Trans- pent

Comparison of the results for catalytic isomerization of pent-l-ene to trans-pent-2-ene with the basic and one-electron donating properties of the catalysts led to the conclusion that two different reaction mechanisms operate in double bond isomerization reactions (a) an ionic mechanism which involves proton abstraction from the alkene molecule by the super base site (pAia = 37 for pentenes) and (b) a free radical mechanism which involves the abstraction of a hydrogen atom from the alkene by the one-electron donor center (Scheme 39). 

There are many instances of the isomerisation of alkenes or alkenilic compounds under the influence of transition metal compound catalyst. Some examples arepent-l-ene into cis- and trans-pent-2-ene (RhCl3... 

In Chapter 7, we used heats of hydrogenation to compare the relative stabilities of alkenes. For example, the heats of hydrogenation of pent-l-ene and fran.s-pent-2-ene show that the disubstituted double bond in trans-pent-2-ene is 10 kJ/mol (2.5 kcal/mol) more stable than the monosubstituted double bond in pent-l-ene. 

The stereochemistry about the C-C double bonds of II was assigned by comparison of NMR chemical shifts with those of model compounds (ii) (Chart I). Poly(l,l-diphenyl-l-sila-ci5-pent-3-ene) (VI) (mp 130-136 °C) undergoes phenylthio-radical-catalyzed isomerization 12, 13) to s. mixture of poly(l,l-diphenyl-l-sila-ci5-pent-3-ene) and poly(l, 1-diphenyl-1-sila-trans-pent-3-ene) (VII) (mp 62-70 °C). NMR chemical shifts support these assignments, as shown in Scheme I. 

There is no direct experimental evidence for this complex decomposition and it may well occur by several steps [107]. However, substantial yields of unsaturated carbonyl compounds are formed particularly at high pressures [78] under initial reaction conditions where cool flames propagate. For example, the cool-flame oxidation of 2-methylpentane at 525 °C and 19.7 atm in a rapid compression machine [78] yields no less than 14 unsaturated carbonyl compounds viz acrolein, methacrolein, but-l-en-3-one, pent-2-enal, pent-l-en-3-one, pent-l-en-4-one, trans-pent-2-en-4r one, 2-methylbut-l-en-3-one, 2-methylpent-l-en-3-one, 4-methylpent-l-en-3-one, 2-methylpent-l-en-4-one, 2-methylpent-2-en-4-one, 2-methyl-pent-2-enal and 4-methylpent-2-enal. Spectroscopic studies of the preflame reactions [78] have shown that the unsaturated ketones account for ca. 90 % of the absorption which, occurs at 2600 A. At lower initial temperatures and pressures acrolein and crotonaldehyde are formed from n-pentane [69, 70] and n-heptane [82], and acrolein is also formed from isobutane [68]. 

The specific degradation of epoxides of cis- and trans-pent-2-enes is discussed in Chapter 6, Section 6.1.3. 

Robinson annulation of the n-butylthiomethylene derivative (480) of (+)-2-methyl-4-isopropenylcyclohexanone (479) with trans-pent-5-en-2-one has recently been shown to provide a mixture ( 1 1) of (+)-nootkatone (481) and (-)-7-epinootkatone (482). In previous synthetic studies (Vol. 2, p. 108 Vol. 

By reference to these model systems it has been shown that trans-pent-3-en-2-one exists mainly in the trans form and that on cooling, the population of the trans form increases. 

Scheme 6.2 Possible structures of puckered metallacycle intermediates in the productive metathesis reactions of trans-pent-2-ene.

In the case of 41 (experimental conditions 41 —100 torr, oxygen—10 torr, H2—2 mCi, volume of the pyrex vessel 500 ml, storage period 250-263 days, temperature 90 °C), besides tritiated 41 (6.2%), tritiated methane (15.2%), ethene (1.9%), propene (14.2%), cis-but-2-ene (2.3%) and 2-methylbut-2-ene + trans-pent-2-ene (8.1%) have been isolated. In the case of reaction of with bicyclo[2.1.0]pentane (42), the yields of the tritiated... 

Predict the preferred conformation of the isomeric (cis- and trans-) pent-3-en-2-ones, A, R = CH3. How would you expect the conformational picture to change as R becomes successively larger ... 

The two Schrock carbenes (complexes 2 and 3) are not stereoselective With trans-pent-2-ene the resulting olefin always has the thermodynamic cis/trans ratio while with a>pent-2-ene there is formation, at the beginning of the reaction, of cis and trans olefins in quite the same amounts. 

C5H12Cl3NPt, Trichloro(ir-trans-pent-2-enylammonium)platinum(II),... 

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