Cyclodimerization

Due to their tendency to dimerize in different thermal conditions, the formal [2 -I- 2] cycloaddition reaction of methylenecyclopropane derivatives and their 

When heated at 190°C, T-halo- 96 and 453 (Table 36, entries 1 and 2) and ethoxymethylenecyclopropane (454) (entry 3) gave the head-to-head dimerization in good yield as a mixture of cis and trans isomers, with the latter slightly prevailing [27], 

In contrast, perfluoromethylenecyclopropane (105) failed to give any thermal dimerization (cesium fluoride promotes its dimerization, but not in a [2 -I- 2] fashion) [29], while tetrafluoroethylene and other 1,1-difluoroalkenes readily undergo thermal [2 -t- 2] dimerization [123]. 

Entry X Y MCPs Conditions Product Yield Diastereomeric ratio Reference 

The reported proposed sequence also offers two additional alternative mechanisms for the cyclodimerization of BCP (3), involving either intermediate 463 or 464 [6a, 13b]. However, they appear less likely, requiring successive three-membered ring fissions and formations. Alternatively, a thermally allowed concerted [jt2s + jc2a -I- r2a] pericyclic reaction involving the Walsh type molecular orbital of cyclopropane [124] has been proposed (Fig. 4) [13b]. 

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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. 

Cycloaddition of norbornadiene with allene takes place to yield the cyclobutene derivative 10[5], Cyclodimerization of 1,2-cyclononadiene (11) affords a mixture of stereoisomers of the cyclobutane derivatives 12[6,7],... 

Production of ethylbenzene from butadiene has been iavestigated by many researchers. It consists of two steps cyclodimerization of 1,3-butadiene to 4-vinylcyclohexene and dehydrogenation of the vinylcyclohexene to ethylbenzene. 

A general type of stabilization for iminoboranes is a cyclodimerization, which yields diazadiboretidines (RBNR )2 that are isoelectronic with cyclobutadienes. 

Dioxopiperazines are amongst the most ubiquitous of natural products (75FOR(32)57) and they are formally derived by the cyclodimerization of a-amino acids (69CCC4000) or their esters. A number of methods are available for their oxidation to the corresponding pyrazines. Treatment of 2,5-dioxopiperazines with triethyl- or trimethyl-oxonium fluorobor-ate followed by oxidation with DDQ, chloranil or iodine results in pyrazine formation, usually in high yields (Scheme 63) (72JCS(P1)2494). 

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... 

Irradiation of Z-but-2-ene 8 initiates a cyclodimerization reaction, even without a photosensitizer." This cycloaddition proceeds from a singlet state and is likely to be a concerted, one-step reaction. Bond formation occurs suprafacial with respect to both reactants, whereupon only the tetramethylcyclobutanes 9 and 10 can be formed ... 

The cyclodimerization of 1,3-butadiene was carried out in [BMIM][BF4] and [BMIM][PF(3] with an in situ iron catalyst system. The catalyst was prepared by reduction of [Fe2(NO)4Cl2] with metallic zinc in the ionic liquid. At 50 °C, the reaction proceeded in [BMIM][BF4] to give full conversion of 1,3-butadiene, and 4-vinyl-cyclohexene was formed with 100 % selectivity. The observed catalytic activity corresponded to a turnover frequency of at least 1440 h (Scheme 5.2-24). 

The process which was developed hy DOW involves cyclodimerization of hutadiene over a proprietary copper-loaded zeolite catalyst at moderate temperature and pressure (100°C and 250 psig). To increase the yield, the cyclodimerization step takes place in a liquid phase process over the catalyst. Selectivity for vinylcyclohexene (VCH) was over 99%. In the second step VCH is oxidized with oxygen over a proprietary oxide catalyst in presence of steam. Conversion over 90% and selectivity to styrene of 92% could he achieved. ... 

All methods37 using the cyclodimerization of two dipyrrolic building blocks for the synthesis of porphyrins are restricted regarding symmetry. This approach is limited to the synthesis of porphyrins which are centrosymmetrically substituted or porphyrins which possess symmetry in one or both halves of the structure. 

