A-heteroatom stabilized

The Andersen sulphoxide synthesis allows one also to synthesize a variety of a-heteroatom substituted sulphoxides starting from a-heteroatom stabilized carbanions and (—)-(S)-276. The selected examples shown in Scheme 3 are the best illustration of the generality of this approach. The reaction of enolates or enolate like species with (—)-(S)-276 has been used for the synthesis of optically active a-carbalkoxy sulphoxides. For example, treatment of (—)-(S)-276 with the halogenomagnesium enolates of -butyl acetate, t-butyl propionate or t-butyl butyrate resulted in the formation of ( + )-(R)-t-butyl p-toluenesulphinylcarboxylates 298367 (equation 163). 

The majority of Al-heterocyclic carbenes and their metal complexes are obtained from cyclic azolium derivatives (Fig. 8). Nevertheless, the first complex with a heteroatom stabilized carbene ligand, although it was not recognized as such, was prepared by Tschugajeff et al. as early as 1925 [157]. It was later identified as a heterocarbene complex [158], Tschugajeff reacted the nucleophihc proton base... 

Increasing the solvation causes the C,—Li contact to be gradually given up whereas the electrostatic contact of the lithium to the a-carbon is maintained. However, the estimation of the solvation number in the solid state shows the presence of two THF molecules per lithium. This study suggests that specific solvation increases the ability of the sulfur group to localize the negative charge on the a-carbon atom (a-heteroatom stabilization). 

Additions of a-Heteroatom-stabilized Carbanions (Non-acyl Anion Equivalents) 115... 

A rare species of salts consisting of a heteroatom-stabilized carbocation and a heteroatom-stabibzed carbanion has been formed by deprotonating methyl (Z)- or (E)-3-hydroxy-2,3-dimesitylpropenoate with tetrakis(dimethylamino)methane the resonance stabilization of the cation [(CH3)2N]3C+ and enolate anion, which is of E-configuration exclusively, since the guanadinium ion is incapable of forming a chelate, prevents a spontaneous O- or C-alkylation.12... 

A method for the synthesis of eight-membered carbocycles based on a new tandem tungsten-catalysed cycloisomerization-cyclopropanation reaction has been reported (Scheme 99).143 This is a catalytic reaction in which a heteroatom-stabilized Fischer... 

Substituents with a-heteroatoms stabilize carbenium ions via resonance, which leads formally to oxonium ions [Eq. (3)]. 

A nonbonding lone pair on a heteroatom stabilizes a carbocation more than any other interaction. A resonance structure can be drawn in which there is a ir bond between the positively charged heteroatom and the adjacent cationic center. The heteroatom must be directly attached to the electron-deficient C for stabilization to occur if not, destabilizing inductive effects take over. Even if the heteroatom is directly attached, if geometric constraints prevent overlap between the two orbitals, as in the bicyclic carbocation shown, then no stabilization occurs, and inductive effects take over. 

Common error alert When a heteroatom stabilizes a carbocation by sharing its lone pair, it still has its octet and hence is neither electron-deficient nor electrophilic (Remember that when an electronegative atom has a formal positive charge and its full octet, the atoms adjacent to it are electrophilic.) Electronegative atoms like N, O, and F are capable of stabilizing carbocations by resonance exactly because they do not surrender their octet when they participate in resonance. 

Low energy (55 eV) electron irradiation of multilayers of methanol on silver has been used in conjunction with temperature-programmed desorption of the products as a method to identify radiolysis products. In this case several products such as methoxymethanol were identified . The primary photochemical process in the methanol system is thought to involve the fission of a C—H bond forming a heteroatom stabilized cation such as 6. In addition to methoxymethanol glycolaldehyde (5) or methyl formate are also detected. Apparently the methanol system is unreactive to UV irradiation either at 240 nm or at 355 nm . 

The two examples portrayed below, each occurring via different mechanistic pathways, give one a sense of the breadth of the chemistry. In the first (Eq. 2), different nucleophiles, viz., azide and methanol, add across a double bond [5]. The more nucleophilic of the pair adds to the less substituted carbon to afford a heteroatom-stabilized cation that is then intercepted by the second nucleophile, ultimately delivering the product (Eq. 3). The second example portrays the replacement of a C-H bond on anthracene with acetonitrile to afford the acetamide adduct after hydrolysis of the nitrilium ion intermediate (Eq. 4) [6]. 

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