Buta-l,2,3-trienes

It is interesting to note that the yield of 39 can be increased to 95 % in the presence of a Ni(0) catalyst (see Section I.3.I.2.).43 In unsymmetrically substituted buta-l,2,3-trienes, head-to-head dimerization takes place as well, as seen in the examples below.44 The intermediate cyclopropy-lidenecumulenes 41 are formed from vinylidene insertions into alkenes and reactions occur at the terminal unsaturated site of the cyclopropylidene group. Structures 42 were confirmed by X-ray crystallographic analysis and revised the original assignment of the head-to-tail structures for these derivatives.45... 

If one compares the solvolyses of 2-bromo-l,l-diphenyl-4-(p-methoxyphenyl)-but-l-en-3-yne (57) and4,4-diphenyl-l-bromo-l-(p-methoxyphenyl)-buta-l,2,3-trienes (58, X = Br) in aqueous ethanol (equation 21), the destabilization of the intermediate cation 59 by the large inductive effect of the triple bond as compared to its conjugative effect is evident. Only in the case of 58 could the substitution product butatrienyl enol ether 60 be isolated in 40% yield, while it was only detected by UV and IR spectroscopy in the solvolysis product of 57. The faster observed reaction rate of 58 as compared to 57 was ascribed to a difference in their ground-state energies. ... 

A synthesis of substituted buta-l,2,3-trienes 168 from the double dehydrohalogenation of gem-dichlorocyclopropanes 167 has been described. This reaction does not involve cyclopropylidene but likely proceeds through the chlorocyclopropene-vinylcarbene-l-chlorobuta-... 

Buta-l,2,3-trienes. Newkome et al. were able to prepare the unstable (Z)-and (E)-l,4-diphenyl-l,4-di(2-pyridyl)butatrienes, (2) and (3), by didehydrox-ylation of (1) with P214 in anhydrous pyridine. Reduction of (1) with the more usual reagents (SnCl2 or PBra) resulted mainly in formation of (4). 

Cyclo-octa-l,5-diyne, prepared in 2% yield by dimerization of buta-l,2,3-triene, has been examined by X-ray diffraction and shown to be almost planar with C—C C bond angles of 159.3°. Irradiation of cyclo-octa-l,5-diyne gave buta-l,2,3-triene, and treatment with ba gave cyclo-octa-tetraene. Cyclo-octa-l,5-diyne underwent Diels-Alder reactions with two molecules of buta-1,3-diene and 2,3-dimethylbuta-1,3-diene, but gave complex mixtures of products when treated with bromine, iodine, dimethyl acetylenedicarboxylate, or tcne. ... 

Buta-l,2,3-trienes (66) have been alkylated by organocuprates to give mainly the E-1,3-diene regioisomer (67), but side products such as the other regioisomer of (67) and the corresponding acetylene are present as substantial impurities. ... 

Zinc can also defluorinate fluorocarbons via a SET pathway. Fluorinated buta-1,2,3-trienes have been stereoselectively defluorinated to divinylalkyne derivatives 6141 and fluorine is eliminated from the CF(CF3)2 group in polyfluoroaromatic compounds.143 The products of the defluorination of perfluoro(l,2,3,4,5,6,7,8-octahydronaphthalene) (7) depend on the polarity of the solvent.142 The higher the polarity of the solvent the more extensive is the defluorination... 

Thus, 7,7-difluorobicyclo[4.1.0]hepta-l,3,5-triene (l,l-difluoro-l//-cyclopropabenzene, 2) was first made by the addition of l,2-dibromo-3,3-difluorocyclopropene to buta-1,3-diene, followed by double dehydrobromination with potassium hydroxide in triethylene glycol at 80 °C and low pressure. Reaction of the adduct of the 1,2-dichloro analog 1 (X = Cl) similarly gave the difluorocyclopropabenzene (2). In subsequent work, potassium /er/-butoxide was used as base and the reaction has been carried out in tetrahydrofuran at low temperatures, which gives a better yield of product. ... 

