Ring strain cyclopentane

Cyclopentane and cycloheptane have about the same modest amount of ring strain (27 kJ mol-1). 

Cyclopentane and cycloheptane have almost equal ring strains (6.5 and 6.4) and a 15 membered cycloalkane again has a very slight amount of ring strain. 

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 . ... 

Pinacol rearrangement driven by the release of the ring strain of a cyclobutane ring has been employed in an extremely efficient manner to form cyclopentanone derivatives. Experimentally. the Lewis acid mediated aldol condensation of benzaldehyde with l,2-bis(trimethyl-siloxy)cyclobutcne at —78 C gave the pinacol 1 in its silylated form.35,36 Subsequent treatment of this pinacol with trifluoroacetic acid at room temperature afforded 2-phenyl-cyclopentane-l,3-dione (2) in 97% yield.35,36... 

The aminolyses of the carbonates derived from cyclopentane-1,2-diol (38) and cyclohexane-1,2-diol (39) by hexylamine at 70 °C were much slower than that of ethylene carbonate, the latter (39) being about twice as reactive as the former (38). Computational calculations confirmed that ring strain was the main determining factor.40... 

The regioselectivity in diene addition reactions can also be influenced by ring strain effects in cyclization reactions. The regioselectivity is highly predictable in those cases, in which addition to the preferred diene center forms the preferred ring size. Thus, the cyclization of radical 15 proceeds readily to form the ct s-disubstituted cyclopentyhnethyl radical 16 with high selectivity. Similarly, cyclization of 17 affords exclusively bicyclic radical 18, in which the additional cyclopentane ring has been formed by addition to the terminal position of the butadiene subunit. This preference for 5-exo cyclizations onto dienes is not even dismpted by substiments at the C1 or C4 positions of the diene system, as seen for radical 19, which cyclizes to 20 (equation lO). This is in contrast to alkyl radical cyclizations to alkenes, in which major amounts of 6-endo cyclization is observed for 5-substituted systems. ... 

The heats of polymerization of two cyclic siloxanes have been measured the six-membered ring, hexamethyltrisiloxane, and the five-membered paraffin-siloxane , 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane (shown in Table 8.) For the corresponding cyclopentane the heat of polymerization is —5 2 kcal/mole, whilst for the paraffin-siloxane it is —10 0 kcal/mole. Thus, the effect of replacement of —(CHgla— by —Si(Mc2)—0—Si(Mc2)— in the cyclopentane molecule appears to either increase the ring strain in the monomer, or to stabilize the polymer, to the extent of some... 

The pseudorotation concept has been introduced by K. S. Pitzer [49] to describe the continuous interconversions between an infinite number of indefinite puckered conformations of the cyclopentane ring. PseudorotatiOTi [50] allows cyclopentane to relieve the ring strain, which would be induced by a 120° bond angle and the torsional strain by an eclipsed methylene group, if it were to adopt a planar conformation. A barrier to the planarity of cyclopentane of 22 kJ/mol has been reported [51]. The concept of pseudorotation has been applied for the first time to sugar furanoses by Hall et al. [52] studying the conformational analysis of pentofuranosyl fluorides. 

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