4- cyclopentane

Cyclopentane is appreciably less strained than cyclobutane and cyclopropane, and the strain energy relative to cyclohexane is ca. 6.45 kJ mol-1 per CH2 group. In order to lessen the torsion strain that would occur in a planar conformation, in which every C-H bond is involved in two eclipsing interactions, cyclopentane adopts a puckered conformation (see Dunitz, Further Reading). This has four carbons approximately planar, with the fifth carbon bent out of this plane in such a way that the molecule resembles a small near-square envelope 9. A Newman projection of 9 is shown in 10. 

In this envelope-type conformation the puckering moves rapidly around the ring so that each carbon in rapid succession becomes the flap of the envelope. This process is known as pseudorotation (see Fuchs, Further Reading). 

Cyclopentane adopts an envelope, or puckered, conformation the puckering rotates rapidly around the ring in a process called pseudorotation. 

In Chapter 1 the conformations of ethane, propane and butane were considered and then extended to cyclohexane and its (more stable) chair and 

Cyclohexane was originally postulated to be non-planar by the German chemist Sachse in 1890. Definitive proof was provided in ca. 1950 by Hassel in Oslo, from analysis of X-ray structures, and by Barton in London from correlations with steroid reactivities (see also Dunitz and Weser, Further Reading). The distinction between axial and equatorial positions was also made at that time (see Barton3). 

A study very similar in nature to the one described above for cyclobutane has been undertaken by Ausloos et for the photolysis of cyclopentane at 1470, 1236 and 1048-1067 A. The photolysis mechanism is not as clear-cut as with cyclobutane, and it appears that more reactions contribute to the photolysis, viz. 

At 1470 A the most important primary reaction is (2), with most of the cyclo-pentene being stabilized. The hydrogen is eliminated molecularly as confirmed by NO-inhibited reactions and by the yield of mainly Hj and D2 from cyclo-CsHio-cyclo-CsDio mixtures. The amount of hydrogen formed is greater than the yield of cyclopentene this indicates decomposition of the excited cyclopentene. The pentadienes observed probably arise in this manner, as also does part of the ethylene observed. There is, however, more ethylene formed than can be accounted for by the decomposition of cyclopentene, and hence reaction (3) probably occurs to some extent at 1470 A. 

The photolysis of cyclopentane at 1048-1067 A was found to produce photoionization with an ionization efficiency of about 0.43. The presence of CjHjo ions was shown by the H2 transfer reaction to propene, viz. 

In addition, ring-opening of the CsHjo ions was found in cyclo-CjHjo-NO mixtures which lead to pentene-2, presumably through the charge-transfer reaction with NO. Pentene-2 was not a product at 1236 or 1470 A, for similar mixtures. The presence of CsHs ions was also observed in the cyclopentane photolysis at 1048-1067 A, and has been attributed to the decomposition of hot cyclopentane ions, viz. 

The decomposition of the neutral excited cyclopentane molecules is very similar to that at 1236 A. 

In the envelope conformation four of the carbon atoms are coplanar. The fifth carbon is out of the plane of the other four. There are three coplanar carbons in the halfchair conformation, with one carbon atom displaced above that plane and another below it. In both the envelope and the half-chair conformations, in-plane and out-of-plane carbons exchange positions rapidly. Equilibration between conformations of cyclopentane is very fast and occurs at rates similar to that of rotation about the carbon-carbon bond of ethane. 

The (a) planar, ib) envelope, and (c) half-chair conformations of cyclopentane. 

In 1978, a German-Swiss team of organic chemists reported the synthesis of a cycloalkane with 96 carbons in its ring (cyclo-C96H,92). 

PROPERTIES RESEARCH GRADE PURE GRADE TECHNICAL GRADE m GRADE 

Acidity, distillation reeidue neutral neutral neutral 

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The majority of acids contained in the diesel cuts are cyclic and come from cyclopentane or cyclohexane. They are better known as naphthenic acids / ... 

Aromatics - Benzene, Methylbenzene (Toluene) Dimethylbenzene (Xylenes) Naphthenes - Cyclopentane, Cyclohexane... 

It is important that the synthesis should be carried out as quickly as possible, particularly the washing with alkah at 0°, since the latter tends to convert the product into cyclopentane-a-hydroxycarboxyhc acid. 

Cyclopentene-l-carboxaldehydes are obtained from cyclohexene precursors by the sequence cyclohexene - cyclohexane-1,2-diol -> open-chain dialdehyde - cyclopentane aldol. The main advantage of this ring contraction procedure is, that the regio-and stereoselectivity of the Diels-Alder synthesis of cyclohexene derivatives can be transferred to cyclopentane synthesis (G. Stork, 1953 G. BUchi, 1968). 

