Pent-3-ene-2-one

The behaviour of compounds 49 where R = H and CH3 from is even more complicated. For example, 4-methylamino-3-(4-X-phenylazo)pent-3-ene-2-one (49) contains two isomers. The tautomeric form of both isomers was confirmed with the help of 2D H-15N GHMQC spectra. The minor isomer is very similar to the tautomeric form of azo-enamine ( j(15N, H) = 89.6 Hz) the major isomer possesses a somewhat stronger character of hydrazono-ketimine form ( /(15N,1H) = 74.5 Hz about 80% azo and 20% hydrazone). These conclusions are confirmed by the values of <5(15NH2) in the minor isomer it is 6(15N) = —269.0 ppm (i.e. almost the same as in the starting enaminone with... 

Identification of the Fluorescent Species. Figure 2 compares the fluorescence excitation spectra of the polymers with the absorption spectrum of a simple ,/3-unsaturated carbonyl compound (pent-3-ene-2-one) (13). The three spectra are very similar. Figure 2 shows also that the fluorescence from the polymers in the region 300-400 nm cannot be caused by the presence of polynuclear aromatic hydrocarbons such as naphthalene as postulated earlier by Carlsson and Wiles (13). Furthermore, as shown below, the excitation spectrum also differs significantly from that of a fully saturated aldehyde or ketone. 

The behaviour of compounds 49 where R = H and CH3 from is even more compUcated. For example, 4-methylamino-3-(4-X-phenylazo)pent-3-ene-2-one (49) contains two isomers. The tautomeric form of both isomers was confirmed with the help of 2D GHMQC spectra. The minor isomer is very similar... 

Careful re-investigation of the photolysis products of 3,5-dimethyl-y-pyrone in trifluoroethanol (reported last year ") by preparative t.l.c. (Si02) has revealed the presence of two minor, but mechanistically significant, products. One, identified as l,3-dimethyl-6-oxabicyclo[3.1.0]pent-3-en-2-one (145), is the first isolable example of the hitherto elusive cyclopentadienone epoxides, whidi have long been postulated as intermediates in these photolyses. This dienone epoxide is the photo-precursor of the final product, i.e. the isomeric 3,6-dimethyl-a-pyrone, but not of the trifluoroethanol adduct (146). This latter product, as suggested previously, derives directly from the zwitterionic precursor of the bicyclopent-3-en-2-one (145). The other product proved to be cyclopent-l-ene-3,5-dione (147), a photo-rearrangement product of (145). 

There are two early examples in the literature where apparently an intermediately formed bisacceptor-substituted methylenecyclopropane was attacked by a nucleophile. 2-Bromo-3-(dicyanomethyl)pent-3-ene (22) was converted to (2,3-dimethylcyclopropylidene)malonodini-trile (23) in the presence of an excess of triethylamine in dichloromethane at room temperature. 23 could not be isolated but reacted further with 22 to give 6-ethylidene-l,2,5-trimethyl-spiro[2.4]heptane-4,4,7,7-tetracarbonitrile (24) in 70% yield.Also, treatment of 1-[bis(ethoxycarbonyl)methyl]-2,2-dimethylcyclopropanol or its precursor 3-chloro-3-methyl-butan-2-one with the sodium salt of diethyl malonate in tetrahydrofuran under reflux gave small amounts of adducts such as l,l-bis bis[(ethoxycarbonyl)methyl] -2,2-dimethylcyclo-propane and l-[bis(ethoxycarbonyl)methyl]-l-[(ethoxycarbonyl)methyl]-2,2-dimethyl-cyclopropane via dehydration and nucleophilic attack. ... 

For conjugated dienes, the heat of hydrogenation is less than the sum for the individual double bonds. For example, fran.v-pcnta-1,3-dicne has a monosubstituted double bond like the one in pent-l-ene and a disubstituted double bond like the one in pent-2-ene. The sum of the heats of hydrogenation of pent-l-ene and pent-2-ene is —242 kJ (—57.7kcal), but the heat of hydrogenation of fra s-penta-l,3-diene is only —225 kJ/mol (—53.7 kcal/mol), showing that the conjugated diene has about 17 kJ/mol (4.0 kcal/mol) extra stability. 

