1.5- Cyclooctadiene dimerization

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],... 

The dimerization of isoprene has been accompHshed by methods other than heating. Thus isoprene has been dimerized by uv radiation in the presence of photosensitizers to give a complex mixture of cyclobutane, cyclohexene, and cyclooctadiene derivatives (36,37). Sulfuric acid reportedly... 

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

A novel route to azelaic acid is based on butadiene. Butadiene is dimerized to 1,5-cyclooctadiene, which is carbonylated to the monoester in the presence of an alcohol. Hydrolysis of this ester foUowed by a caustic cleavage step produces azelaic acid in both high yield and purity (56). 

Butadiene could be oligomerized to cyclic dienes and trienes using certain transition metal complexes. Commercially, a mixture of TiCU and Al2Cl3(C2H5)3 is used that gives predominantly cis, trans, trans-1,5,9-cyclododecatriene along with approximately 5% of the dimer 1,5-cyclooctadiene ... 

A derivative of cyclopentyne has been trapped in a matrix. Although cycloheptyne and cyclohexyne have not been isolated at room temperatures, Pt(0) complexes of these compounds have been prepared and are stable." The smallest cyclic allene" so far isolated is l-/err-butyl-l,2-cyclooctadiene 107." The parent 1,2-cyclooctadiene has not been isolated. It has been shown to exist transiently, but rapidly dimerizes." " The presence of the rert-butyl group apparently prevents this. The transient existence of 1,2-cycloheptadiene has also been shown," and both 1,2-cyclooctadiene and 1,2-cycloheptadiene have been isolated in platinum complexes." 1,2-Cyclohexadiene has been trapped at low temperatures, and its structure has been proved by spectral smdies." Cyclic allenes in general are less strained than their acetylenic isomers." The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air." ... 

Conjugated dienes can be dimerized or trimerized at their 1,4 positions (formally, [4 4- 4] and [4 4-4 4-4] cycloadditions) by treatment with certain complexes or other transition metal compounds. " Thus butadiene gives 1,5-cyclooctadiene and 1,5,9-cyclododecatriene. " The relative amount of each product can be controlled by use of the proper catalyst. For example, Ni P(OC6H4—o-Ph)3 gives predominant dimerization, while Ni(cyclooctadiene)2 gives mostly trimerization. The products arise, not by direct 1,4 to 1,4 attack, but by stepwise mechanisms involving metal-alkene complexes. " ... 

Another type of dimerization was observed by Japanese authors198. In the presence of Ni°, compounds like bis(l,5-cyclooctadiene) nickel(0), diphenyl and di-n-propyl cyclopropenone, and cyclohepteno cyclopropenone are transformed to tetra-substituted p-benzoquinones (261/262) by formal (2 + 2) or (3 + 3) cycloaddition of two cyclopropenone moieties effected by metal complexing. 

That the sequence shown in Scheme 3 is not the only pathway available for H—NiY formation is indicated by the isolation of 1,3-cyclooctadiene from the reaction products of the dimerization of propene with the n-cyclooctenylnickel system (25) (80) it seems reasonable that the H—NiY species 22 in this case is at least in part formed through direct elimination from 25 without prior monomer insertion into the Ni—C—bond [Eq. (6)] ... 

Codimerization of butadiene with dicyclopentadiene (example 8, Table II) was shown to proceed via a crotyl-nickel complex (62). Ring contraction of cyclooctadiene (example 10, Table II) appears to be a hydride promoted reaction. The hydride-promoted dimerization of norbomadiene to -toly 1 norbornene (example 9, Table II) appears to be quite different from dimerization via a metallacycle (see Table I, example 16). 

The coals used were PSOC 1098 Illinois 6 and Beulah-Zap North Dakota lignite from the Argonne coal bank. The analytical data of these coals are shown in Table I. The ratio of catalyst to coal was approximately 0.6 mmoles of metal per gram of coal. The organometallic catalysts were molybdenum(II) acetate dimer, Mo2(OAc)4, obtained from Strem, molybdenum(II) allyl dimer Mo2(OAc)4, was prepared by die method of Cotton and Pipal (25). The NiMo supported catalyst was prepared by addition of bis(l,5-cyclooctadiene) Ni(0) (Strem) to sulfided Mo on alumina (. Cp2Mo2( l-SH)2(p.-S)2, referred to as MoS2(OM), was prepared by modification of method of Dubois et al. (26), and Cp2Mo2Co2( i3-S)2(li4-S)(CO)4, CoMo(OM) was prepared by the method of Curtis et al. (27). Pentacarbonyl iron was obtained from Aldrich,... 

Nickel(0)-catalyzed [2-1-2] cycloaddition of cyclobutene to bicyclopropylidene (1) gave rise to the bis(spirocyclopropane)-annelated bicyclo [2.2.0]hexane derivative 232, the main product was 1,5-cyclooctadiene (233), formed by dimerization of cyclobutene with subsequent rearrangement (Scheme 54) [8,144]. 

Regioselective electrophilic bromination of the carbazole 1025 afforded the 6-bromocarbazole 1026. Cleavage of the methyl ether to 1027 and subsequent nickel-mediated coupling using a dimeric 7i-prenylnickel bromide complex prepared in situ from prenyl bromide 925 and bis(l,5-cyclooctadiene)nickel(0) led directly to siamenol (89) (546) (Scheme 5.151). 

Die Photodimerisierung von 1,2-Diphenyl-cyclobuten (27) wurde zu-erst von White und Anhalt (24) beschrieben. Das Dimere (28) (ohne stereochemische Angaben) wird neben Diphenyl-Acetylen, Athylen und dem Cyclooctadien-Derivat (29) gebildet, wenn die i-Octan-L6sung von (27) mit Licht der Wellenlange 254 m(x bestrahlt wird. 

Figure 5.14. STM images of the well-ordered monolayers formed from the reaction of (a) cyclopentene and (b) 1,5-cyclooctadiene with Si(100)—2 x 1. The images in (a) are collected on a vicinal silicon surface, as described in the text, (i) and (ii) are before and after saturation exposure to cyclopentene, respectively. The images in (b) include (i) the 1,5-cyclooctadiene molecule, (ii) and (in) STM images of saturation coverge of 1,5-cyclooctadiene, and (iv) a model of the molecule bonded across a Si-Si dimer. Both molecules form well-ordered monolayers on Si(100)-2x 1. Figure adapted from Ref. [218] with kind permission of Springer Science and Business Media.

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