Pentenes addition 4- carbenes

In laser-impulse experiments with chlorophenyldiazirine the carbene could be observed by UV spectroscopy. On addition of defined amounts of alkene the rate of cyclopropanation was measured directly. The rate constants with various alkenes were (lO moF s ) 1-hexene, 1.3 ( )-2-pentene, 34 2-methyl-2-butene, 77 2,3-dimethyl-2-butene, 130 (80JA7576>. 

The stereospecificity of addition suggests a singlet(10) carbene although the ground state of cyclopentadienylidene is known to be a triplet. Attempts to produce a triplet species, which would be expected to react nonstereospecific-ally, in a 4-methyl-cw-2-pentene matrix at 77°K or by dilution of mixtures of the azo compound and olefin with hexafluorobenzene or octafluorocyclo-butane (inert diluents) were unsuccessful. It was concluded that the singlet carbene produced upon photolysis reacts more rapidly with the olefinic... 

It is evident from Table 11 that the rate of addition of the triplet bis(methoxy-carbonyl) carbene is somewhat slower than that of the singlet. Another important general rule may also be deduced from Table 11 the triplet carbene adds to dienes about 3—4 times faster than to olefins the reactivity ratio of 2,3-dimethyl-butadiene-1,3/pentene-l is 9.6 for triplet and 2.8 for the singlet. This ratio may be compared with that for diphenylcarbene (1,3-butadiene/hexene-l), which is > 100. 

CH3)2C=C(CH3)2 to 3.3 X 10 M-h- for the addition of CH3OCCI to trans-pentene. This million-fold variation testifies to the great modulating power of carbenic substituents on carbenic reactivity. 

Given our ability to measure absolute rate constants for carbene additions, variable temperature studies readily afford activation parameters. Initial studies of CeHsCX additions gave very low values, 1 kcal/mol for reactions with 1-hexene and frani-pentene. Most surprisingly, the values for CgHsCCl additions to (CH3)2C=C(CH3)2 and (CH3)2C=CHCH3 were negative (—1.7 and —0.8 kcal/ mol, respectively). The reaction rates increased as temperature decreased. The preexponential (A) factors were low (2-6 x 10 s ), indicative of an unfavorable... 

Extreme cases were reactions of the least stabilized, most reactive carbene (Y = CF3, X = Br) with the more reactive alkene (CH3)2C=C(CH3)2, and the most stabilized, least reactive carbene (Y = CH3O, X = F) with the less reactive alkene (1-hexene). The rate constants, as measured by LFP, were 1.7 x 10 and 5.0 X lO M s, respectively, spanning an interval of 34,000. In agreement with Houk s ideas,the reactions were entropy dominated (A5 —22 to —29e.u.). The AG barriers were 5.0 kcal/mol for the faster reaction and 11 kcal/ mol for the slower reaction, mainly because of entropic contributions the AH components were only —1.6 and +2.5 kcal/mol, respectively. Despite the dominance of entropy in these reactive carbene addition reactions, a kind of de facto enthalpic control operates. The entropies of activation are all very similar, so that in any comparison of the reactivities of alkene pairs (i.e., ferei)> the rate constant ratios reflect differences in AA//t, which ultimately appear in AAG. Thus, car-benic philicity, which is the pattern created by carbenic reactivity, behaves in accord with our qualitative ideas about structure-reactivity relations, as modulated by substiment effects in both the carbene and alkene partners of the addition reactions. " Finally, volumes of activation were measured for the additions of CgHsCCl to (CH3)2C=C(CH3)2 and frani-pentene in both methylcyclohexane and acetonitrile. The measured absolute rate constants increased with increasing pressure Ayf ranged from —10 to —18 cm /mol and were independent of solvent. These results were consistent with an early, and not very polar transition state for the addition reaction. 

The direct photolysis and benzophenone-sensitized decomposition of methyl diazomalonate has been shown to produce two different spin states of biscarbo-methoxycarbene. In the direct photolysis experiments stereospecific addition to olefins and a high yield of insertion products in the presence of alkanes demonstrate the singlet character of the carbene in the sensitized decomposition the yield of insertion products decreases, and addition to cis- or ra 5 -4-methyl-2-pentene yields about 88 % trnwj-cyclopropane. In this case, as with most carboalkoxy-carbenes, the addition reaction to olefins can proceed by two paths ... 

With the ability to measure absolute rate constants, we are in a position to follow changes in as a function of reaction temperature, and thus to derive activation energies for the additions of carbenes to alkenes. Our first experiments involved PhCCl and the initial 4 alkenes of Table 7. [95] With trans-pentene and 1-butene, we found activation energies of 1 kcaFmol for the addition of PhCCl. Surprisingly, for tetramethylethylene and trimethylene, was negative (-1.7 and -0.8 kcal/mol, respectively). [95] In these cases, the observed rate of carbene to alkene addition increased as temperature decreased The preexponential (A) factors for the additions were low (2 x 10 - 6 x 10 M" s" ), indicating substantially negative activation entropies for these reactions. 

A stepwise 1,7-vinyl shift was proposed to account for the reaction after carbene addition to make the cyclopropene. Deuterium labeling studies were consistent with this pathway. Interestingly, upon heating to 145°C the phenyl-substituted 5.2.0 tetraene isomerized to 2-phenylindene in what appears to be a cyclization of the cycloheptatriene moiety followed by opening of the bicyclo[2.1.0]pentene and a 1,5-hydrogen shift (Scheme 10.9). 

This catalyst, however, turned out to be considerably less active in ring opening metathesis polymerization of norbom-2-ene and in metathesis polymerization of c/s-2-pentene. Better results were obtained when a ruthenium-based metathesis type catalyst was immobilized via an A -heterocy-clic carbene [57]. Additional improvements in performance were obtained when the catalyst was immobilized on a monolithic silica rods ... 

Addition of cis-2-pentene to a mixture of the carbene complexes 9 and 11 yielded a mixture of compounds which when analyzed by H NMR spectroscopy, show signals corresponding to the expected methyl and ethyl substituted carbenes. 

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