Alkanes, addition cyclopropanes

The synthesis of bicyclo[n. 1.0]alkanes (82) from various 1-chlorovinyl p-tolyl sulfoxides (80) and lithium enolate of ieri-butyl acetate, propionate and hexanoate through the adducts (81) are summarized in Table 3. As shown in Table 3, addition reaction of tert-butyl carboxylates to 1-chlorovinyl p-tolyl sulfoxides (80) proceeds smoothly to afford the adducts (81) in high to quantitative yields. Cyclopropanation of 81 with i-PrMgCl... 

Cyclopropane is generally more reactive than other alkanes. Reactions that open the cyclopropane ring release 115 kJ (27.6 kcal) per mole of ring strain, which provides an additional driving force for these reactions. 

The direct photolysis and benzophenone-sensitized decomposition of methyl diazomalonate has been shown to produce two different spin states of biscarbo-methoxycarbene. In the direct photolysis experiments stereospecific addition to olefins and a high yield of insertion products in the presence of alkanes demonstrate the singlet character of the carbene in the sensitized decomposition the yield of insertion products decreases, and addition to cis- or ra 5 -4-methyl-2-pentene yields about 88 % trnwj-cyclopropane. In this case, as with most carboalkoxy-carbenes, the addition reaction to olefins can proceed by two paths ... 

Bicyclobutane is unique in that it does not obey the strain additivity rule of small-ring bicyclo[n.m.O]alkanes. For example, the strain energy of bicyclo[2.1.0]pentane, using the Franklin group increment is 53.6 kcal mol" This should be compared to the sum of the strain energies in cyclobutane (26.4) and cyclopropane (27.6 kcal mol" ). Yet, the sum of two cyclopropanes (55.2 kcal mol" is ca. 9 kcal mol" lower than that of bicyclobutane. 

In 1981, Noels reported that rhodium(II) carboxylates, originally developed as cyclopropanation catalysts, smoothly catalyze the addition of ethyl diazoacetate (21 equation 9) to a variety of alkanes. " While some differentiation between possible sites of insertion is observed, selectivity is not as high for this carbenoid process as it is for the free radical processes illustrated above. 

Bromine is an electrophile which will undergo an addition reaction with alkenes in the dark. The lack of a reaction in the dark with Br2 indicates that the molecule is not an alkene. In the presence of light, bromine will undergo a substitution reaction with alkanes. Therefore, the molecule is most likely an alkane and the only alkane with the molecular formula C3H6 is cyclopropane. 

In addition to these exchange reactions, a number of alkane/alkane and al-kane/arene exchange reactions could be studied as equilibria (benzene, toluene, cyclopropane, methane, ethane, neopentane, cyclohexane). Determination of equilibrium constants allowed calculation of AG° values and estimation of relative metal-carbon bond energies. Wolczanski concluded that the differences between metal-carbon bond energies and the corresponding carbon-hydrogen bond energies were essentially the same [82]. 

The regioselectivity of the addition of copper-activated alkanes to activated cyclopropanes is not always maintained. An exception is the cleavage of the cyclopropyl bond in a very complex polycycle upon treatment with lithium dimethylcuprate which adds in a 1,5-fashion across the cyclopropane bond. ° ... 

The lack of alkane C-C activation also arises from the fact that two relatively weak M-C bonds are formed in the process [Eq. (6.105)]. However, in strained molecules such as small-ring cycloalkanes (cyclopropanes, cyclobutanes), relief of strain is an additional favorable factor. Furthermore, C-C activation can be rendered thermodynamically more favorable when an extra driving force is available, such as formation of an aromatic structure or by utilizing an activating functionality. 

Photolysis (200-260 nm) of diazomethane (CH2N2) produces highly reactive methylene ( CH2), which can insert into primary, secondary, and tertiary C-H bonds of an alkane with almost equal ease, as well as undergo addition to double bonds. The cyclopropanation of alkenes can, however, be achieved by carrying out the photolytic decomposition of diazomethane in the presence of metal salts.4,5 Palladium(II) acetate has been reported to be a very effective catalyst for the cyclopropanation of alkenes using diazomethane6 (vide infra). 

Cyclopropane is one notable exception to the generalization that cyclic and acyclic compounds undergo the same reactions. Although it is an alkane, cyclopropane undergoes electrophilic addition reactions as if it were an alkene. 

---