C-H bonds secondary

Only 20—40% of the HNO is converted ia the reactor to nitroparaffins. The remaining HNO produces mainly nitrogen oxides (and mainly NO) and acts primarily as an oxidising agent. Conversions of HNO to nitroparaffins are up to about 20% when methane is nitrated. Conversions are, however, often ia the 36—40% range for nitrations of propane and / -butane. These differences ia HNO conversions are explained by the types of C—H bonds ia the paraffins. Only primary C—H bonds exist ia methane and ethane. In propane and / -butane, both primary and secondary C—H bonds exist. Secondary C—H bonds are considerably weaker than primary C—H bonds. The kinetics of reaction 6 (a desired reaction for production of nitroparaffins) are hence considerably higher for both propane and / -butane as compared to methane and ethane. Experimental results also iadicate for propane nitration that more 2-nitropropane [79-46-9] is produced than 1-nitropropane [108-03-2]. Obviously the hydroxyl radical attacks the secondary bonds preferentially even though there are more primary bonds than secondary bonds. 

Methylene from diazirine has higher energy of vibration than the product from photolysis of ketene, but it is more discriminating in insertion reactions into primary and secondary C—H bonds. 

What are the reasons for the observed reactivity order of alkane hydrogens toward radical chlorination A look at the bond dissociation energies given previously in Table 5.3 on page 156 hints at the answer. The data in Table 5.3 indicate that a tertiary C—H bond (390 kj/mol 93 kcal/mol) is weaker than a secondary C-H bond (401 kj/mol 96 kcal/mol), which is in turn weaker than a primary C H bond (420 kj/mol 100 kcal/mol). Since less energy is needed to break a tertiary C-H bond than to break a primary or secondary C-H bond, the resultant tertiary radical is more stable than a primary or secondary radical. 

The rate of this intramolecular isomerization depends on the chain length, with the maximum in the case of a six-atomic transition state, i.e., when the tertiary C—H bond is in the (3-position with respect to the peroxyl group [13]. For the values of rate constants of intramolecular attack on the tertiary and secondary C—H bond, see Table 2.9. The parameters of peroxyl radical reactivity in reactions of intra- and intermolecular hydrogen atom abstraction are compared and discussed in Chapter 6. 

The activation of cyclohexane can also be achieved using an Ir(m) complex. The reactions with -alkanes are unselective and a mixture of products, probably resulting from the activation of primary and secondary C-H bonds, is obtained (Equation (4)).12... 

The primary C-H bond is often activated in these and related systems, whereas secondary C-H bonds are often intact, although in general chemistry the reactivity of a C-H bond is often in the order tertiary > secondary > primary.24... 

The regioselectivity of the C-H insertion is very dependent on the nature of the catalysts. A good example of this is the reaction of 2-methylbutane (Equation (6)).56 As can be seen in the reactions with 2-methylbutane, the silver and copper catalysts TpBf3Cu and Tp(GFs)2Ag resulted in competitive C-H insertions at the tertiary and secondary C-H bonds. In contrast, the more electrophilic silver catalyst TpBf3Ag was less discriminating and all four possible products were formed, comparable to the earlier results with the rhodium catalysts. 

When the alkyl substituent contains secondary C-H bonds, both ring and side chain oxidation at the secondary C-H bond occur. Thus, ethylbenzene gives... 

Recently, substantial progress has been registered in regard to the regioselective CH oxidations by dioxiranes. Usually, the regioselectivity of the CH oxidation is mainly governed by the reactivity of the C—H bond for example, in the above-mentioned oxidation of c -decalin , the tertiary C—H bond is selectively oxidized in the presence of the secondary C—H bonds. When the reactivities are similar, the regioselectivity is determined by steric factors. For example, the preferential oxyfunctionalization of the tertiary C—H bond at the C-14 position of the steroid 19 by DMD in the presence of the other tertiary C—H bonds at the C-5, C-8 and C-9 positions is due to steric reasons (equation 30) . ... 

