Cyclobutane 1.2- divinyl

Electron-rich bifunctional vinyl ethers (e.g. ethylene glycol divinyl ether) react with electron-poor alkenes (e.g. TCNE) to produce cyclobutanes in good yields via tetramethylene zwitterion intermediates. In some cases, cyclobutanes reacted with the solvent (MeCN) to yield tetrahydropyridines.9 Trifluoromethanesulfonimide is an... 

Die Cycloocta-l,5-diene sind moglicherweise keine primaren Photopro-dukte, sondern durch Cope-Umlagerung der entsprechenden 1,2-Divinyl-cyclobutane entstanden. 

In the divinyl substituted tethered aryl systems shown previously in Sch. 13 the steric interaction between the cyclobutane and the ortho methoxy groups is obvious. The authors indicate that the observed product, the exo-exo isomer, has 6kcal/mol less strain energy than the exo-endo compound. The suggestion that the major photoadduct is formed because it has less steric strain implicates reversibility in a reaction pathway which eventually leads to the most thermodynamically stable product. However, it does not preclude a reversibly formed exciplex or ground state complex. 

Similar oxidative additions involving the inner carbon atoms of the butadiene molecules can generate complexes having the formal structures 7.26 and 7.27. These may also be formed from 7.25 through tautomerization. Regeneration of 7.24 from these species involves elimination of vinyl cyclohexene and divinyl cyclobutane, respectively. 

VCH = vinyl cyclohexene, COD = cyclohexadiene, CDT = cyclododecatriene, DVCB = divinyl cyclobutane)... 

The cyclooligomerization reaction is not confined to BD as the monomer. Activated or monosubstituted 1,3-dienes also react, but reaction rates are usually slow, and selectivity and turnover numbers (TONs) are low. Cyclotrimerization and cyclodimerization of substituted 1,3-dienes - either alone or in admixture with BD - give numerous isomers of substituted CDT, COD, VCH and divinyl-cyclobutane (DVCB). For example, isoprene [34], 1,3-pentadiene [35], 2,3-dimethylbutadiene [36], 1,3-hexadiene [37], and even 1-vinyl-1-cyclopentene [38] do react (eqs. (2)-(6)). 2,4-Hexadiene is inert. 

The use of crystal-to-crystal [2+2] photodimerizations as a means to construct crystalline polymers has been pioneered by Hasegawa and co-workers [32]. To construct the polymers, reactants with two double bonds in the form of 1,4-divinyl-benzenes, such as methyl 4-[2-(4-pyridyl)ethenyl]cinnamate (Scheme 2.3.2) were employed [33]. One-dimensional chains composed of repeat units of cyclobutane... 

Fur die Synthese von trans-1, 2-Dimethyl-l,2-divinyl-cyclobutan (VIII) wurden prapara-tive Bedingungen ausgearbeitet (s. ds. Handb., Bd. IV/4, S. 321). 

As well as [2 + 2], [4 + 4] and [4 + 4 + 4] products, the cyclodimerization of conjugated dienes also yields [4 + 2] cycloadducts47Thus, butadiene gives 4-vinyleyelohexene, ci.v-1,2-divinyl-cyclobutane and 1,5-cyclooctadiene. The influence of the catalyst and reaction conditions on the product distribution has been carefully investigated50- 53. Efforts towards asymmetric induction have concentrated on the stereoselective synthesis of 4-vinylcyclohexene as the sole chiral product. 

Eight-membered rings can be obtained by [4+4]-cycloadditions of 1,3-dienes [1] via diradicals or other intermediates. Synthesis of such compounds has been achieved by thermal, [2] photochemical, [3] and by metal-catalyzed [4] processes these reactions have been the subject of extensive mechanistic [5] and theoretical [5c] studies. Their strategic applications in natural product synthesis have been reviewed. [5d] The thermal version has generated little interest, except in orthoquino-dimethane dimerizations and in cycloreversions the Cope rearrangement of 1,2-divinyl-cyclobutanes [3] is more commonly used. [4+4]-Cycloadditions are also used with 1,3-dipoles or mesoionic heterocycles for the synthesis of six- and seven-membered rings. Sometimes also [6+4]-cycloadditions are... 

Butadiene gives a few percent of /ra/j5-1,2-divinyl-cyclobutane when dimerised without added catalysts at 150°C and 100 atm , while the dimerisa-tion of 2-chlorobutadiene at 35-40°C yields in nearly equal amounts a cyclohexene and a cyclobutane derivatives ... 

