Cyclopropane hydrogens

Fig. 4. Comparison of activity patterns of the group VIII noble metals for cyclopropane hydrogenation and ethane hydrogenolysis. The activities were all determined at hydrogen and hydrocarbon partial pressures of 0.20 and 0.030 atm, respectively (63).

It is now possible to satisfactorily calculate 13CNMR shifts with an average error of only 5 ppm.118 The results for the above compounds are given in Table 7, and it can be seen that they are remarkably good. It has not as yet been possible to derive a simple physical model to explain the variation in chemical shifts, but it seems likely that such a model will be forthcoming. It will then be possible to understand a variety of effects found in NMR spectroscopy, including the upfield shifts of cyclopropane hydrogens and carbons. 

The best-known gas hydrates are those of ethane, ethylene, propane, and isobulaue. Others include methane and I butene, most of the fluorocarbon refrigerant gases, nitrous oxide, acetylene, vinyl chloride, carbon dioxide, methyl and ethyl chloride, methyl and ethyl bromide, cyclopropane, hydrogen sulfide, methyl mercaptan, and sulfur dioxide. 

Insertion of a carbenoid into a C-H bond has also been performed under synthetic conditions. A recent novel approach to spiroacetal pheromone (equation 123) is unfortunately hampered by the absence of regioselectivity in the ultimate cyclopropane hydrogenation step (see Section III.A). 

The preference for the edge-protonated structure emerges from interesting microwave spectroscopy experiments conducted on the cyclopropane-hydrogen fluoride and cyclopropane-hydrogen chloride complexes which reveal a structure like that shown in Figure 54. 

Finally, alloying patterns are justified. Figure 4.6 shows cyclopropane hydrogenation decreasing uniformly with surface Cu concentration. Each nickel atom retains its orbital integrity, and the effect of copper is simply to dilute. For instance, if n surface nickel sites are required to generate an absorbed species, then a surface copper fraction fcu results in an activity decrease. given by... 

The initiating mechanism of the cyclopropane-hydrogen reaction presumably results in an adsorbed n-propyl radical, while by reason of its weaker secondary C—H bond, propane probably dissociates into an isopropyl radical and an H atom. The close similarity between the distributions in the two cases su ested that exchange proceeds through the equilibria... 

In 1968 Winstein generated the pentamethylbicyclo[3,l,0]hexenyl cation 489 from olefine 485 The cited chemical shifts (t scale) show the methylene cyclopropane hydrogens of ion 489 to be deshielded in comparison with the respective protons of the parent olefine by 2-3 ppm. This points to essential participation of the cyclopropane fragment in positive charge delocalization. 

The difference between cyclopropane and alkenes on the one hand and alkanes on the other is nowhere more clearly seen than in the kinetics of their reactions with hydrogen and in the relative activities of metals for those reactions. The ranking of metals activities for cyclopropane hydrogenation, namely,... 

Olefinic and cyclopropane hydrogens are metallated most readily by pcnlyl-sodium, particularly in the presence of potassium t-butoxidc. For example, camphene (1) can be convened into (2) and (3) as formulated in equation (1). Mclullalion of norlricyclene is tomudaled in equation (II). If an activating... 

Cyclopropanes can be attacked by radicals at either hydrogen or carbon (Scheme 16). Only very reactive radicals such as Cl , CF3 , t-BuO or imidyl radicals can abstract cyclopropane hydrogen atomswhile with less reactive species such as Br or I only S 2 substitution at carbon is observed . ... 

PROBLEM 15.49 List the factors that can influence the chemical shift of a hydrogen. Which of these factors do you suppose accounts for the fact that cyclopropane hydrogens appear at unusually high field, near 5 0.2 ppm. 

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