The first preparative use of intramolecular C—H insertion in organic synthesis was based on the observation that on flash vacuum pyrolysis, a conjugated alkynyl ketone such as 1-(1-methyl-cyclopentyl)-2-propynonc is smoothly converted to a mixture of the cyclizcd enones 1 and 223. This elegant reaction apparently proceeds via isomerization of the alkyne to the corresponding alkylidene carbcne, followed by subsequent intramolecular C-H insertion. It should be noted that despite a 3 2 statistical predominance of primary C-H bonds over secondary C—H bonds, a marked preference for insertion into the latter (methylene) is observed. 

This reaction apparently proceeds by way of the normal phosphonate condensation product, the diazoalkylidene, which then spontaneously loses nitrogen to form the transient alkylidene car-bene. Careful work showed that, after statistical corrections were applied, the reactivity of a C-H bond toward insertion was approximately 0.003 for primary C-H bonds (methyl), 1.0 for secondary C-H bonds (methylene), 7.5 for benzylic (methylene) C-H bonds and 18.6 for tertiary C-H bonds. These relative reactivities are very similar to those previously observed for intramolecular C-H insertion by an alkylidene carbenoid generated from a vinyl bromide27. It was shown subsequently that the alkylidene carbene insertion reaction proceeds with retention of absolute configuration28. Using this approach, (l )-3-dimethyl-3-phenyl-l-cyclopentene and (i )-4-methyl-4-phenyl-2-cyclohexcnonc were prepared in high enantiomeric purity. 

Rate constants were determined for CeHsCCl insertions into Si—H, N—H, and C—H bonds.The C—H substrates included cumene (31, X = H, k = 1.7 X lO M s ), ethylbenzene (8.2 x lO M s ), and toluene (7.5 x 10 s ). These C—H insertions are several orders of magnitude slower than the alkene additions of CgHsCCl summarized in Table 7.5. Other interesting substrates include c/5,c/i-l,3,5-trimethylcyclohexane (44), adamantane (37), and cyclohexane (45). On a per-H basis, the rate constants for CeHsCCl insertion were 1.0 x 10, 1.3 x 10, and 0.06 x lO M s, respectively (Fig. 7.17). The tertiary C—H bonds of 44 and 37 are slightly less reactive than the tertiary and benzylic C—H of cumene, but they are 15-20 times more reactive than the secondary C—H bonds of cyclohexane. These observations agree with the charge distributions depicted in transition states 30 and 33. 

Answer. Three factors combine to make this reaction facile (a) activation of the carbonyl group toward nucleophilic addition as a result of coordination to the Lewis acid (aluminum triisopropoxide), as discussed in Chapter 8 (b) activation of the secondary C—H bond as a donor by the presence of the very good X substituent (— —Al, which resembles —O), as discussed in Chapter 4 and (c) opportunity presented by the coordination within the complex shown in Figure B.4,... 

The presence of other hexane isomers and a typical hexane isomer distribution of 26% 2,3-dimethylbutane, 28% 2-methylpentane, 14% 3-methylpentane, 32% n-hexane, far from equilibrium, indicate that the 1-propyl cation (although significantly delocalized with protonated cyclopropane nature) is also involved in alkylation. It yields n-hexane and 2-methylpentane through primary or secondary C—H bond insertion, respectively (Scheme 5.3). 

H202 in superacids at —78°C converts simple straight-chain alkanes into primary alcohol (ethane), or secondary alcohols and ketones (propane, butane).1,62 89 9° Electrophilic hydroxylation of the secondary C—H bond by the incipient hydroxyl cation formed through the protolytic cleavage of hydroperoxo-nium ion accommodates these observations ... 

The results of dry ozonation, namely, regioselectivities and stereoselectivities, are very similar to those in superacidic liquid-phase ozonation. Tertiary C—H bonds in strained systems such as norbomane are inert to dry ozonation.93 Such compounds are oxidized at the secondary carbon to yield a mixture of alcohols and ketones.93 104 Similarly, substituted cyclopropanes exhibit a general preference for the oxidation of the secondary C—H bond in the a-position to the ring 104... 

Propane contains one secondary carbon, and the secondary C—H bonds are weaker than the primary C—H bonds (Table I). Therefore, propane is... 

The selectivity is high at low conversions and decreases as the conversion increases. Thus the sequential reaction of Eq. (3) is the primary reaction pathway for these catalysts, and the activation of propane is mostly by breaking of a secondary C—H bond to form a propyl species, with breaking of a primary C—H bond contributing to a minor extent. 

