E-but-2-ene

The [4 + 2] reaction was shown to be stereospecific by the formation of only one geometric silacyclohex-2-ene isomer (537 or 538) in the reaction of 81 with Z- or E-but-2-ene, respectively, whereas the distribution of products deriving from the [2 + 2] cycloaddition (539-541, equation 186) suggests a non-stereospecific course of the reaction54. 

Musilek, K., Kuca, K., Jun, D., et al., 2006a. Synthesis of the novel series of bispyridinium compounds bearing (E)-but-2-ene linker and evaluation of their reactivation activity against chlorpyrifos-inhibited acetylcholinesterase. Biorg. Med. Chem. Lett. 16, 622-627. 

For example, disproportionation of but-2-yl radicals produces a mixture of butenes as shown (Scheme 1.1 I).138 Thermodynamic considerations suggest thai but-l-ene and but-2-enes should be formed in a ratio of ca 2 98. However, the observed 5 4 ratio of but-1-ene but-2-enes is little different from the 3 2 ratio that is expected on statistical grounds (i.e. ratio of f5-hydrogens in the I- and 3-positions). 

We have just seen that when H-H or Br-Br or H-Br or H-OH is added to but-2-ene, only one product is formed. However, when a hydrogen halide or water is added to an unsymmetrical alkene, i.e. one in which the groups attached to one carbon of the double bond are not identical to the groups attached to the other carbon atom, two products are formed. For example, when hydrogen chloride is added to the unsymmetrical alkene but-l-ene, both 2-chlorobutane and 1-chlorobutane are formed ... 

When one considers bromination of ( )-but-2-ene, the product turns out to be the meso R,S isomer, i.e. a single product. 

Note The choice of the constitutional units is dictated by the seniority rules of [1], i.e., but-l-ene-l,4-diyl is senior to but-2-ene-l,4-diyl. 

Electrophilic ailenes such as alkyl buta-2,3-dienoates react with nonactivated alkenes, e.g. (Z)-and (jE)-but-2-ene,3 hex-l-ene,3 2-ethylbut-l-ene,3 cycloalkenes,4 and cycloocta-1,5-diene,4 in the presence of Lewis acids giving methylenecyclobutanes both regio- and stereospecifically.3 5... 

Chloroheptafluorobut-l-ene (3) isomcrizes in the presence of antimony(V) fluoride to give an E Z mixture of but-2-ene 4.8 4-Chloroheptafluorobut-l-ene also gives the isomeric but-2-cne (see also Section 5.1.2.2.). 

A detailed study of the rearrangement of heptafluoro-2-phenylbut-1-ene (11) to but-2-ene 12 and the E,7. equilibration of 12 showed that lithium and sodium fluorides do not catalyze the rearrangement. Cesium, rubidium, and potassium fluorides are effective catalysts, in that order of decreasing reactivity.25... 

A series of increasingly refined experiments were performed. The final example in the series is outlined in Scheme 12.18, where E/Z equilibration is used to monitor substrate (deca-2,8-diene) and product (but-2-ene) equilibration, and a pair of D3-labelled methyl... 

As mentioned in the above sub-section ( Adsorbed Species ), physisorbed, i,e., weakly bound, species can generally be observed with conventional liquid-state spectrometers. It is important to realise, therefore, that many catalytic reactions which occur via fairly loosely bound species can be perfectly adequately followed by such n.m.r. techniques. The isomerization of alkenes has been reported recently by Nagy etal for but-l-ene to cis- and tnz/is-but-2-ene over mixed Sn-Sb oxide catalysts145 and earlier over zeolites.146 A mechanism has been proposed involving a cyclic intermediate. When the catalyst is NiO/Si02, however, dimerization occurs,147 which in the case of propene leads to a mixture of isomers of hexene, the composition of the mixture depending upon the reaction temperature. 13C-enriched propene was used in this study. 

Fig. 3.5. Relationship between log (retention time) and boiling point for a range of hydrocarbons on squalane at 43°C. A, aliphatics (and benzene) B, alicyclics, 1-4 = C5-Cg n-alkanes 5 = 2-methyl-butane 6 = 2-methylpentane 7 = 2,3-dimethylbutane 8-12 = C4-C8 1-olefins 13,15,17 = trans-(but-2-ene, pent-2-ene and hept-2-ene) 14,16,18 = e/s-(but-2-ene, pent-2-ene and hept-2-ene) 19 = 2-methylbut-l-ene 20 = 2-methylpent-l-ene 21 = 4-methylpent-l-ene 22 = 2-methylbut-2-ene 23 = cyclopentane 24 = cyclohexane 25 = methylcyclopentane 26 = methylcyclohexane 27 = cyclo-pentene 28 = cyclohexene 29 = 4-methylcyclohexane 30 = 1,3-butadiene 31, 32 = trans- and cis-1,3-pentadiene 33 = diallyl 34, 35 = trans- and c s-2-methyl-l,3-pentadiene 36 = cyclopentadiene ...

As discussed in Sect. 2, a-selanylalkyllithiums, generated from selenoacetals, can react with various electrophilic reagents, i. e. chloromethyl isopropyl ether for the synthesis of la-hydroxy vitamin D analogues [25] and with propargylic chloride derivatives for the preparation of alkynols [26]. A synthesis of vinyl-cyclopropane derivatives from l,4-dichloro-but-2-ene was achieved with trans stereoselectivity (>93%) in 68-89% yield. This one-pot cyclization, via an intramolecular allylic substitution, required the presence of two equivalents of u-BuLi [26] (Scheme 23). 

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