Dimerisation

Both sodium and nickel bring about oxidative dimerisations, despite the apparently reducing conditions, the former giving 4,4 -bipyridine and the latter 2,2 -bipyridine. Each reaction is considered to involve the same anion-radical resulting from transfer of an electron from metal to heterocycle, and the species has been observed by ESR spectroscopy, when generated by single electron transfer (SET) from lithium diiso-propylamide. In the case of nickel, the 2,2 -mode of dimerisation may be favoured by chelation to the metal surface. Bipyridyls are important for the preparation of Paraquat-type weedkillers. 

Intermediate, reduced dimers can be trapped nnder milder conditions,and rednced monomers when the pyridine carries a 4-snbstitnent.  

Irradiation of the styrene (175) in a NaY zeolite brings about (2 + 2)-dimerisation to yield (176) and (177). Oxidation of the styrene also occurs during this treatment. Radical cations of the alkenes (178), (179) and (180) 

Irradiation of an acetone solution of the azetine (185) affords (2 + 2)-dimers and although four dimers are possible, only a 1 1 mixture in 52% yield of the head-to-head dimers (186) and (187) was obtained.  

The mechanism of the electron transfer-sensitised dimerisation of acenaphthylene (188) has been studied in considerable detail. Two dimers (189) and (190), formed from the radical cation of acenaphthylene, are obtained in addition to (191) and (192) which incorporate the TCNE sensitiser. Fumaroni-trile behaves in a similar fashion yielding the dimers (189) and (190) as well as cycloadducts. A study of the energetics of electron transfer in acenaphthylene charge transfer photochemistry has been carried out.  

The photochemical dimerisation of (193) results in the formation of some major products, one of which has been identified as the adduct (194). The photochemical dimerisation of single crystals of ( )-2,6-di-Bu -4-[2-(4-methyl-phenyl)-ethenyl]pyrylium salts has been studied.  

The results of a study of the catalytic cycloreversion of the benzene dimers (195) and (196) have been published. 

Since the activation energy for the dissociation of N2O4, is close to the enthalpy change involved, 22 will not vary much with temperature, i.e. the activation energy for (22) will be close to zero. 

---

This compound, which contains atoms arranged tetrahedrally around the boron atom, can readily be isolated from a mixture of dimethyl ether and boron trichloride. On occasions a chlorine atom, in spite of its high election affinity, will donate an electron pair, an example being found in the dimerisation of gaseous monomeric aluminium chloride to give the more stable Al2Clg in which each aluminium has a tetrahedral configuration ... 

Unlike nitrogen monoxide, nitrogen dioxide has properties more typical of an odd electron molecule. It is a coloured (brown), reactive gas which dimerises to the diamagnetic colourless gas dinitrogen tetroxide, N2O4. in which the odd electron is paired. The structure of dinitrogen tetroxide can be represented as a resonance hybrid of ... 

Oxygen, like nitrogen oxide, NO, shows little tendency to dimerise although the presence of the unstable, weakly bonded species, tetratomic oxygen O4, has been reported as a constituent of liquid oxygen. 

Table 1.1. Second-order rate constants kj for the dimerisation of cyclopentadiene in solution and in the gas pha.se at 25°C. ...

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

Synthesis The Diels-Alder reaction is simply the dimerisation of isoprene to giye the naturally occurring terpene A. Now we haye to cleaye one double bond and leaye the other alone. It turns out that epoxidation is selectiye in this case. 

An isotopic effect (H or D) has been demonstrated when starting from 2-methyl-4-phenylthiazole or from 2-methyl-4-phenyl-5-D-thiazole (224) in the dimerisation reaction. 

In the foUowiag cases, only those reactions ia which there is no chain growth, or at most dimerisation, are considered (see Olefin polymers). Alkyl titanium haUdes can be prepared from alkyl aluminum derivatives. The ring stmcture imparts regiospecificity to the ensuing carbometalation (216) ... 

Because chloroprene is a flammable, polymerisable Hquid with significant toxicity, it must be handled with care even in the laboratory. In commercial quantities, precaution must be taken against temperature rise from dimerisation and polymerisation and possible accumulation of explosive vapor concentrations. Storage vessels for inhibited monomer require adequate cooling capacity and vessel pressure rehef faciUties, with care that the latter are free of polymer deposits. When transportation of monomer is required, it is loaded cold (< — 10° C) into sealed, insulated vessels with careful monitoring of loading and arrival temperature and duration of transit. 

