Steric strain, strained hydrocarbons

The Jcc and Jcc couplings of allene and two sterically strained hydrocarbons, cyclopropane and cyclopropene, have been calculated by Ruden et at different levels of electronic-structure theory and compared with each other and with the experimental equilibrium constants obtained from experiment by subtracting the calculated vibrational contributions. 

As was the case for the alkyl hydroperoxides in reaction 4, the enthalpies of the oxy-gen/hydrocarbon double exchange reaction 8 for dialkyl peroxides are different depending on the classification of the carbon bonded to oxygen. For R = Me, Et and f-Bu, the liquid phase values are —4, 24.6 and 52.7 kJmoR, respectively, and the gas phase values are 0.1, 25.7 and 56.5 kJmoR, respectively. For the formal deoxygenation reaction 9, the enthalpies of reaction are virtually the same for dimethyl and diethyl peroxide in the gas phase, —58.5 0.6 kJ moR. This value is the same as the enthalpy of reaction of diethyl peroxide in the liquid phase, —56.0 kJ moR (there is no directly determined liquid phase enthalpy of formation of dimethyl ether). Because of steric strain in the di-ferf-butyl ether, the enthalpy of reaction is much less negative, but still exothermic, —17.7 kJmol (Iq) and —19.6 kJmol (g). 

Thus, on the basis of relative C-H BDEs, (CH3)3C-H (96.4), (CH3)2CH-H (98.6), CH3CH2-H (101.1) and CH3-H (104.8 kcal/mol) [23,26], one reaches the conclusion that the stability of hydrocarbon radicals decreases 3° > 2° > 1 ° > CH3. It is, of course, recognized that the variable degree of steric strain of the molecules within this series limits the quantitative impact of these numbers vis-a-vis radical stability. 

The cation possesses a more favourable geometry than 1,6-methano-[10]annulene, being able to incorporate the 10 -system into a more planar structure. It is evidently more stable than the tropylium ion, as it can be formed from its neutral hydrocarbon precursor 57 by a hydride-transfer reaction with tropylium tetrafluoroborate 136>. The anion, on the other hand, involves substantial steric strain and is not as planar as either the cation or the neutral bridged [lOJannulene. Whereas formation of the cation from 57 was seen to be most favourable, the anion precursor 58 is much less acidic than cyclopentadiene, although in the presence of... 

The energy of some free radicals derived from small strained hydrocarbons has been calculated at the MINDO/3 level. The AH and A// were calculated for several possible fragmentations and are given below. Consider the stereoelectronic and steric... 

Twenty previously unreported polycyclic aromatic hydrocarbons (PAHs) were synthesized by condensing mixtures of 1-phenalenone-type ketones. These PAHs were isolated and identified by using field-ionization mass spectrometry, spectrofluorometry, and UV-visible absorbance spectrometry. The spectrometry and chromatography of these and other PAHs were compared. Correlations between the steric strain and the spectral valley depths between maximums, or chromatographic retention, were found. Steric effects were not the only factors affecting retention resonance stability also appeared to increase retention. 

In connection with the activation of saturated hydrocarbons via homogeneous catalysis, we have examined transition metal catalyzed reactions of various strained hydrocarbon systems that have unique steric and electronic properties. Strained carbon-to-carbon single bonds have considerable TT-bonding character. The chemistry of these substrates should be intermediate between well-documented transition metal chemistry of alkenes and rather unclarified alkane chemistry (1, 2, 3). Our attention has been focused particularly on the stereoselectivity, regioselectivity, and periselec-tivity of the Ni(0)-catalyzed reactions (4-14). 

One can assume that in the transition state of the 1,2-hydride shift in ion (64c) there occurs an essential hmrease in the strain brought about by the unfavourable steric interaction of hydrocarbon fiagments. This assumption is evidenced by the fact that the 1,2-shifl of a far bulkier migrant — a methyl group in ion (64c) does occur despite its obvious endothermicity (cf. Ref. 

One simple difference in the conformations of pure hydrocarbons and heterocyclic rings results from the fact that C-heteroatom bonds are of different lengths than C-C bonds. Bonds to O and N are shorter, often causing increased steric strain. Bonds to S are significantly longer. 

Vq can be taken as being equal to the ethane barrier (2.S-2.9 kcal/mol). The potential energy diagram for rotation about the C-C bond of ethane is given in Fig. 3.1. The ethane barrier may be taken as a standard rotational barrier for acyclic hydrocarbons when analyzing the contribution of torsional strain to the total steric strain. The stereoelectronic origin of the ethane barrier was discussed in Chapter 1. Any steric interactions which are present in more highly substituted systems will make an additional contribution to the barrier." ... 

Lithium in ethylamine reduces hindered secondary and tertiary alcohol acetates to the hydrocarbon, release of steric strain apparently acting to overcome the more commonly observed acyl-oxygen cleavage. Cyclo-octatetraene [COT] is readily derived from 1,5-cyclo-octadiene by oxidation (O2) of the salt derived by reaction with PhNa-TMED, or by CdCla oxidation of the dilithium salt of COT dianion. 

Because E -constants for complex groups are obtainable at present only by an empirical procedure82 a corresponding analysis is not possible for aryl-, vinyl-, and other nonflexible a-type radicals. This difficulty was overcome recently by the development of a new set of steric substituent parameters. 9f for front strain phenomena. These constants are defined as the difference in heat of formation for the hydrocarbons 30 and 31. The heats of formation are calculated for this purpose by... 

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