Cyclodimerization 1,3-dienes

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

The use of zeolites is particularly advantageous for self-Diels-Alder reactions of gaseous dienes because it reduces the polymerization of the reactant. An example is the cyclodimerization of 1,3-butadiene to 4-vinylcyclohexene [20a] carried out at 250 °C with satisfactory conversion when non-acidic zeolites, such as large-pore zeolites Na-ZSM-20, Na- S and Na-Y, are used. 

The first stannenes were obtained as reactive intermediates that were identified by their reaction products.570,571 For example, Me2Sn=C(SiMe3)2 (Scheme 20) was prepared by the 1,2-elimination of LiBr, or by a retro-Diels-Alder reaction, and was characterized by cyclodimerization, by ene reactions with alkenes, and by cycloaddition with 1,4-dienes.572... 

The sterically hindered base 2,6-bis(tert-butyl)pyridine does not inhibit cyclization triaryl-amine retards this reaction photosensibilized one-electron oxidation of a diene leads to the same products, which are formed in the presence of ammoniumyl salt. As shown, in majority of cases, only the cation-radical chain mechanism of the diene-diene cyclization is feasible (Bauld et al. 1987). Meanwhile, cyclodimerizations of 2,4-dimethylpenta-l,3-diene (Gassman and Singleton 1984) and l,4-dimethylcyclohexa-l,3- or -1,4-diene (Davies et al. 1985) proceed through both mechanisms. 

Diels-Alder reactions with alkynes as dienophiles have been known for a long time. Iminoboranes, however, will more readily cyclodimerize than react with dienes, and even the cyclodimers are generally superior to dienes in the competition for excess iminoborane. Among many attempts, therefore, only two reactions with iminoboranes as dienophiles have been successful, and in both of them the diene is cyclo-pentadiene [Eq. (55)] (9, 14). 

In a series of papers published in the 1960s by Schmidt and coworkers, there is no description of any successful and stereospecific reaction of 1,3-diene compounds, e.g., topochemical polymerization, other than cyclodimerization. They... 

The strain associated with small-ring compounds often engenders chemical properties not encountered in larger ring systems. Methylenecyclopropanes and cyclobuta-1,3-dienes are known to undergo thermal cyclodimerization. 

Cyclobutadienes represent very reactive alkenes that undergo both [2 + 2] as well as [4 + 2] cycloadditions. Both the cyclodimerizations, mixed [2 + 2] cycloadditions and Diels-Alder reactions of these reactive species have been reviewed (see Houben-Weyl, Vols. 4/4, p 231 and E 17 f, Section 10B). In most instances the initially formed cyclodimer is tricyclo[4.2.0.02-5]octa-3,7-diene (36) and has the all cis-syn configuration. This is attributed to the concerted [4n -I- 2n] cycloaddition mechanism in which stereochemical control is affected by secondary orbital interactions. 

Cyclic allenes undergo head-to-head cyclodimerization to give tricyclic 1,2-alkylidenccyclobu-tanes. Unsaturated cyclic allenes represent molecules with increased strain and often dimerize spontaneously. Whereas cyclonona-1,2-diene (28) requires heating of a neat sample to 138°C,38 the tetraene 31 has a half-life of 10 minutes at 0°C.39 The more strained cycloocta-l,2,4,6-te-traene (34) could not be isolated and only the dimeric cyclobutane 35 was formed on thermolysis of the tosylhydrazone precursor 33.40... 

Metal-catalyzed cyclodimerization has been restricted to buta-1,3-dienes, norbornadienes (see Houben-Weyl, Vol. 4/4, pp 295-299) and more recently to strained alkenes such as cyclopropenes and methylenecyclopropanes (see Sections, 3., l.B.2.2.2. and 2.B.2.4.2.). 

Nickel(0)-catalyzed dimerization of isoprene (3 2-methylbuta-l.3-diene) has been used as the key step in the synthesis of (+)-grandisol (8), a male boll weevil pheromone. It is interesting to note that the cyclobutane 4 was formed by unsymmetrical and head-to-head cyclodimerization and that cycloadduets 5-7 were also formed in this transformation.9... 

The reaction mechanism of nickel-catalyzed cyclodimerization has been investigated.10 The dimerization of buta-1,3-dienes is also possible with palladium salts.11... 