The rigorous temperature of these procedures can be avoided when 5-bromo-5 -(bromomcth-yl)dipyrrylmethene hydrobromides 12 a, 5-bromo-5 -(bromomethyl)dipyrrylmethene perbrom-ides 12b or a mixture of both are used, instead of 5-bromo-5 -methyldipyrrylmethene hydrobromides 6. These compounds 12 can be cyclodimerized by heating in formic acid to produce porphyrins. 

Treatment of diallenyl sulfone 354 with n-butyllithium resulted in a cyclodimerization to afford 2,6-dithiaadamantane derivative 356. This dimerization is considered to be initiated by formation of the a-sulfonyl carbanion 355 and to proceed through a carbanion walk or carbanion tour process426. 

Hofmann elimination to correspondingly substituted cyclopentadienones which, depending on the nature and the nucleophilicity of the base as well as the nature of the substituents RLand Rs, undergo [2+2] or [4+2] cyclodimerization or in situ Michael addition to yield compounds 69, 70, and 71, respectively (Scheme 14) [44,70]. 

The dimerization of 1,3-cyclohexadiene gives 30% adduct after 20 h at 30 °C [32]. In the presence of a catalytic amount of tris(p-bromophenyl) aminium hexachloroantimonate (ArsN SbCle Ar = /iBrC6H4) in CH2CI2 at 0°C, the cyclodimerization occurs in 15 min with 70% yield with a greater diastereos-electivity endojexo = 5 1) than that observed under thermal... 

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. 

An alternative mode for dehydrobenzoannulene decomposition was recently reported by Vollhardt et al. [58]. Non-planar hybrid 81, prepared in low yield via cyclodimerization of known triyne 82 [Eq.(3)], reacted explosively at ca. 250°C to give a nearly pure carbon residue. Solvent extraction of the black powder failed to yield soluble materials such as fullerenes however, analysis of the residue by TEM showed formation of bucky onions and bucky tubes [59], in addition... 

Cyclodimerization is observed in the vanadium-catalyzed reaction of the arylidene malononitriles 39 in the presence of MesSiCl, giving 40 diastereos-electively as shown in Scheme 23 [59]. 

When phenyl (Ph) groups replaced both Me3Si groups, again a rather unstable 1,2-disilacyclobutane dimer appeared to be formed,90 as shown by NMR data but when f-butyl replaced a Me3Si group, the silene failed to dimerize.87 Thus, it is evident that whether or not head-to-head [2 + 2] cyclodimerization occurs depends on the bulk of the substituents on both sp2-hybridized silicon and carbon. 

Finally, Eq. (28) documents a unique situation in which a bis-silene was observed to undergo both head-to-tail and head-to-head [2 + 2] cyclodimerizations. Photolysis of the bisdiazoalkane 80 in benzene apparently... 

The first evidence for a phosphasilene was reported in 1979.19 Thermolysis of 1,2-phosphasiletane 1 at 100°C gave the transient silylidenephos-phane Me2Si=PPh 2, which undergoes head-to-tail [2 + 21-cyclodimerization, as well as an insertion reaction into the Si—P bond in 1, leading to 3 and 4 (Scheme 1). 

Oxidation of arylphospholes (17) by peroxides led to phosphole oxides (84) that dimerized to the corresponding phosphanorbomene derivatives (85) (Scheme 23) [36, 38, 66], As in earlier cases, the cyclodimerization took place in a regio- and stereospecific manner. The interesting observation was that, due to the bulky P-substituent, oxidation was slower and the phosphole oxides (84) became relatively stable hence, they could be characterized by NMR. 

Semiempirical and ab initio calculations were performed on the cyclodimerization of 1-methylphosphole oxide (86). The relative order of the values of the heat of formation for the transient states leading to the possible isomers (87-94) confirmed that the formation of the isomer prepared (87) is indeed favored to a high extent (Scheme 24) [67], The selectivity can be explained by steric reasons and kinetic factors. 

Methylenecyclopropane itself undergoes the cyclodimerization at elevated temperatures (200-250°C) to give predominantly the head-to-head dimer 445 vs its head-to-tail isomer 446 (Scheme 61) [116,117], albeit in poor yield. 

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]. 

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