1.3- dienes from cyclobutenes

Preparation. Reppe et al. prepared this diene from cyclooctatetraene by the reactions formulated, but regarded it as 1,2-diacetoxy-A -cyclobutene. Criegee et al. established the true structure, and Hill and Carlson report a short preparative... 

MO correlation diagram for conrotatory interconversion of 1,3-buta-diene and cyclobutene (C2 symmetry). (Adapted from reference 14.)... 

A cyclobutene ROCM sequence was also used in a synthesis of racemic sporochnol (410), a naturally occurring feeding deterrent toward herbivorous fish (Scheme 80) [170]. Exposing cyclobutene 406 (0.01 M in boiling 1,2-dichloroethane) in the presence of ethylene to second-generation catalyst C (8 mol%) led to 1,5-diene 407 in 73% yield, along with 9% of the homodimer derived from 407 by involving the less hindered double bond. Site-selective hy-... 

Thiopyranopyrrolizines can be prepared readily from the enamine 170 upon treatment with DMAD. Alternatively, heating of the thiacyclooctadiene derivative 171 in methanol gives the same tricycle 172, but this time as a 5 2 mixture with the (Z,E)-isomer of the precursor 171. These reactions probably involve the the intermediacy of an unstable cyclobutene and/or a zwitterionic diene, as shown in Scheme 51 <1984JA1341>. 

Dimethyl tricyclo[4.2.2.02 -5]deca-3,7-diene-9,10-dicarboxylate adds bromine and iodine only to the less hindered double bond to give the syn 1,2-addition product of the cyclobutene moiety79. The product composition from this compound depends on the temperature and the solvent. At high temperatures, the 1,2-addition predominates over the transannular reaction, but this predominance is small in a solvent like chloroform and is lost in a protic solvent such as acetic acid (equation 47). 

It appears likely that the peak reported by the earlier workers as having the same retention time as methylenecyclobutane, was in fact penta-1,4-diene. Some support for this comes from the observation that highly energetic bicyclopentane (produced by the addition of methylene to cyclobutene) isomerizes to cyclopentene and to penta-1,4-diene at almost equal rates, but no methylenecyclobutane is formed (Elliott and Frey, 1965b). 

Close examination of Table 6 reveals several points of interest. The isomerization of 3-methylcyclobutene yields only diene with none of the cis-isomer. With the transition state suggested for the isomerization of cyclobutene, either isomer could be formed. However, the methyl group away from the ring rather than towards it, with less steric repulsion. 

Discussion. The combination of BCB with BMI significantly improves the properties of the BMI, apparently by tying up the maleimide in a Diels-Alder reaction with the transient diene formed from the opening of the cyclobutene ring. Both the thermal and the thermomechanical properties are improved almost to the level of the BCB... 

The relevant products are the dienes (23) which are the major products. The cyclobutenes are very likely formed from an intermediate bicyclo[1.1.0]butane. Three pathways leading to 23 are considered A, B, and C (Scheme 7.12). 

For example, acetolysis of r/n /-6-tosyloxytricyclo[5.2.0.02 5]nona-3.8-diene proceeded smoothly at 35 C with stereospeeific rearrangement to c.Y0..mj-9-acetoxylricyclo[4.2.1,02 5]nona-3,7-diene (l).30 Interestingly, the rate of acetolysis of the substrate was considerably enhanced kre[25 C = 6.8 x 104) as compared with that of its bicyclic counterpart 2.30 An important conclusion from these studies is that the anchinteric assistances by cyclobutene in the form of homoallylic participation is effective in the stabilization of the carbocation intermediate. It was found in another study that cyclobutene is better than cyclobutane in terms of anchimeric assistance.31... 

POLYHYDROGEN FLUORIDE REAGENT 1-FLUORO-ADAMANTANE. Dichloroalkane synthesis is shown in cis-DI-CHLOROALKANES FROM EPOXIDES cis-1,2-DICHLORO-CYCLOHEXANE. Nitrile functionality can be introduced from a ketone, as in NITRILES FROM KETONES CYCLOHEXANENITRILE, or from a reactive diene, as shown in 2,3-DICYANOBUTADIENE AS A REACTIVE INTERMEDIATE BY in situ GENERATION FROM 1,2-DICYANO CYCLOBUTENE 2,3-DICYANO-1,4,4a,9a -TETRAHYDRO-FLUORENE. 

Consequently, it is not surprising that the rate ratio for solvolysis of 99 100 was found to be greater than 2.5 x 106 and that at 150°C 100 did not solvolyze at all.395 This evidence is kinetic. Unlike the cases of the cyclobutene—1,3-diene and cyclohexadiene—1,3,5-triene interconversions, the direct product here is a cation, which is not stable but reacts with a nucleophile and loses some of its steric integrity in the process, so that much of the evidence has been of the kinetic type rather than from studies of product stereochemistry. However, it has been shown by investigations in super acids, where it is possible to keep the cations intact and to study their structures by nmr, that in all cases studied the cation that is predicted by these rules is in fact formed.396... 

In some cases the nature of the products resulting from photolysis of dienes and trienes depends on whether the reaction occurs from the excited singlet state or the triplet state. The triplet states can be populated exclusively by suitable sensitizers. For example, 3-methylene-1, 5-hexadiene gives a cyclobutene derivative (A) from the singlet state and tricyclo ring compound (B) from triplet state ... 

The products obtained upon direct excitation of butadiene,291 norbornadiene,292 cyclohexadiene,293 and other dienes294 are also completely different from those obtained by photosensitization, again demonstrating quite different chemical reactivity of excited singlet and triplet dienes. However, 1,l -bicyclohexenyl295 and 1,3-cycloocta-diene298 represent exceptions, undergoing both direct and sensitized photoisomerizations to cyclobutene derivatives. 

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