Earlier examples of copper(I)-photocatalyzed cycloalkene cyclodimerizations have been summarized in Houben-Weyl, Vol. 4/5a, pp 280-292 as was the copper(I) chloride photocata-lyzed isomerization of cycloocta-1,5-diene to tricyclo[3.3.0.02 6]octane (see Houben-Weyl, Vol. 4/5 a, p 231). This same reaction has recently been used for the preparation of 4-oxatctra-cyclo[6.3.0.02,(,.07,1 L]undecanes.10... 

For synthetic purposes, the conversion of diallyl ether (11, X = O) to 3-oxabicy-clo[3.2.0]heptane (12, X = O),11 of ethyl JV,7V-diallylcarbamate (11, X = NC02Et) to A7-car-boethoxy-3-azabicyclo[3.2.0]heptane (12, X = NC()2Et)12 and of 4-hydroxyhepta-l,6-diene (13) to 3-hydroxybicyclo[3.2.0]heptane (14)13 have become the most useful copper(I)-photocat-alyzed (intramolecular) cyclodimerizations of alkenes. 

Similarly, the complex 3 was formed from 3,3-dimethylcyclopropene and 2,2 -bipyridyl( 4-cy-cloocta-l,5-diene)nickel, which on treatment with maleic anhydride at 25 °C gave anti-3,3,6,6-tetramethyltricyclo[3.1.0.02,4]hexane (4) in >90% yield. Displacement of the hydrocarbon ligand from 3 with 3,3-dimethylcyclopropene proceeds at > 90"C. Since the complex 3 is regenerated in this step, 3 is the catalyst in the cyclodimerization of the cyclopropene.120... 

Cyclodimerization of isoprene to 1,5-dimethylcycloocta-l,5-diene and disproportion with a rhenium oxide catalyst and isobutene produce 2,6-dimethylhepta-l,5-diene. The diene is hydroformylated to citronellal, which after hydrogenation produces citronellol (137). 

For example, the dinitrosyliron(O) complex can be formed cathodically. This complex is able to catalyze the cyclodimerization of conjugated dienes (Scheme 8) 259,260) in case of 1,3-butadiene, 20000 turnovers are obtained per hour with... 

Scheme 8. Electro-generated dinitrosyliron(O) complex as catalyst for the cyclodimerization of conjugated dienes...

Cyclooligomerization. Ever since the first cyclodimerization reaction of 1,3-butadiene to yield 1,5-cycloctadiene catalyzed by a so-called Reppe catalyst was reported45 [Eq. (13.12)], cyclooligomerization of conjugated dienes has been intensively studied ... 

Homogeneous nickel complexes proved to be versatile catalysts in dimerization and trimerization of dienes to yield different oligomeric products.46-55 Depending on the actual catalyst structure, nickel catalyzes the dimerization of 1,3-butadiene to yield isomeric octatrienes, and the cyclodimerization and cyclotrimerization to give 1,5-cyclooctadiene and all-trans-l,5,9-cyclododecatriene, respectively46 56 [Eq. (13.13)]. Ziegler-type complexes may be used to form cis,trans,trans-1,5,9-cyclododecatriene37,57 58 [Eq. (13.14)], which is an industrial intermediate ... 

By using the hypersensitive molecular mechanistic probe 2-(2-methoxy-3-phenylcy-clopropyl)-5-methylhexa-2,4-diene in the 2 + 2-photocycloaddition of [60]fullerene, it was shown that the reaction proceeds via a biradical and not a dipolar intermediate.6 Zirconium-induced cyclodimerization of heteroaryl-substituted alkynes produces tetrasubstituted cyclobutenes with high regio- and stereo-selectivity.7 The ruthenium-... 

Catalyzed oligomerization and co-oligomerization of conjugated dienes have been performed with a wide range of transition-metal complexes. Catalytic cyclodimerizations of conjugated dienes have also been performed selectively [27]. Thus, a catalytic amount of CpRuCl(diene) and Ag(0S02CF3) led to the formation of 1,5-cyclooctadiene, dimethylcyclooctadienes, and 6-methyl-... 

Very interesting and potentially useful tandem processes shown in Scheme 34 [107-109] involve the conjugated diene cyclodimerization shown in Eq. 2 in Scheme 2 followed by trapping of the resultant 7r-allypalladium derivatives with an amine or alcohol. In this chapter, the vast and important... 

Even in the absence of Lewis acid functions, zeolites can accelerate gas phase Diels-Alder reactions. This rate enhancement, for instance in the butadiene cyclodimerization, is attributed to a concentration effect inside the zeolite pores. The effect is however not zeolite-specific any adsorbent with affinity for dienes, such as a carbon molecular sieve, displays similar effects (5). 

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