But-2-enes

Numbers refer to the positions of the double bond for example, butylene-1 (or butene-1 or but-l-ene) is CH3CH2CH= or but-2-ene) is CHgCH CHCHg,... 

The palladium-promoted conversion of 1,3-dienes to pyrroles proceeds via 4-acetoxy-2-alkenylpalladium complexes (Scheme 50g) (81CC59), and a similar pathway may be involved in the palladium mediated reaction of but-2-ene-l,4-diol with primary amines to give A-substituted pyrroles (74CC931). 

The possibility of electrocyclic valent tautomerism of the dithiins 55 with the product of their ring opening, but-2-ene-l,4-dithione (56), has been considered (85KGS1443 96T12677). However, all the experimental data provide evidence for the dithiyne form 55 (85KGS1443 96T12677). 

The synthetic utility of the alkene metathesis reaction may in some cases be limited because of the formation of a mixture of products. The steps of the catalytic cycle are equilibrium processes, with the yields being determined by the thermodynamic equilibrium. The metathesis process generally tends to give complex mixtures of products. For example, pent-2-ene 8 disproportionates to give, at equilibrium, a statistical mixture of but-2-enes, pent-2-enes and hex-3-enes ... 

Irradiation of Z-but-2-ene 8 initiates a cyclodimerization reaction, even without a photosensitizer." This cycloaddition proceeds from a singlet state and is likely to be a concerted, one-step reaction. Bond formation occurs suprafacial with respect to both reactants, whereupon only the tetramethylcyclobutanes 9 and 10 can be formed ... 

We should also note that 1UPAC changed their naming recommendations in 1993 to place the locant indicating the position of the double bond immediately before the -ene suffix rather than before the parent name but-2-ene rather than 2-butene, for instance. This change has not been widely accepted by the chemical community, however, so we ll stay with the older but more commonly used names. Be aware, though, that you may occasionally encounter the newer system. 

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). 

Further evidence against the formation of a free carbonium ion in the alkylation reaction is obtained from the fact that in the presence of boron trifluoride-phosphoric acid catalyst, but-l-ene, but-2-ene, and i-butene react at different rates with alkylbenzenes, yet they would each give the same carbonium ion. In addition, only the latter alkene gave the usual activation order (in this case the hyper-... 

There is clear evidence that the addition of a sulfonyl radical to an olefin is a reversible reaction. This has been demonstrated for the case of but-2-ene, where cis-trans isomerization of the olefin accompanies addition63,64. Furthermore, 18a and 18b radicals generated by the reaction of Bu3Sn with erythro- and tfireo-2-bromo-3-(phenylsulfonyl)butane, respectively, eliminate benzenesulfonyl radicals to some extent, even at — 67 °C, to form 2-butenes in a non-stereospecific manner, providing also indication that the rotation about the central C—C bond is faster than the /1-cleavage process65. 

The subsequent hydrogenation of butadiene to but-l-ene and but-2-ene is kineti-cally insignificant, and these hydrocarbons have no influence on the rate of the first step. H2S, however, does influence the rate. Briefly, the reaction proceeds over a site where a sulfur atom in the catalyst is missing (see Chapter 9 for details). A high pressure of H2S simply reduces the number of these vacancies and therefore adversely affects the rate. 

Simple second-order kinetics have also been found for the Co(III) sulphate oxidation of the hydroperoxide of 2-methyl-but-2-ene in aqueous solution, although the reaction also shows both acid-inverse and acid-independent routes . Co(IlI) in aqueous sulphuric acid oxidises tert-huiy hydroperoxide with kinetics... 

A few other routes to new silenes involving photochemical processes have been reported, most of which have the structure RR Si=CH2110-112 and are listed in Table I. Photolysis of l,l-dimethyl-2-phenyl-l-silacyclo-but-2-ene 22 in a glass at 77 K led to the siladiene 23, which absorbed at 338 nm,113 as shown in Eq. (17) ... 

Related studies on the thermal conversion of 1,1-dimethyl-l-silacyclo-but-2-ene to 1,1-dimethyl-1-silabutadiene and its reaction with ethylene and cis- and fra/is-2-butene were also described.219... 

A-Methylphthalimide (288) undergoes photoaddition in acetonitrile to ds-but-2-ene (289) to give d.s-l,6,7-trimethyl-3,4-benzo-6,7-dihydroazepine-2,5-dione (290).238 Evidence supports a concerted [ 2 + J2] pathway to the intermediate 291. Similar additions to other alkenes have been reported.239 Electron transfer quenching has been shown to compete with cycloaddition... 

---