Cyclopentane isomerization

The stereochemical integrity of the alkene is usually lost during the reaction. Sequential formation of the new a bonds makes possible a rotation about single bonds within the biradical before its closure. Then the same mixture of stereoisomers can be obtained by photoaddition on either (Z)-or ( )-alkenes. With cycloalkenes such as cyclopentane, isomerization cannot occur and the cis-anti-cis adduct is the main product of 2 + 2 photocycloaddition. 

Donath11 has extended these calculations to cyclohexane and cyclopentane and obtained comparably good agreement. It seems safe to conclude that these long-range electron correlation effects account for the isomerization energies of the paraffins which had heretofore remained unexplained. 

As the number of carbon atoms increases, the number of possible isomers becomes larger. Whereas there are only two isomeric butanes, there are three isomeric pentanes. With hve carbons, in addition to the open-chain compounds shown, a stable ring compound known as cyclopentane is also possible. Five- and six-membered carbon rings are very stable because the bonds between carbon atoms in these size rings are close to the 109° angle preferred by carbon. Three- and four-membered hydrocarbon rings are also known, but they are less stable because of the required distortion of the bond angle. 

In the case of the cyclopentane oxirane 424, fluorination [KHFj, Me0(CH2)20H] proceeded to give preferably one fluoroalcohol, 426 (61%) over the isomeric one, 425 (7%), possibly by the influence of the benzyloxy-methyl group. Similarly (KHF2, ethylene glycol, 160°), another oxirane, 427, was converted into 428 (30%). 2-C-(Fluoromethyl)-/ T6>-inositol (429)... 

Likewise it is possible to differentiate between substituted and unsubstituted alicycles using inclusion formation with 47 and 48 only the unbranched hydrocarbons are accommodated into the crystal lattices of 47 and 48 (e.g. separation of cyclohexane from methylcyclohexane, or of cyclopentane from methylcyclopentane). This holds also for cycloalkenes (cf. cyclohexene/methylcyclohexene), but not for benzene and its derivatives. Yet, in the latter case no arbitrary number of substituents (methyl groups) and nor any position of the attached substituents at the aromatic nucleus is tolerated on inclusion formation with 46, 47, and 48, dependent on the host molecule (Tables 7 and 8). This opens interesting separation procedures for analytical purposes, for instance the distinction between benzene and toluene or in the field of the isomeric xylenes. 

Compounds called cycloalkanes, having molecules with no double bonds but having a cyclic or ring structure, are isomeric with alkenes whose molecules contain the same number of carbon atoms. For example, cyclopentane and 2-pentene have the same molecular formula, C5H, but have completely... 

Cyclopentane has the low chemical reactivity which is typical of saturated hydrocarbons, while 2-pentene is much more reactive. Similarly, ring structures containing double bonds, called cyclo-alkenes, can be shown to be isomeric with alkynes. 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

Cyclohexene does not polymerize by either route except when it is part of a bicyclic structure as in norbornene. Stereochemistry in the ROMP of norbomene is complicated since the polymer, LXVI in Sec. 7-8, has possibilities of isomerism at both the ring and the double bond. Most polymerizations by the typical ROMP initiators yield cis stereochemistry at the cyclopentane ring with varying amounts of cis and trans placements at the double bond [Ivin, 1987]. Metallocene initiators yield predominantly double-bond polymerization with 1,2-placement [Janiak and Lassahn, 2001]. 

As discussed in Sect. 3.4, the synthon 20 was prepared from the dibromo-heptonolactone, which in turn was obtained from the cheap commercially available D-g(ycero-D-gM/o-heptono-l,4-lactone (Table 1). The other isomeric dibro-moheptonolactones shown in Table 1, which were prepared from the heptonates, obtained from chain extension of o-mannose and o-galactose, respectively, were also converted into unsaturated bromodeoxyheptonolactones. Finally, we obtained 2-0-acetyl-7-bromo-3,7-dideoxy-D-x7(o-hept-2-enono-l,4-lactone and the corresponding D-/yxo-isomer by the Kiliani extension of o-gulose. These substrates were all cyclized to cyclopentane lactones, stereoisomers of 65 [98]. 

Ojima has reported a rhodium-catalyzed protocol for the disilylative cyclization of diynes with hydrosilanes to form alkylidene cyclopentanes and/or cyclopentenes. As an example, reaction of dipropargylhexylamine with triethyl-silane catalyzed by Rh(acac)(GO)2 under an atmosphere of CO at 65 °G for 10 h gave an 83 17 mixture of the disilylated alkylidene pyrrolidine derivative 92b (X = N-//-hexyl) and the disilylated dihydro-1/ -pyrrole 92c (X = N-//-hexyl) in 76% combined yield (Equation (60)). Compounds 92b and 92c were presumably formed via hydrosilyla-tion and hydrosilylation/isomerization, respectively, of the initially formed silylated dialkylidene cyclopentane 92a (Equation (60)). The 92b 92c ratio was substrate dependent. Rhodium-catalyzed disilylative cyclization of dipro-pargyl ether formed the disilylated alkylidene tetrahydrofuran 92b (X = O) as the exclusive product in low yield, whereas the reaction of dimethyl dipropargylmalonate formed cyclopentene 92c [X = C(C02Et)2] as the exclusive product in 74% isolated yield (Equation (60)). 

The ability of a catalyst to promote isomerization plays two roles in reforming it increases the amount of branched chain paraffins in the product and it converts naphthene hydrocarbons with cyclopentane rings into cyclohexane ring naphthenes which are necessary for the formation of aromatics by dehydrogenation. 

The octane number improvement obtained by isomerization of paraffin hydrocarbons is not great since the amounts of the more highly branched paraffins formed at equilibrium are small at the temperatures employed in catalytic reforming (5). Naphthene isomerization, on the other hand, plays a more important role in reforming. In most naphthas about 50% of the naphthene hydrocarbons are of the cyclopentane type (4) so that in order to obtain the maximum aromatic formation, isomerization of these rings to cyclohexane rings must be promoted by the catalyst. 

Several years ago, one of the authors found that nickel, platinum, and some other hydrogenating agents, when deposited on fresh synthetic silica-alumina cracking catalyst, made a new catalyst that would isomerize paraffin and naphthene hydrocarbons in the presence of hydrogen at elevated pressures and nominal temperatures. Table I shows some early typical results calculated from mass spectrometer analyses of the products obtained by passing methyl cyclopentane, cyclohexane, and n-hexane over a catalyst composed of 5% nickel in silica-alumina at the indicated reaction conditions. Isomerization of a number of other hydrocarbons has also been studied and reported elsewhere (2). 

Among the isomeric C7 alkyl cyclopentanes, the approximate relative amounts of the monoalkyl and the sum of the dialkyl isomers are ethyl isomer, 1 sum of the five dimethyl isomers, 10. 

Cyclopropanes and cyclobutanes are not produced by isomerization of cyclopentane. Cycloheptane is irreversibly isomerized to methylcyclohexane by promoted aluminum chloride14 or bromide.15... 

Cyclohexane-methylcyclopentane isomerization21 can be depicted as in Scheme 4.2. The isomerization of substituted cyclopentanes and cyclohexanes to polymethylcyclohexanes similarly occurs by way of a series of consecutive steps... 

When cycloalkanes (cyclopentane, cyclohexane) alkylate benzene, cycloalkylben-zenes, as well as bicyclic compounds (indan and tetralin derivatives) and products of destructive alkylation, are formed.191192 Cyclohexane reacts with the highest selectivity in the presence of HF—SbF5 to yield 79% cyclohexylbenzene and 20—21% isomeric methylcyclopentylbenzenes.191... 

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