Tertiary C-H bonds

In mammalian liver microsomes, cytochrome P-450 is not specific and catalyzes a wide variety of oxidative transformations, such as (i) aliphatic C—H hydroxylation occurring at the most nucleophilic C—H bonds (tertiary > secondary > primary) (ii) aromatic hydroxylation at the most nucleophilic positions with a characteristic intramolecular migration and retention of substituents of the aromatic ring, called an NIH shift,74 which indicates the intermediate formation of arene oxides (iii) epoxidation of alkenes and (iv) dealkylation (O, N, S) or oxidation (N, S) of heteroatoms. In mammalian liver these processes are of considerable importance in the elimination of xenobiotics and the metabolism of drugs, and also in the transformation of innocuous molecules into toxic or carcinogenic substances.75 77... 

We ascribed the driving force for Eqs. (6.3) and (6.4) to be the high BDE values of the O-H bonds formed in hydrogen abstractions, which are particularly favorable over weaker C-H bonds (tertiary alkyl, benzyl, RCO-H, etc.). 

In order to develop a quantitative interpretation of the effects contributing to heats of atomization, we will introduce other schemes that have been advocated for estimating heats of formation and heats of atomization. We will discuss two schemes and illustrate them with the example of alkanes. Laidler [11] modified a bond additivity scheme by using different bond contributions for C-H bonds, depending on whether hydrogen is bonded to a primary (F(C-H)p), secondary ( (C-H)g), or tertiary ( (C-H)t) carbon atom. Thus, in effect, Laidler also used four different kinds of structure elements to estimate heats of formation of alkanes, in agreement with the four different groups used by Benson. 

Similarly by comparing the bond dissociation energies of the two different types of C—H bonds m 2 methylpropane we see that a tertiary radical is 30 kJ/mol (7 kcal/ mol) more stable than a primary radical... 

Thermal, Thermooxidative, and Photooxidative Degradation. Polymers of a-olefins have at least one tertiary C-H bond in each monomer unit of polymer chains. As a result, these polymers are susceptible to both thermal and thermooxidative degradation. Reactivity in degradation reactions is especially significant in the case of polyolefins with branched alkyl side groups. For example, thermal decomposition of... 

Insertions into tertiary C—H bonds can be carried out with moderate yields. Among other less common reactions improved by the use of PTC-generated CCI2 are the carbylamine synthesis (RNH2 — R NC) (33). Alkylidene carbene (R2C=C ) and alkenyUdene carbene (R2C=C=C ) adducts have also been prepared (34,35). 

Polyolefins such as polyethylene and polypropylene contain only C—C and C—H bonds and may be considered as high molecular weight paraffins. Like the simpler paraffins they are somewhat inert and their major chemical reaction is substitution, e.g. halogenation. In addition the branched polyethylenes and the higher polyolefins contain tertiary carbon atoms which are reactive sites for oxidation. Because of this it is necessary to add antioxidants to stabilise the polymers against oxidation Some polyolefins may be cross-linked by peroxides. 

The absence of both secondary and tertiary C—H bonds leads to a high measure of oxidative stability. Oxidation does take place when thin films are heated in air to temperatures above 300°C and causes cross-linking but this is of little practical significance. The absence of double bonds gives a very good but not absolute resistance to ozone. 

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. 

These differences have been attributed to various factors caused by the introduction of new structural features. Thus isopentane has a tertiary carbon whose C—H bond does not have exactly the same amount of s character as the C—H bond in pentane, which for that matter contains secondary carbons not possessed by methane. It is known that D values, which can be measured, are not the same for primary, secondary, and tertiary C—H bonds (see Table 5.3). There is also the steric factor. Hence, it is certainly not correct to use the value of 99.5 kcal mol (416 kJ mol ) from methane as the E value for all C—H bonds. Several empirical equations have been devised that account for these factors the total energy can be computed if the proper set of parameters (one for each structural feature) is inserted. Of course these parameters are originally calculated from the known total energies of some molecules that contain the structural feature. 

Compounds containing susceptible C—H bonds can be oxidized to alcohols. " Nearly always, the C—H bond involved is tertiary, so the product is a tertiary alcohol. This is partly because tertiary C—H bonds are more susceptible to free-radical attack than primary and secondary bonds and partly because the reagents involved would oxidize primary and secondary alcohols further. In the best method, the reagent is ozone and the substrate is absorbed on silica gel. Yields as high as 99% have been... 

The addition of acetic acid (0.5 equiv. to the substrate) to the catalyst system led to increased activity (doubling of yield) by maintaining the selectivity with 1.2 equiv. H2O2 as terminal oxidant. Advantageously, the system is characterized by a certain tolerance towards functional groups such as amides, esters, ethers, and carbonates. An improvement in conversions and selectivities by a slow addition protocol was shown recently [102]. For the first time, a nonheme iron catalyst system is able to oxidize tertiary C-H bonds in a synthetic applicable and selective manner and therefore should allow for synthetic applications [103]. 

Iron phthalocyanine is an efficient catalyst for intermolecular amination of saturated C-H bonds. With 1 mol% iron phthalocyanine and 1.5 equiv. PhlNTs, amination of benzylic, tertiary, and ally lie C-H bond have been achieved in good yields (Scheme 31). With cyclohexene as substrate, the allylic C-H bond amination product was obtained in 75% yield, and the aziridination product was found in minor amount (17% yield) [79]. 

The oxidation by Cr(VI) of aliphatic hydrocarbons containing a tertiary carbon atom has been studied by several groups of workers. Sager and Bradley showed that oxidation of triethylmethane yields triethylcarbinol as the primary product with a primary kinetic isotope effect of about 1.6 (later corrected by Wiberg and Foster to 3.1) for deuterium substitution at the tertiary C-H bond. Oxidations... 

There is some increase in selectivity with functionally substituted carbenes, but it is still not high enough to prevent formation of mixtures. Phenylchlorocarbene gives a relative reactivity ratio of 2.1 1 0.09 in insertion reactions with i-propylbenzene, ethylbenzene, and toluene.212 For cycloalkanes, tertiary positions are about 15 times more reactive than secondary positions toward phenylchlorocarbene.213 Carbethoxycarbene inserts at tertiary C—H bonds about three times as fast as at primary C—H bonds in simple alkanes.214 Owing to low selectivity, intermolecular insertion reactions are seldom useful in syntheses. Intramolecular insertion reactions are of considerably more value. Intramolecular insertion reactions usually occur at the C—H bond that is closest to the carbene and good yields can frequently be achieved. Intramolecular insertion reactions can provide routes to highly strained structures that would be difficult to obtain in other ways. 

Selective oxidations are possible for certain bicyclic hydrocarbons.285 Here, the bridgehead position is the preferred site of initial attack because of the order of reactivity of C—H bonds, which is 3° > 2° > 1°. The tertiary alcohols that are the initial oxidation products are not easily further oxidized. The geometry of the bicyclic rings (Bredt s rule) prevents both dehydration of the tertiary bridgehead alcohols and further oxidation to ketones. Therefore, oxidation that begins at a bridgehead position... 

Insertions of dichlorocarbene into tertiary C-H bonds were observed even if these were not activated by neighboring phenyl or ether groups. Yields are 3-29% under the conditions used and this insertion does not require a thermoexcitation of the dichlorocarbene as was assumed earlier.136... 

The effects of substitutents on the y-carbon on the efficiency of the type II cleavage are presented in Table 3.15.<89) These data indicate that the rate constant of cleavage increases as the strength of the y C—H bond decreases, that is, from a primary to a secondary to a tertiary hydrogen atom. The substitution of groups capable of radical stabilization, such as — or... 

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