Primary carbon atom direct substitutions

Carbon atoms are classified according to their degree of substitution by other car bons A primary carbon is directly attached to one other carbon Similarly a secondary carbon is directly attached to two other carbons a tertiary carbon to three and a qua ternary carbon to four Alkyl groups are designated as primary secondary or tertiary according to the degree of substitution of the carbon at the potential point of attachment... 

In the kinetic studies described in Tables 5.1 and 5.2, the reactions were forced to follow either the SnI or the Sn2 mechanism to yield the relevant rate data. In general, as we said earlier, the two mechanisms compete. This means that the faster mechanism predominates. So, methyl and primary alkyl substrates generally react by the Sn2 mechanism, and tertiary substrates almost always react by the SnI mechanism. Secondary substrates can react via either mechanism depending upon the conditions. Exceptions to this rule include compounds with one or more phenyl groups directly attached to the carbon atom undergoing substitution. 

The reaction of an alkyl halide or los3 late with a nucleophiJe/base results eithe in substitution or in diminution. Nucleophilic substitutions are of two types S 2 reactions and SN1 reactions, in the SN2 reaction, the entering nucleophih approaches the halide from a direction 180° away from the leaving group, result ing in an umbrella-like inversion of configuration at the carbon atom. The reaction is kinetically second-order and is strongly inhibited by increasing stork bulk of the reactants. Thus, S 2 reactions are favored for primary and secondary substrates. 

Several trivial but highly useful reactions are known to convert one acceptor-substituted allene into another. For example, the transformation of allenic carboxylic acids is possible both via the corresponding 2,3-allenoyl chlorides or directly to 2,3-allen-amides [182,185], Allenylimines were prepared by condensation of allenyl aldehydes with primary amines [199]. However, the analogous reaction of allenyl ketones fails because in this case the nucleophilic addition to the central carbon atom of the allenic unit predominates (cf. Section 7.3.1). Allenyl sulfoxides can be oxidized by m-CPBA to give nearly quantitatively the corresponding allenyl sulfones [200]. The reaction of the ketone 144 with bromine yields first a 2 1 mixture of the addition product 145 and the allene 146, respectively (Scheme 7.24). By use of triethylamine, the unitary product 146 is obtained [59]. The allenylphosphane oxides and allene-... 

Finally, another important heuristic principle is to carry out the systematic disconnection of substituted nucleophilic heteroatoms (O, N, S) directly attached to the carbon skeleton, especially if they are attached to a primary sp carbon atom (HP-8) ... 

Aldono-1,4-lactones, having six or more carbon atoms also form an exocyclic aceteoxonium ion between the primary and the neighbouring hydroxy group when treated with hydrogen bromide in acetic acid. By the opening of this ion, bromine is easily introduced at the primary carbon (Scheme 1, entry II). Pento-nolactones also yield 5-bromo-5-deoxy-lactones when treated with HBr-HOAc, although the reaction requires 1-4 days. Since no exocyclic acetoxonium ion can be formed, a direct substitution probably takes place in these cases. 

Lithium alkyls or aryls add to pseudoazulenes 3398100 and 39.113-114 The carbanion always reacts with the carbon atom opposite the heteroatom in the six-membered ring (Scheme 16). After hydrolysis of primary intermediate 121 dihydro products (122) are formed, which can be transformed to pseudoazulenes in the usual manner (see Section III,A,2). The structure of primary intermediate 121 is confirmed by its alkylated (or acylated) products (123). Even these compounds can easily be dehydrogenated to pseudo-azulene. In this way. substituted pseudoazulenes can be formed that are not obtainable otherwise. The direction of addition to the pseudoazulenes is the same as in azulenes.241... 

Different chlorine-substituted fragments in partially substituted complexes have similar reactivities, and the direction of the reaction is not determined by the electron density on the carbon atoms (i.e., their electrophilic properties) but by more specific effects an intramolecular activation via hydrogen bonds in the case of sterically unhindered primary amines and sterical hindrances in the case of secondary and sterically hindered primary amines. The reaction with sterically unhindered primary amines occurs via the route An through the substitution of a halogen atom from an already monofunctionalized fragment. In the case of secondary and sterically hindered primary aliphatic amines, the reaction proceeds via the route Bn with sterically controlled substitution. 

This article, therefore, will deal only with the chemistry of heteroaromatic compounds directly substituted on a ring carbon with a boron atom. The primary chemical literature published between 1965 and the middle of 1987 is surveyed, but patents are not included. Earlier relevant literatures are also referenced. 

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