C.H. Cyclopentene

Michael addition to the salt to form an alkylidenecarbene, which undergoes intramolecular C—H insertion to form a cyclopentene. 

Water-soluble mthenium vinyUdene and aUenylidene complexes were also synthetized in the reaction of [ RuCl2(TPPMS)2 2] and phenylacetylene or diphenylpropargyl alcohol [29]. The mononuclear Ru-vinylidene complex [RuCl2 C=C(H)Ph)(TPPMS)2] and the dinuclear Ru-aUylidene derivative [ RuCl(p,-Cl)(C=C=CPh2)(TPPMS)2 2] both catalyzed the cross-olefin metathesis of cyclopentene with methyl acrylate to give polyunsaturated esters under mild conditions (Scheme 7.10). 

Exclusive formation of furans clearly indicates that the intramolecular 1,5-insertion into the C—H bonds of methylene groups cannot compete with that into OH bonds of enols. However, the stereochemistry of enolized carbene intermediates plays an important role in this reaction as acetylace-tone affords a mixture of furan 37 and cyclopentene 38 in a 64 36 ratio in 61% yield (Scheme 14). 

Interestingly, two of the other species in Table 3 are nitrolates, i.e. ethers of a-nitrooximes, an otherwise thermochemically unprecedented class of compounds. We already have briefly discussed one, 3-nitroisoxazoline, and the second is 1-nitroacetaldehyde 0-(l,l-dinitroethyl)oxime (ONo-ld-dinitroethyl acetonitronate), MeC (NOala—O—N=C(N02)Me. The latter acyclic species is a derivative of 1,1-dinitroethanol—we know of the enthalpy of formation of no other a-nitroalcohol or derivative. Nonetheless, we may ask if the two calorimetric data are internally consistent. Consider the condensed phase reaction 47, which involves formal cleavage of the O — bond in the nitroisoxazoline by the C—H bond of the dinitromethane. It is assumed that the isoxazoline has the same strain energy as the archetypal 5-atom ring species cyclopentane and cyclopentene, ca 30 kJ mol . ... 

Alkylidenes (alkylidene carbenes) are valence isomers of alkynes. They have been prepared by alkyne pyrolysis, by homologation of ketones, and by generation of alkenyl anions bearing oc-leaving groups. Generated by any of these means, an alkylidene will insert intramolecularly into a remote C- H bond to form a new C—C bond and thus a cyclopentene. A concerted two-electron process, this reaction proceeds with retention of absolute configuration at the C - H site. 

This reaction apparently proceeds by way of the normal phosphonate condensation product, the diazoalkylidene, which then spontaneously loses nitrogen to form the transient alkylidene car-bene. Careful work showed that, after statistical corrections were applied, the reactivity of a C-H bond toward insertion was approximately 0.003 for primary C-H bonds (methyl), 1.0 for secondary C-H bonds (methylene), 7.5 for benzylic (methylene) C-H bonds and 18.6 for tertiary C-H bonds. These relative reactivities are very similar to those previously observed for intramolecular C-H insertion by an alkylidene carbenoid generated from a vinyl bromide27. It was shown subsequently that the alkylidene carbene insertion reaction proceeds with retention of absolute configuration28. Using this approach, (l )-3-dimethyl-3-phenyl-l-cyclopentene and (i )-4-methyl-4-phenyl-2-cyclohexcnonc were prepared in high enantiomeric purity. 

Butadiene. The reaction of methylene with butadiene was studied by Frey44 under experimental conditions similar to those in the case of allene, except that lower pressures were required to avoid butadiene polymerization. Products formed by attack of methylene on the C—H bonds were cis and vinyl-cyclopropane resulting from addition of CH2 to the carbon-carbon double bond underwent collisional deactivation or isomerization to cyclopentene and C dienes, with the exception of isoprene. 

The C—C=C angle in alkenes normally is about 122°, which is 10° larger than the normal C—C—C angle in cycloalkanes. This means that we would expect about 20° more angle strain in small-ring cycloalkenes than in the cycloalkanes with the same numbers of carbons in the ring. Comparison of the data for cycloalkenes in Table 12-5 and for cycloalkanes in Table 12-3 reveals that this expectation is realized for cyclopropene, but is less conspicuous for cyclobutene and cyclopentene. The reason for this is not clear, but may be connected in part with the C-H bond strengths (see Section 12-4B). 

