Tricyclic dimer

In the crystal of 1,4-dicinnamoylbenzene (1,4-DCB) (see Fig. 12), the distances between the intermolecular photoadductive carbons are 3.973 and 4.086 A for one cyclobutane ring, and 3.903 and 3.955 A for the other. The two topochemical pathways may occur competitively in a single crystal of 1,4-DCB at the initial stage of reaction. Then, both intramolecular photodimerization and intermolecular photopolymerization of the diolefinic mono-cyclobutane intermediate occur competitively to give tricyclic dimer 21,22,23,24-tetraphenyl-l,4,ll,14-tetraoxo-2(13),12(13-diethanol, [4.4] para-cyclophane or oligomers (Hasegawa et al., (1985). On photoirridation at room temperature the 1,4-DCB crystal gives >90% of the tricylic... 

Cyclic dienes which are locked in the cisoid conformation, e.g. (82), are found to react very much faster than acyclic dienes in which the required conformation has to be attained by rotation about the single bond (the transoid conformation is normally the more stable of the two). Thus cyclopentadiene (82) is sufficiently reactive to add to itself to form a tricyclic dimer, whose formation—like most Diels-Alder reactions—is reversible. 

For rigid alkenes, triplet sensitisation brings about photocycloaddition via the 3(Jt,7t ) state. These reactions are neither concerted nor stereospecific. Cyclopentene produces a tricyclic dimer ... 

Unsaturated cyclic sulfones, e.g. 4,5-dihydrothiophene 1,1-dioxide and 3,4,5,6-tetrahy-drothiopyran 1,1-dioxide, afford mixtures of tricyclic dimers, both in sensitized irradiations and in [60Co] y-irradiation.23... 

We have found a rare reaction where two topochemical processes occur competitively in a single crystal, that is, a competitive photocyclo-dimerization and -polymerization in the crystal of 1,4-dicinnamoylbenzene 3 (15). On photoirradiation with a mercury lamp (100W) at 20°C for 9 h, the crystals of 3 (2.00g), dispersed in heptane (400 ml), are transformed into amorphous substances consisting of a tricyclic dimer, 21,22,23,24-tetraphenyl-1,4,11,14-tetraoxo-2 (3), 12 (13) -diethano(4,4Jparacyclophane, 4 (isolated yield 57%) (16), a mixture of oligomers (ca. 30%) and unreacted 3 (7%). 

Hasegawa et al. [66] studied the photodimerization of methylchalcone-4-car-boxylate at different temperatures and for different irradiation times. The observed maximum yield occurs at about 258 K, implying that the transport of energy that competes with thermal activation is favorable at this temperature. The photochemical conversion of 1,4-dicinnamoylbenzene (1,4-DCB) at different temperatures shows that the tricyclic dimer yield reaches a maximum at 298 K, the reaction taking place even at 256 K (Fig. 6) [67],... 

The energy of the double bond may be raised by other means, e.g. by strain or by antiaromaticity [224]. Miller, a pioneer in the field of organofluorine chemistry, generated tetrakis(trifluoromethyl)cyclobutadiene and showed that it reacts to form a tricyclic dimer [242] which can be converted to the corresponding cubane and cuneane derivatives by ultraviolet radiation [243] (Figure 7.72). 

Besides polymeric material, a mixture of three ene reaction products is obtained (18% combined yield) in the thermal reaction, which proceeds at 20°C within several minutes. Similar results are found when zeolites with a large pore diameter (5-13 A) are used as catalysts. After irreversible addition of the cyclopropene to the zeolite, only polymeric material is obtained. Zeolites with 3- or 4-A pores give rise to a product spectrum similar to that obtained with phosphane-free palladium(O) catalyst systems, i.e. 90-97% of the tricyclic dimers are formed from 1- or 3-methylcyclopropene at temperatures between — 35 and 35 C. The palladium(II) chloride catalyzed reaction almost quantitatively yields a 2 1 mixture of the isomeric 1,2- and l,4-dimethyl-evo-tricyclo[3.1.0.0 ]hexane (lA and IB) from 1-methylcyclopropene. The reaction is strongly exothermic and therefore must be carried out in dilute solutions with effective... 

The cyclodimerization of 3,3-dimethylcyclopropene to form 3,3,6,6-tetramethyl-exo-tri-cyclo[3.1.0.0 ]hexane (4) can also be achieved with pentacarbonyliron in toluene at 95"C in 3 hours (no yield given) or in tetrahydrofuran at 60 °C with catalytic amounts of hexacarbonyl-molybdenum (77% yield, 93% conversion) or hexacarbonyltungsten (86.5% yield). With nonacarbonyldiiron a stoichiometric ring-opening reaction and formation of tricarbonyl(2-methyl-l-propenylketene)iron has been observed (see Section 2.C. for details). A 40% yield of the tricyclic dimer 4 is obtained under the catalytic action of ( / -cycloocta-l,5-diene)(t -cyclopentadienyl)cobalt(I). ... 

With methyl 2-alkylcycloprop-2-enecarboxylates, on the other hand, only [2-1-1] cycloaddition to the tricyclic dimers of cyclopentadiene (18) occurs. Only the strained double bond in the norbornene substructure is cyclopropanated, and the exo-adducts are formed. The Z/E isomeric ratios (6 1 and 2.5 1) are comparable to the ratios obtained in the reactions with norbornadiene vide supra). A mechanism proposing ring opening of the cyclopropene and attack on the metal-coordinated alkene in a concerted way has been suggested in order to explain the stereoselectivity of the reaction. ... 

If, in an early stage of the reaction, a second cyclopropene molecule is coordinated to the nickel, homo-cyclodimerization leading to tricyclic dimers of type 28 may also occur. To prevent the formation of 28, the stationary concentration of the cyclopropene in the reaction mixture must be small, i.e. the cyclopropene must be added slowly. This is especially critical if the electron-poor alkenes are only weakly bound, as is the case with methyl acrylate and the 3-alkyl-substituted acrylates. When acrolein or acrylonitrile are employed, the cycloaddition reaction is inhibited due to the formation of stable bis(alkene)nickel complexes. 

An interesting case of an intramolecular dimerization reaction occurs on treatment of the ethylene-bridged MCP 12 with a nickel/phosphane catalyst system. The tricyclic dimer 13 is formed in 33% yield.The only obvious rationale for this reaction is a proximal ring cleavage occurring at one of the methylenecyclopropanes and subsequent addition to the alkene of the other. 

Dechlorination of 3,4-dichloro-l,2,3,4-tetramethylcyclobutene with lithium amalgam led to the formation of a tricyclic dimer of tetra-methylcyclobutadiene [5], reasonably implicating the cyclobutadiene as an unobserved intermediate ... 

When unsaturated fatty acids are subjected to thermolytic cleavage, dimers, polymers and cyclic compounds are produced. The dimers include alicyclic mono-, di-, and triene dehydrodimers, saturated dimers with cyclopentene structures, tetrasubstituted cyclohexanes and bicyclic and tricyclic dimers (Nawar, 1985). Cyclic mononmers exhibit toxicity in experimental animals (Van Gastel et al, 1984), therefore, generation of cyclic monomers at high temperature frying with oils containing polyunsaturated fatty acids should be carefully monitored. 

The poorer enantioselectivities observed in the Sharpless epoxid-ation of allylic alcohols following the use of 1 1 complexes of various sugar diols compared with diisopropyl tartrate has been ascribed to the observed formation of tricyclic dimers between the former and titanium tetraisopropoxide, whereas the latter gives a monocyclic 2 1 complex. 

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