Now retain this favorite reaction but abandon all assumptions. If one even assumes the solid catalyzed dimerisation reaction of ... 

Also the mirror image of the strueture I, eorreetly denoted as exo-3,10-dihydroxy-3,5,8,10-tetra-methyltrieyelo[6.2.2.0 ]dodeea-5,l 1-diene-4,9-dione, would be possible sinee enantiomers are not differentiated by NMR. A retro-Diels-Alder fragmentation of I to CsH/oO explains why the moleeular ion eorresponding to the moleeular formula C16//20O4 is not deteeted in the mass spee-trum. The metabolite I eould be formed by Diels-Alder dimerisation of 1,5-dimethyleyelohexa-l,3-dien-5-ol-6-one J as the primary metabolite which acts as diene and dienophile as well... 

The monomers can be prepared by isomerisation of 4-methylpent-2-ene or reaction of tri-isobutylaluminium with ethylene but commercial interest appears to centre on the dimerisation of propylene Figure 11.12). 

The simplest polyamines are the aliphatic types such as diaminoethane, but these readily carbonate when exposed to the atmosphere as a thin him, so adducts (pre-reacted epoxy polyamines) are preferred. An alternative system is the polyaminoamides which are made by reacting dimerised fatty acids with an excess of polyamine. These themselves act as corrosion inhibitors and are noted for excellent adhesion. 

The importance of cross-talk in GR actions is indicated by the construction of a GR dimerisation-deficient mutant mouse in which GR is unable to dimerise and therefore bind to DNA, thus separating the DNA-binding (transactivation) and inflammatory gene repression (transrepression) activities of glucocorticoids. In these animals dexamethasone was able to inhibit AP-1- and NF-kB-mediated gene transcription,... 

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

A plausible pathway is that the aromatisation of the cyclohexadienone 92 by a proton shift is accelerated in the presence of Ac20 under formation of acetate 93. The simultaneously generated acetic acid then cleaves the acetate to form the free phenol 94 (Scheme 44). This effect was observed for the first time during studies towards the total synthesis of the lipid-alternating and anti-atherosclerotic furochromone khellin 99 [64].The furanyl carbene chromium complex 96 was supposed to react with alkoxyalkyne 95 in a benzannulation reaction to give the densely substituted benzofuran derivative 97 (Scheme 45). Upon warming the reaction mixture in tetrahydrofuran to 65 °C the reaction was completed in 4 h, but only a dimerisation product could be isolated. This... 

Bei der Reduktion von 1,2-Dibrom-benzocyclobuten wird als einziges Dimerisations-produkt 6a,lOb-Dihydro-(benzo-[a]-biphenylen) isoliert5 ... 

M. Y. Fioshin u. A. P. Tomilov, Electrochemical Dimerisation - a promising Method for the Synthesis of organic Compounds, Khim. Prom. 1964. 649. 

Organic Fluorides. Part XIII. The High-temperature Dimerisation of Chlorotrifluoroethylene, M. W. Buxton, D. W. Ingram, F. Smith, M. Stacey, and J. C. Tatlow, /. Chem. Soc., (1952) 3830-3834. 

Polysaccharides can regulate weak interactions between protein molecules. A recent example is the effect of low molecular weight heparin molecules on the weak dimerisation of the plasminogen growth factor NKl, or at least a mutant thereof [135]. 

Cyclopenta[fc]dioxanes (44) are accessible from the reaction of the dioxenylmolybdenum carbene complex (43) with enynes <96JOC159>, whilst an intramolecular and stereoselective cyclisation of (Ti5-dienyl)tricarbonyliron(l+) cations affords chiral frans-2,3-disubstituted 1,4-dioxanes <96JOC1914>. 2,3-Dimethylidene-2,3-dihydro-1,4-benzodioxin is a precursor of the 3,8-dioxa-lff-cyclopropa[i]anthracene, which readily dimerises to dihydrotetraoxaheptacene (45) and the analogous heptaphene <96AJC533>. 

Both o- and p- substituted bis(methylthio)arylmethylium salts, derived by the 5-methylation of substituted dithiobenzoates, dimerise following deprotonation to give 1,4-dithianes. Only with the former isomer is the quinone methide generated leading to a cyclooctene derivative <96LA1159>. 

---