As shown above, insertion of alkylidene carbenes is highly regioselective. However, when the normal 1,5-C-H insertion pathway is blocked, 1,4- or 1,6-C-H insertion takes place [Eq. (109)]. Thus, the cyclobutene 121 [192] and the six-membered enol ether 123 [193] were obtained in modest yields. Intramolecular insertion into carbon-carbon double bond provides a method for synthesis of cyclopenten-annulated dihydropyrrole 124, which results from homolytic scission of a methylenecyclopropane intermediate [194]. 

The cyclopentene annulations can also occur in the reactions of alkynyliodo-nium salts with nitrogen- and sulfur nucleophiles (Scheme 61). Specifically, azi-docyclopentene 155 is formed upon treatment of octynyliodonium tosylate 154 with sodium azide in dichloromethane [123]. The reaction of alkynyliodonium salt 156 with sodium toluenesulfinate results in the formation of substituted indene 157 via alkylidene carbene aromatic C-H bond insertion [124]. 

The ruthenium-catalyzed addition of C-H bonds in aromatic ketones to olefins can be applied to a variety of ketones, for example acetophenones, naphthyl ketones, and heteroaromatic ketones. Representative examples are shown in the Table 1. Terminal olefins such as vinylsilanes, allylsilanes, styrenes, tert-butylethy-lene, and 1-hexene are applicable to this C-H/olefin coupling reaction. Some internal olefins, for example cyclopentene and norbornene are effective in this alkylation. The reaction of 2-acetonaphthone 1 provides the 1-alkylation product 2 selectively. Alkylations of heteroaromatic ketones such as acyl thiophenes 3, acyl furans, and acyl pyrroles proceed with high yields. In the reaction of di- and tri-substitued aromatic ketones such as 4, which have two different ortho positions, C-C bond formation occurs at the less congested ortho position. Interestingly, in the reaction of m-methoxy- and m-fluoroacetophenones C-C bond formation occurs at the congested ortho position (2 -position). 

These highly reactive yet stable species are strong electrophiles of tetraphilic character, since nucleophiles may attack three different carbon atoms (a,/ ,a ) and iodine. In most reactions the first step is a Michael addition at fi-C with formation of an alkenyl zwitterionic intermediate (ylide) which normally eliminates iodoben-zene, generating an alkylidene carbene then, a 1,2-shift of the nucleophile ensues. The final result is its combination with the alkynyl moiety which behaves formally as an alkynyl cation. The initial adduct may react with an electrophile, notably a proton, in which case alkenyl iodonium salts are obtained also, cyclopentenes may be formed by intramolecular C-H 1,5-insertion from the alkylidenecarbenes ... 

Maas and co-workers have elaborated the approach to l-oxa-2-silacyclo-pentene derivatives based on cyclization of a siloxyalkylidenecarbene. Thus, as a result of intramolecular aliphatic C — H insertion of the carbene derived from diazo ketone 576, l-oxa-2-sila-4-cyclopentenes 577 were obtained in high yields. Here, the insertion may occur in primary, secondary, and tertiary C—H bonds (87CB635 94JOM115). When no C—H bond is adjacent to the Si atom, as in substrate 578, the cyclopropanation of a benzene ring with concomitant ring expansion may result in oxasilacyclopentene 579 (86CC1782). Photolysis or catalytic decomposition of bissilanes 580 furnishes fused bicyclic heterocycles 581 in moderate yields (90JOM229). 

Finally, one rather different example of vinylic C-H activation has appeared using a first row transition metal. Field reported that irradiation of Fe(dmpe)2H2 in the presence of an olefin at -80°C leads to the formation of vinyl hydride products. These insertion adducts were observed with cyclopentene, ethylene, and 1-pentene. Upon warming to room temperature, the r 2-olefin complexes formed at the expense of the C-H insertion adducts (Eq. 39) [129,130]. 

Evidence for the intermediacy of the carbene in the corresponding photochemical reaction has also been obtained.These results support theoretical predictions." Chemical support is seen in the isomerization of l-butyl-3,3-dimethylcyclopropene (4), the cyclopentene 5 apparently arising from an insertion of the methylenecarbene 6 into a C —H bond. ... 

Dimethyl-[ I -fluoro-2,2,3,3-tet-ramethyl-cyclopropyl]- -fluorosulfonale 1666 C,H F,Sn Cyclopentene... 

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