In contrast we found that the addition of lithium to the central double bond of buta-trienes 722 takes place very easily The resulting 2,3-dilithio-l,3-butadienes 123 which are stable towards excess lithium interestingly are cleaved to 1,1-dilithio-1-alkenes 81 and 725 in the presence of mercury(II) chloride (see Sect. 3.3). 

Polyacetylene reacts with chlorine rapidly to give a white polymer that is equivalent to poly(l,2-dichloroethylene) and no stereospecificity has been reported on the chlorinated polymer. The addition of halogens is an important general reaction of carbon-carbon double bonds in a stereospecific and regiospecific sense. For instance, the electrophilic additions of chlorine to ethylene, buta-1,3-diene and hexa-l,3,5-triene have been shown to proceed by 1,2-trans (anti), 1,4-cis (syn) and 1,6-trans (anti) attack, respectively. This stereospecificity has been rationalized with a mixing rule of a-n orbital interaction. The reaction of a long conjugated polyene like polyacetylene with chlorine may produce an atactic chlorinated polyene, because random l,2n additions occur to result in a random addition product. ... 

The size of the K-system chosen has important implication on the structural and functional aspects of metal binding. To explore the size effect calculations were performed on the cation-ir complexes of Li+ and Mg + with the Jt-face of linear and cyclic unsaturated hydrocarbons [45]. In the case of the acyclic Jt-systems, we started with the simplest system, e.g. ethylene followed by buta-1,3-diene, hexa-l,3,5-triene, and octa-1,3,5, 7-tetraene with 2, 3 and 4, conjugated jt units, respectively. These linear systems with two and more number of jt units can have various conformations wherein the jt units can have cis, trans or a combination of both cis and trans orientations. Similarly for cyclic systems cyclobutadiene, benzene, cyclooctateraene, naphthalene, anthracene, phenanthrene and naphthacene have been included. Thus a wide range of sizes for aromatic systems have been covered. 

Sol 2. (c) On heating, potassium salt of l,2-di(( )-buta-l,3-dienyl)cyclo-hexanol transforms into cyclotetradeca-3,7,9-trien-l-one by an oxy [5,5] sigmatropic process. The rearrangement does not proceed by a sequence of [3,3] shifts it is indeed a result of [5,5] shift. 

MO studies have shown that the Diels-Alder reaction of substituted selenocar-bonyl compounds with buta-1,3-diene or 2-methoxybuta-1,3-diene proceeds through a concerted, asynchronous transition state. Q ,jS-Unsaturated seleno ketones and seleno aldehydes readily undergo 4 - - 2-cycloaddition with alkenes and 4 - - 2-dimerization. The reaction of phosphaacetylene (138) with buta-1,3-diene produces triphosphatri-cyclooctenes (139) through a sequence involving Diels-Alder, ene, and intramolecular 4 -I- 2-cycloadditions (Scheme 53). The 4 + 2-cycloaddition of phosphaalkynes with 5,8-bis(trimethylsilyl)cycloocta-l,3,6-triene (140) readily yields the tricyclodecadiene (142) via the bicyclic intermediate (141) (Scheme 54). "... 

The use of Cr(0)-promoted 6 r - - 2 r-cycloaddition in the synthesis of pharmacologically active natural products has been reviewed. The 6 -I- 3-cycloaddition of dimethylaminofulvene (194) with benzoquinones provides an efficient route to the 3-oxabicyclo[4.3.0]nonane system (195) (Scheme 76). Density-functional theory calculations provide evidence for a feasible concerted antara-antara cycloaddition for the 6-I-4-reaction of c -hexa-l,3,5-triene with buta-1,3-diene. Azulene-1,5-quinones (196) and azulene-l,7-quinones undergo a variety of 6-1-4- and 2-1-4-cycloaddition reactions with cyclic dienes and cycloheptatrienes to produce cycloadducts, e.g. (197) and (198) (Scheme 77) ... 

Other examples where the intramolecular variant has been employed include the direct formation of bridgehead bicyclo[3,n,l]alkenes from 2-(alkenyl)buta-1,3-dienes and the preparation of rrans-perhydronaphthalenes, bearing an angular methyl group, from acyclic trienes. ... 

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