acidic porous thin-film composite membranes. Desalination, 144, 417—418. 

This is a general situation for thermal, concerted additions those involving Ane + 2ne systems proceed readily, e.g. the Diels-Alder reaction, whereas those involving 2ne + 2ne systems, e.g. the cyclo-dimerisation of alkenes, do not. We might, however, expect that photochemical cyclodimerisation of alkenes would be symmetry... 

Use of less sterically hindered examples of 5 in combination with MAO allows for active catalysts for the linear (head-to-head) dimerisation of a-olefins such as 1-butene, 1-hexene, 1-decene and Chevron Phillips C20-24 a-olefin mixture (Scheme 4) [47], The mechanism for dimerisation is thought to involve an initial 1,2-insertion into an iron-hydride bond followed by a 2,1-insertion of the second alkene and then chain transfer to give the dimers. Structurally related cobalt systems have also been shown to promote dimerisation albeit with lower activities [62], Oligomerisation of the a-olefms propene, 1-butene and 1-hexene has additionally been achieved with the CF3-containing iron and cobalt systems 5j and 6j yielding highly linear dimers [23],... 

Some readers may regret that we have taken our title so literally that we have not included the rate-constants of dimerisation, e.g., for diphenylethylene nor those for initiation by preformed cations (except marginally) nor the rate-constants for the addition of aryl and di-aryl cations to alkenes, established so elegantly by Wang and Dorfman (1980) and by Mayr and his collaborators (Mayr, 1990 Bartl et al., 1991 and numerous earlier papers). In our view these subjects are sufficiently interesting and important to be reviewed together in their own right. 

The impossibility of a conjugated alkene in 15 as well as 13, makes this an exceptional case in aldols of aldehydes. Without the branch at the a-carbon, the product is more usually a conjugated enal.6 So the linear isomer 17 of 16 with the same base gives the enal 18 in good yield.7 The true product of the dimerisation is the anion 19. This is in equilibrium with the enolate that allows an ElcB elimination of water 20 to give the enal 18. 

The easiest way to do the disconnections is to reverse the rearrangement and there are two ways to do this 42a and 42b. Diol 44 can be made by pinacol dimerisation of cyclopentanone 43 while diol 45 would be the product of dihydroxylation of the alkene 46. 

Unlike ordinary alkenes, ketenes do 2 + 2 cycloadditions with themselves—the dimerisation above—and with other alkenes.1 Reaction of dichloroketene with cyclobutadiene 11 to give the... 

Scheme 1.1. Some characteristic reactions of radicals. A and B are any two types of reactive radical. A = B reaction [1]+ is a dimerisation. The reverse [ 1 ] is bond homolysis and may be induced thermally or photochemically. Reaction [2] represents addition to an alkene derivative. Reaction [3], hydrogen atom transfer, is one of the most important displacement reactions of radicals.

It is not that this cycloaddition is energetically unprofitable—the cyclobutane is lower in energy than the two alkenes—so there must be a high kinetic barrier to the cycloaddition of one alkene to another. This is a deeply important point, and it is just as well that it is true—if alkenes and other double-bonded compounds could readily dimerise to form four-membered rings, there would be few stable organic molecules, and life would be impossible. 

Photodimerisation of Alkenes. When alkenes are irradiated, they often dimerise to give cyclobutanes. In a dimerisation reaction, the set of orbitals on the left of each pair in Fig. 8.1 and the set on the right will have identical... 

In the absence of nucleophilic species, the radical cations generated in the photo-induced electron-transfer reactions may undergo other reactions. Thus, 1-phenoxypropene undergoes cis-trans isomerisation on irradiation in the presence of electron acceptors such as dicyanobenzene (Majima et al., 1979). Some alkenes undergo dimerisation giving cyclobutanes on irradiation in the... 

Acenaphthylene forms [2 + 2]-cycloaddition products with electron-deficient alkenes such as acrylonitrile and methyl acrylate (Nakamura et al., 1978). Cycloaddition is favoured by carrying out the reaction in a micellar solution owing to the inefficiency of the dimerisation in such an organised system. 

Miller , Brown and others - have noted the tendency of polyfluoro-alkenes and polyfluoroalkynes to form relatively stable cyclobutane- and cyclobutene-type ring systems. The cyclic dimerisation of tetrafluoroethylene has already been noted, the activation energy for the thermal dissociation of perfluorocyclobutane being some 12 kcal.mole" higher than for the corresponding hydro-... 

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