The first step of the activation of butane and cyclohexane has been assumed to be the cleavage of a secondary C—H bond, with minor contributions from primary C — H bonds in the case of butane. This picture is supported only by indirect evidence. When the relative rates of reaction of various alkanes were compared on a V-Mg oxide and Mg2V207 catalyst (Table VIII), it was found that alkanes with only primary carbons (ethane) reacted most slowly. Those with secondary carbons (propane, butane, and cyclohexane) reacted faster, with the rate being faster for those with more secondary carbon atoms. Finally, the alkane with one tertiary carbon (2-methylpropane) reacted faster than the ones with either a single or no secondary carbon (26). From these data, it was estimated that the relative rates of reaction of a primary, secondary, and tertiary C—H bond in alkanes on the V-Mg oxide catalyst were 1, 6, and 32, respectively (26). 

Direct evidence about the first step of activation of butane was obtained on a V-P oxide catalyst in the butane oxidation to maleic anhydride based on deuterium kinetic isotope effect (34). It was found that when a butane molecule was labeled with deuterium at the second and third carbon, a deuterium kinetic isotope effect of 2 was observed. No kinetic isotope effect was observed, however, if the deuterium label was at the first or fourth carbon. By comparing the observed and theoretical kinetic isotope effects, it was concluded that the first step of butane activation on this catalyst was the cleavage of a secondary C—H bond, and this step was the rate-limiting step. 

Here it should be noted that secondary C-H bond rupture is only slightly more probable than the scission of primary bonds, despite the fact that D(iso-C3H7—H) is 5-6 kcal./mole lower than D(m-C3Ht—H) (70,71). Hence, the bond-dissociation energy does not appear to be the major determining factor in the primary mode of decomposition. However, the results obtained by Palmer and Lossing (73) for the isobutane reaction do indicate that methyl substitution on the secondary position in propane causes C-H bond cleavage to occur preponderately at the tertiary site. 

Considering all possibilities, the protolytic cleavage of propane can be summarized according to Scheme 5.42. Since the butyryl cation was not detected, path a (involvement of n-propyl cation 34) can be excluded. Pathway b (protonation of the secondary C—H bond) is kinetically disfavored compared with protonation of the more electron-rich C—C bond (pathways c andrf). The large amount of methane leaves only path c as the major activation route of propane leading to the formation of ethylcarboxonium ion 112 (formation of the ethyl cation followed by carbonylation with CO). 

Bicyclo[2.2.1]heptane (norbomane) and bicyclo[2.2.2]octane, when treated with nitronium tetrafluoroborate in nitrile-free nitroethane, unexpectedly gave no nitro products. Instead, only bicyclo[2.2.1]heptane-2-one and bicyclo[2.2.2]octan-l-ol were isolated, respectively.500 Observation of bicyclo[2.2.1]heptane-2-yl nitrite as an intermediate and additional information led to the suggestion of the mechanism depicted in Scheme 5.48. In the transformation of norbomane the first intermediates are the 2-norbornyl cation 126 formed by hydride abstraction and nonclassical cation 127 formed through insertion of N02+ into the secondary C—H bond. In the case of bicyclo [2.2.2]octane, the oxidation of bridgehead tertiary C—H bond takes place and no further transformation can occur under the reaction conditions. Again these electrophilic oxygenation reactions testify to the ambident character of the nitronium ion. 

Dehydrodimerization. On excitation with a mercury vapor lamp, mercury is converted to an excited state, Hg, which can convert a C—H bond into a carbon radical and a hydrogen atom. This process can result in dehydrodimerization, which has been known for some time, but which has not been synthetically useful because of low yields when carried out in solution. Brown and Crabtree1 have shown that this reaction can be synthetically useful when carried out in the vapor phase, in which the reaction is much faster than in a liquid phase, and in which very high selectivities are attainable. Secondary C—H bonds are cleaved more readily than primary ones, and tertiary C—H bonds are cleaved the most readily. Isobutane is dimerized exclusively to 2,2,3,3-tetramethylbutane. This dehydrodimerization is also applicable to alcohols, ethers, and silanes. Cross-dehydrodimerization is also possible, and is a useful synthetic reaction. 

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