Cyclodimer

Essentially the same procedure may be used to produce mixtures of cyclodimers from isoprene,4 1,3-cyclopentadiene,4 and 1,3-cyclohexadiene.7 Separation of all products is somewhat difficult in most cases but has always been possible by preparative vapor phase chromatography. Despite the problems that may be involved in separation of desired products in some instances, photocyclization frequently is the method of choice for preparation of 1,2-dialkenylcyclobutanes if they can be made major products of photoreactions. Starting materials are readily available, and the preparations are easily carried out on the scale described. There is little doubt that the method is the best for preparation of trans-1,2-divinyleyclob u tane. 

Cyclodimer 3 proved to be somewhat difficult to manipulate, thus contributing to the complexity of its characterization. The bowed diacetylenic linkages revealed in the X-ray data impart surprising physical characteristics to the molecule. The energy-rich hydrocarbon was sufficiently strained that it decomposed explosively upon grinding (i. e. preparing a Nujol mull) or when heated above 80°C. At room temperature, crystals blackened within a few days and apparently auto-polymerized, even when stored under vacuum in the dark. Only dilute solutions of 3 in benzene or pyridine were fairly stable over time, especially when stored cold under an inert atmosphere. 

The formal cyclodimer of o-iodoethynylbenzene, diyne (7), was prepared in 1974 by Sondheimer s group using analogous bromination/dehydrobromina-tion chemistry [13]. The highly strained molecule was comparatively stable, decomposing around 110°C on attempted melting. Slow decomposition of the solid was observed after 2 days, when unprotected from light and air. 

Dichloromethylene)cyclopropane (455) (entry 4) undergoes the cyclodimerization in milder conditions to afford the corresponding dispirooctane 459 in good yield [117,118a], while (difluoromethylene)cyclopropane (456) required higher temperatures to afford the cyclodimer 460 in low yield (entry 5) [9]. 

In all the latter cases the easier dimerization reaction is connected with the particular stability of the intermediate diradical species. This is also the reason for the recently found facile dimerization of the 1-donor substituted allylidene-cyclopropane 136a (Scheme 66) [127]. Allylidenecyclopropane 136a cyclodimer-izes to the expected cyclobutane 467 in very mild thermal conditions, due to the stabilization of the intermediate 466. At higher temperature (120 °C) both 136a and 467 give a more complex mixture of products, with the cyclooctadiene dimer 468 being the prevailing one (Scheme 66) [127],... 

However, the cyclodimer 471 is produced as a side-product during the preparation of the parent allylidenecyclopropane 470 from the corresponding 1-bromo-allylidene derivative 469 by a metalation-protonation procedure (Scheme 67). 

Stilbene-like olefins can be irradiated directly to afford cyclodimers... 

The enantioface and also the configuration (s-trans, s-cis) of the prochiral butadienes involved in the several elementary steps are of crucial importance for the stereocontrol of the cyclo-oligomer formation. Oxidative coupling, for example, can occur between two cA-butadienes, two /rum-butadienes or between cis- and /nmv-butadiene with either the same or the opposite enantioface of the two butadienes involved. The several stereoisomers are exemplified for the [Ni°(butadiene)2L] active catalysts for cyclodimer formation, that are schematically depicted in Fig. 1, together with the related stereoisomers of the ry ri fC1) and bis(r 3) octadienediyl-Ni11 species 2a and 4a, respectively. For each of the individual elementary steps there are several stereochemical pathways, which are exemplified in Fig. 1 for the... 