The same discrepancy is found in the abilities of metals to afford one of the other isomers in hydrogenation of cw-penta-1,3-diene. Half-reduction over Co leads to 90% yield of rrans-pent-2-ene, whereas over Cu 70% pent-l-ene is obtained even though the disubstituted double bond is more difficult to hy ogenate than the terminal one ... 

Transfer constants can, in principle, be derived from the relative rates of production of telomers of successively higher n values. Thus for Scheme 15.2 one can derive eqn. (23), where w = 7, 10, or 13 depending on which series of telomers is being considered. Likewise for Scheme 15.3, eqn. (24) can be obtained, where m= 12, 14, or 16, again depending on the series of telomers see Katz (1977a) for derivations. Published data on telomer ratios that have been extrapolated to zero time indicate ( 43 + values of about 1 for pent-2-ene/cyclopentene (Herisson... 

From a practical point of view, commercial diisobutene is a mixture of 2,4,4-trimethyl pent-2-ene, used in the cross-metathesis reaction, and of 2,4,4-trimethyl pent-l-ene (not modified by the metathesis reaction), in a relatively large amount (20 to 25%). In order to valorize the process, it is necessary to use commercial diisobutene and so to convert the terminal non useful olefin into the internal useful one. This can be done by mixing the metathesis catalyst (W03/Si02) with an isomerization catalyst (typically magnesia) together in the reactor. Typically, with a 1 3 mixture of W03/Si02 and MgO, at 370°C, under a 30 bar pressure and with a ethylene to diisobutene ratio of 2. A conversion of 65% of diisobutene is achieved with a neohexene selectivity around 85%. 

If there is possibility of formation of more than one alkene due to the availability of more than one 3-hydrogen atoms, usually one alkene is formed as the major produet. These form part of a pattern first observed by Russian ehemlst, Alexander Zaitsev (also pronounced as Saytzeff) who In 1875 formulated a rule which can be summarised as in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms. Thus, 2-bromopentane gives pent-2-ene as the major product. 

Consider a telomer being formed from a cyclopentenyl polymer growing under the pairwise mechanism (Scheme 12.14) with growth being curtailed by cross-metathesis under two extreme conditions (i) with only pent-2-ene present (C4 C5 C6 = 0 100 0) and (ii) with a fully equilibrated mixture of acyclic monoalkenes (C4 C5 C6 = 1 2 1). Under condition (i), one would expect the formation of only hierarchical telomers (n = 1,2,3,4,5, etc.) of the type (C2)-[(cyc-C5) ]-(C3) as the pent-2-ene is split into a C2 and a C3 unit across the growing cyclo polyene. In contrast, under condition (ii), one would expect each hierarchical telomer to be formed in a 1 2 1 ratio of (C2)-[(cyc-C5)n]-(C2) (C2)-[(cyc-C5) ]-(C3) (C3)-[(cyc-Q)n]-(C3)> depending on whether there is cross-metathesis with C4, C5 or C6 (ratio = 1 2 1). The outcome will thus depend on how quickly the pent-2-ene is equilibrated by homo-metathesis to yield the C4, C5 and C6 mixture. Analysis of the rate of pent-2-ene homo-metathesis showed that it was not fast. Indeed, it proceeded at approximately the same rate as the telomerisation reaction. One would thus expect the telomer product early in the reaction to be essentially pure (C2)-[(cyc-C5) ]-(C3) species. Then, as C4 and C6 increase in concentration relative to C5, formation of the (C2)-[(cyc-C5) ]-(C2) and (C3)-[(cyc-C5) ]-(C3) telomers should increase proportionally. This was not found to be the case. 

Two criticisms of this mechanism can be made. First, these activation energies are overall activation energies for a two-step process for the decomposition of different alkylperoxy radicals [106] see opposite page. For the formation of 2-methyltetrahydrofuran both steps will involve cyclization and will have pre-exponential factors [104] of ca. lO sec", whereas the formation of pent-2-ene involves only one such step and a second step for which [39] A = 10 sec". Since the strain energy involved in the isomerizations of each of the alkylperoxy radicals is the same (ca. 6.5 kcal. mole" ) the activation energies of this step will only differ by the difference in primary and secondary C—H strengths (ca. 3.5 kcal. mole" ). It is difficult, therefore, to see how the overall activation energies for the formation of pent-2-ene and 2-methyltetrahydrofuran can be approximately equal. 

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