Some straightforward, efficient cyclopentanellation procedures were developed recently. Addition of a malonic ester anion to a cyclopropane-1,1-dicarboxylic ester followed by a Dieckmann condensation (S. Danishefsky, 1974) or addition of iJ-ketoester anions to a (l-phenylthiocyclopropyl)phosphonium cation followed by intramolecular Wittig reaction (J.P, Marino. 1975) produced cyclopentanones. Another procedure starts with a (2 + 21-cycloaddition of dichloroketene to alkenes followed by regioselective ring expansion with diazomethane. The resulting 2,2-dichlorocyclopentanones can be converted to a large variety of cyclopentane derivatives (A.E. Greene. 1979 J.-P. Deprds, 1980). 

Synthetically useful stereoselective reductions have been possible with cyclic carbonyl compounds of rigid conformation. Reduction of substituted cyclohexanone and cyclopentan-one rings by hydrides of moderate activity, e.g. NaBH (J.-L. Luche, 1978), leads to alcohols via hydride addition to the less hindered side of the carbonyl group. Hydrides with bulky substituents 3IQ especially useful for such regio- and stereoselective reductions, e.g. lithium hydrotri-t-butoxyaluminate (C.H. Kuo, 1968) and lithium or potassium tri-sec-butylhydro-borates or hydrotri-sec-isoamylborates (=L-, K-, LS- and KS-Selectrides ) (H.C. Brown, 1972 B C.A. Brown, 1973 S. Krishnamurthy, 1976). 

The most common stereoselective syntheses involve the formation and cleavage of cyclopentane and cyclohexane derivatives or their unsaturated analogues. The target molecule (aff-cts)-2-methyl-l,4-cyclohexanediol has all of its substituents on the same side of the ring. Such a compound can be obtained by catalytic hydrogenation of a planar cyclic precursor. Methyl-l,4-benzoquinone is an ideal choice (p-toluquinone M. Nakazaki, 1966). 

We shall concentrate in our short account of PG syntheses on solutions to both problems as provided in the literature. Some fundamentals of cyclopentane synthesis have been discussed in section 1.13.3 (see also 4.5.1). 

Cyclopentene derivatives with carboxylic acid side-chains can be stereoselectively hydroxy-lated by the iodolactonization procedure (E.J. Corey, 1969, 1970). To the trisubstituted cyclopentene described on p. 210 a large iodine cation is added stereoselectively to the less hindered -side of the 9,10 double bond. Lactone formation occurs on the intermediate iod-onium ion specifically at C-9ot. Later the iodine is reductively removed with tri-n-butyltin hydride. The cyclopentane ring now bears all oxygen and carbon substituents in the right stereochemistry, and the carbon chains can be built starting from the C-8 and C-12 substit""" ... 

The following acid-catalyzed cyclizations leading to steroid hormone precursors exemplify some important facts an acetylenic bond is less nucleophilic than an olelinic bond acetylenic bonds tend to form cyclopentane rather than cyclohexane derivatives, if there is a choice in proton-catalyzed olefin cyclizations the thermodynamically most stable Irons connection of cyclohexane rings is obtained selectively electroneutral nucleophilic agents such as ethylene carbonate can be used to terminate the cationic cyclization process forming stable enol derivatives which can be hydrolyzed to carbonyl compounds without this nucleophile and with trifluoroacetic acid the corresponding enol ester may be obtained (M.B. Gravestock, 1978, A,B P.E. Peterson, 1969). 

The furanose rings of the deoxyribose units of DNA are conformationally labile. All flexible forms of cyclopentane and related rings are of nearly constant strain and pseudorotations take place by a fast wave-like motion around the ring The flexibility of the furanose rings (M, Levitt, 1978) is presumably responsible for the partial unraveling of the DNA double helix in biological processes. 

Hydrosilylation of I-vinyl-1-cyclohexene (77) proceeds stereoselectively to give the (Z)-l-ethylidene-2-silylcyclohexane 78, which is converted into (Z)-2-ethylidenecyclohe.xanol (79)[74]. Hydrosilylation of cyclopentadiene affords the 3-silylated 1-cyclopentene 80. which is an allylic silane and used for further transformations[75.75a]. Cyclization of the 1,3,8, lO-undecatetraene system in the di(2.4-pentadienyl)malonate 69 via hydrosilylation gives the cyclopentane derivative 81. which corresponds to 2.6-octadienylsilanc[l8,76]. 

Oxindoles can be prepared from Af,p-acylphenylhydrazines by a reaction which is analogous to the Fischer cyclization. This is known as the Brunner reaction. The reaction is typically conducted under strongly basic conditions. For example, heating Af-phenylcyclopentanecarbonylhydrazide with CaO gives a 70% yield of spiro-cyclopentane oxindole[l]. 

Cyclopentane and cyclohexane are present m petroleum but as a rule unsubsti tuted cycloalkanes are rarely found m natural sources Compounds that contain rings of various types however are quite abundant... 

The previous compound is isopropylcyclopentane Alternatively the alkyl group can be named accord mg to the rules summarized in Table 2 7 whereupon the name becomes 1 -methylethyl)cyclopentane Parentheses are used to set off the name of the alkyl group as needed to avoid ambiguity... 

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