Fig. 1. Stereoisomeric forms of the [Ni°(ri2-butadiene)2L] active catalyst complex la of the C8-cyclodimer reaction channel and the related stereoisomers of the r 3,ri1(C1), 2a, and bis(r 3), 4a, octadienediyl-Ni11 species. SF and OF denotes the coordination of two c/s-butadienes (cc), of two tr

Formation of cis-l,2-DVCB and ds,dv-COD commences through the formation of a a-bond between the terminal substituted carbons, C3, C6, and the terminal unsubstituted carbons, C1, C8, of the two r 3-allylic groups along 4a —> 9a and 4a —> 10a, respectively (Fig. 8). The transition states TS[4a-9a] and TS[4a 10a] occur at a distance of 1.9 and 2.1 A for the newly formed C-C bond and decay into 9a and 10a, respectively, where the cyclodimers are each coordinated to Ni° by two olelinic double bonds. The several stereochemical pathways are connected with activation barriers that differ significantly. Moderate barriers have to be overcome for 4a —> 9a... 

Overall, steric and electronic factors, which are seen to be small, are found to work in opposite directions and, to some degree, cancel each other out. Consequently, the intrinsic free activation barriers and reaction free energies (AG nt, AG nt), respectively, span a small range for catalysts I-IV and differ by less than l.Okcalmol-1. Thus, oxidative coupling represents the one process (beside allylic isomerization, cf. Section 5.3) among all the critical elementary steps of the C8-cyclodimer channel, that is least influenced by electronic and steric factors. 

Among catalysts I-IV, which predominantly catalyze the generation of cyclodimer products, the overall lowest intrinsic free-energy barrier of 20.5 kcal mol-1 (AG nt) for 2a -> 8a appears for catalyst IV with L = P(OPh)3, where both electronic and steric factors are seen to assist the formation of VCH to a similar amount. Reductive elimination involves a higher intrinsic barrier (AG nt) for catalysts bearing moderately bulky, donor phosphines,... 

It is worth noting, that the mechanistic conclusions for the competing 4a — 9a and 4a — 10a routes drawn for the generic catalyst are corroborated for the real catalysts I-IV. The formation of cis-1,2-D VCB and cis,cis-COD is connected with very similar total activation barriers (i.e., relative to the favorable bis(r 3-,vyra) isomer of 4a) for each of the individual catalysts. Furthermore, cis,cis-COD is clearly seen to be the thermodynamically preferred product of the two cyclodimers. The difference in the thermodynamic stability between the [Ni°(ri4-cyclodimer)L] products 9a and 10a is most remarkable for IV with L = P(OPh)3 and amounts to 6.7 kcal moF1 (AG). This confirms the conclusion (cf. Section 4.6.1), that cis,cis-COD is generated as the predominant product along the reductive elimination routes that commence from the bis(ri3) precursor 4a. 

The cyclodimers are liberated from the respective elimination products 8a and 10a via successive substitution processes with incoming butadiene, that regenerates the active catalyst la in an overall exergonic process. For the rate determining reductive elimination step of the C8-channel free-energy activation barriers of 20.1-24.1 kcalmol-1 are predicted for catalysts TTV, that are in excellent agreement with experimental estimates.43 Thus, moderate reaction conditions are required for the catalytic cyclodimerization of 1,3-butadiene.6... 

Heimbach et al.s conducted a careful experimental investigation of the influence of the ancillary ligand on the cyclodimer product distribution. Strong n-acceptors that are sufficiently space-demanding were found to catalyze the formation of cis,cis-COD almost exclusively. The VCH portion... 

Cyclononatriene, the smallest cyclic [3]cumulene isolated so far, polymerizes when its solutions are concentrated57. On the other hand, several radialenes have been isolated which represent cyclodimers of seven- and eight-membered 1,2,3-trienes (Scheme 9). 

The nickel-catalyzed [4 + 4]-cycloaddition of butadiene to form cyclooctadiene was first reported by Reed in 1954.90 Pioneering mechanistic and synthetic studies largely derived from the Wilke group advanced this process to an industrially important route to cyclodimers, trimers, and other molecules of interest.91-94,943 95,96 While successful with simple dienes, this process is not useful thus far with substitutionally complex dienes as needed in complex molecule synthesis. In 1986, Wender and Ihle reported the first intramolecular nickel-catalyzed [4 + 4]-reaction of... 

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