Sp3 carbon atoms

For substituted anilines (Thompson and Williams, 1977) and for 1-naphthylamine and a series of derivatives thereof (Castro et al., 1986a), k2 and the ratio Ar 2/Ar3 have been determined for nucleophilic catalysis with Cl-, Br-, SCN-, and SC(NH2)2. The values of k2 correspond fairly well to those found for the diazotization of aniline, but those of Ar 2/Ar3 increase markedly in the above sequence (Table 3-1). As k3 is expected to be independent of the presence of Cl- or Br- and to show little dependence on that of SCN- or thiourea, the increase in k 2/k3 for this series must be due mainly to 2. Indeed, the value of log(Ar 2/Ar3) shows a linear correlation with Pearson s nucleophilicity parameter n (Pearson et al., 1968). This parameter is based on nucleophilic substitution of iodine (as I-) in methyl iodide by various nucleophiles. The three investigations on nucleophilic catalysis of diazotization demonstrate that Pearson s criteria for bimolecular nucleophilic substitution at sp3 carbon atoms are also applicable to substitution at nitrogen atoms. 

The bonds between an oxygen and an sp3 carbon atom in alcohols, ethers, or esters are quite resistant to hydrogenolysis. Elevated temperatures and pressures are required to induce C-O bond cleavage and the high temperature can cause the cleavage of the C—C bonds, too. 

There follows in Sect. Ill a detailed discussion of substituent effects and their dependence on different geometric relationships of carbon atoms with respect to the perturbing group. This, however, is strictly confined to sp3 carbon atoms, though allowing for all conceivable kinds of substituents, including unsaturated ones (phenyl, cyano, etc.). 

Finally, chemical-shift variations originating in van der Waals compression are noteworthy, although very few reports have come to our attention discussing signal shifts of sp3 carbon atoms in terms of van der Waals interactions. Schill and co-workers (89) found downfield shifts of up to 1 ppm in the [2]-catenane 5 when its 13C resonances are compared with those of the two isolated subunits. From the increase of these chemical-shift differences with increasing distance from the nitrogen atom, these authors concluded that the strongest van der Waals interactions occur in that part of the heterocycle which is opposite to the amide moiety in space (cf. Scheme 4). 

Although Fe(CO)rcomplexed butadiene does not react with dibromocarbene, the corresponding penta-1,3-dienes undergo C-H insertion at the sp3 carbon atom (27-38%) [15], Metal carbonyl-complexed cyclohexadiene and 1-methoxycarbonyl-1,2-dihydropyridine also react in a similar manner in 31 and 54% yield, respectively. 

The chemical shifts of the fullerene sp2 and sp3 carbon atoms are typically in the regions 155-135 ppm and 80-50 ppm, respectively. However, in cases where the latter carbon atoms are bound to an especially electronegative heteroatom, then they resonate at much lower fields, e.g., for C60O 8(Csp3) = 91 ppm (67, 68). 

For any open-chain single bond that connects two sp3 carbon atoms, an infinite number of conformations are possible, each of which has a certain energy associated with it. For ethane there are two extremes, a conformation of highest and one of lowest potential energy, depicted in two ways as ... 

There appear to be two C5-dehydrocyclizations over platinum-on-carbon catalyst. Activation energy differences suggest that the reaction involving an sp2 and an sp3 carbon atom (a cyclization in which the new bond is formed between an aliphatic and an aromatic carbon atom) is different from cyclizations involving two sp3 carbon atoms (in which the new bond is formed between aliphatic carbon atoms of two side-chains). 

This type of reaction is related to the well-known SN2 reaction, widely encountered in organic chemistry. The essential feature of this type of a reaction is the presence of a leaving group attached to the sp3 carbon centre. The reactions may be controlled in a number of ways. The metal could either co-ordinate to the leaving group and stabilise the anionic species as it is formed, or it could co-ordinate to some other group and increase the S+ charge on the sp3 carbon atom and hence activate it towards the initial attack by the nucleophile (Fig. 4-36). 

The degradation pattern of alkenes has been amply illustrated, particularly for tetrazepam and cholesterol. In addition to the activation of the a-sp3 carbon atom, alkenes can give epoxidation and polymerization... 

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