Related to the PE spectra of vinyl and divinyl cyclobutane are those of ethinyl and cis- and trans-diethinyl-cyclobutane 84 to 86 and derivatives of them. The PE spectra of all three compounds have been recorded A strong interaction is found between the tr system and the four-membered ring. In the case of 85 and 86... 

AAButadiene Dimerization. The production of vinylcyclohexene is of a significant industrial importance as it is a route to styrene manufacture via the dehydrogenation of VCH. Cu zeolites, particularly Cu(I) Y zeolites proved to be active, selective and stable. They prevented the formation of cycloocta 1,5 diene and divinyl cyclobutane and also the formation of trimers. MAXWELL et al. (47) have studied the deactivation of this cuprous catalyst and found that the lower the acidity, the longer the lifetime. Consequently they devised a preparation procedure that involved reduction of cupric ions with NH3 at moderate temperature so that the Bronsted acidity generated by the reduction of cupric ions is neutralized by ammonia. As the temperature is increased little NH3 reamined as NH4 ions. These observations confirm that indeed this is not a Bronsted acid catalysis and confirm the involvement of the cuprous ions. 

The rate of the reaction is influenced by the energy of the starting materials, and strained compounds react at much lower temperatures than do unstrained molecules. The 1,2-divinyl derivatives of cyclopropane, cyclobutane, and cyclopentane illustrate this point. c -Divinylcyclopropane rearranges to qrcloheptadiene... 

Fig. 12.6 Catalytic conversion of 1,3-butadiene into cyclic dimers. VCH = 4-vinylcyclohexene COD = as-1,5-cyclooctadiene DVCB = cis-1,2-divinyl-cyclobutane. When L = P(OC,H PH, ), at 60 C, COD = 94.5% L = PPhEt, COD = 38.2% DVCB = 12.6% VCH = 46.0% of total products.

Photosensitized reaction of 1,3-butadiene gives a mixture of cis- and trans-1,2-divinyl cyclobutanes and 4-vinyl cyclohexene [22],... 

The ratio of the products, divinyl cyclobutanes to cyclohexene depends on the triplet energy (Et) of the sensitizer used. With sensitizer Et > 60 kcal/mol, both s-trans and s-cis are excited having a preference to s-trans. With sensitizer Et 55 kcal/mol, the s-cis is preferentially excited. Excited s-trans gives only cyclobutanes, while the excited s-cis conformer gives both cyclobutane and cyclohexene. For instance, sensitizer benzophenone of Et 68.5 kcal/mol gives divinylcy-clobutanes and cyclohexene in a ratio of 92 8, whereas sensitizer benzil of Et 53.7 kcal/mol, gives divinylcyclobutanes and cyclohexene in a ratio of 55 45 [22]. 

Recently, Billups et al. reported an unusually short, nonphotochemical synthesis of grandisol based on the production of c/s-1,2-divinyl cyclobutanes from 1,3-dienes under influence of zero-valent nickel complexes. ... 

Explain why (a) in butadiene dimerization with a nickel catalyst small amounts of vinyl cyclohexene and divinyl cyclobutane are formed (b) ROMP and RCM are thermodynamically favored reactions (c) Pd-catalyzed cross-coupling reactions can be carried out under ligandless conditions by adding tetralkyl ammonium salts (d) in CTOss-coupling reactions the choice of the added base may play a critical role. 

Ans (a) A reaction similar to (7.2.4.1) and (7.2.4.2), with nickel formally <7-bonded to 3,6-carbon atoms rather than 1,8 or 3,8, will give divinyl cyclobutane (b) in ROMP excess strain energy of the monomer is released, while in RCM change in overall entropy is positive (c) from 7.60, catalytic intermediates are formed according to reaction 7.4.1.7 (d) with different bases, the rates of the individual steps of the catalytic cycles and the resting state of the catalyst may be different, e.g., 7.48, for Cy NMe, but 7.51 for Cs COj. 

CuOTf-catalyzed photocycloaddition of the tetraene 96 produces a mixture of the compounds 98,99, and 100 (Scheme 25). The 1,2-divinyl cyclobutanes 97 initially formed from [2 + 2]-addition of the tetraene 96 undergo further reaction on prolonged irradiation in the presence of CuOTf to form these products. The tricyclic compound 100 arises from intramolecular 2jt + 2Jt addition of cyclooctadiene derivative 99. In fact, the transformation of cis,c/s-l,5-cyclooctadiene 101 to the tricychc compound 102 on irradiation in the presence of CuCl was the first example of an intramolecular Cu(I)-catalyzed photocycloaddition reaction. ... 

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