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Reactions at an sp-Carbon Atom

Reactions at an sp -Carbon Atom 4.4.2.1. Affecting a Carbonyl Group... [Pg.742]

Modem synthetic practice frequently requires the use of methods mote specific than those outlined above. Much attention has been focused on the mixed Claisen or Dieckmann reaction, i.e. the acylation of one ester by another, or its intramolecular equivalent, the regioselective cyclization of an unsymmetri-cal diester. A similar problem arises with the acylation of unsymmetrical ketones. This chapter thus describes the inter- and intra-molecular carbon-carbon bond-forming reactions in which a delocalized enolate anion (or close equivalent) reacts at an sp carbon atom in an addition-elimination sequence, as well as the acid-catalyzed equivalent employing an enol. In Table 1 we list the potential nucleophiles and the electrophiles that have been employed in these reactions, although not every possible combination has been reduced to synthetic practice. Table 2 gives details of acid-catalyzed acylations (see Section 3.6.4.3). [Pg.797]

This chapter will discuss the concept of a nucleophile that reacts at an sp carbon atom to form a new molecule where another atom or group has replaced one in the molecule that is attacked. This type of reaction is called a substitution (see Chapter 6, Section 6.6, for a brief introduction). The Lewis base analogy of a two-electron donor is used to explain the reactivity of nucleophiles, but the target is the sp carbon of an alkyl halide, or related compounds. This reaction can occur in two fundamental ways one by direct displacement and the other by ionization to a carbocation followed by attack of the nucleophile. [Pg.504]

If we had done this reaction with an aliphatic compound, for example, cyclohexylamine, we would still make the diazonium salt, but aliphatic diazonium salts are much less stable than aromatic ones, and this would decompose with loss of molecular nitrogen at -50 °C. Aromatic diazonium compounds are more stable, because the aryl cation that would be produced by nitrogen loss is at an sp carbon atom, which, as we noted in Chapter 11, is very unfavorable. However, molecular nitrogen is a fabulous leaving group the N=N triple bond has lots of enthalpy, and the process also benefits from the entropy of the loss of a mole of gas. So the diazonium salt decomposes slowly to give an aromatic cation, which is then captured by any available nucleophile (Figure 13.2). [Pg.552]

The structural X-ray elucidation of the o -adduct derived from the reaction of benzeneruthenium(II) complex with n-butyllithium has revealed that the cyclohexadienyl ring possesses an envelope conformation, in which the n-butyl group occupies the exo-position at the sp -carbon atom (Scheme 66) [11, 206]. [Pg.40]

Chloroadenallene 45 gave, after refluxing with triethylphosphite for 30 min, g-2 -phosphonate 47 in 30% yield (Scheme 7). A routine dealkylation afforded phosphonic acid 48 (60%). Thus, the usual reaction at the reactive allylic terminal was suppressed in favor of a nucleophilic attack at the sp carbon atom. The mechanism may include an intramolecular dealkylation as indicated in formula 49, although an intermolecular reaction is also possible. [Pg.85]

In this section, we considered two different mechanisms by which a substitution reaction at an sp hybridized carbon atom occurs. It is important to remember that (1) 5 1 and 5 2 reactions compete with each other, and (2) the exact details of any particular reaction might place it somewhere between these two mechanistic extremes. With an appropriate selection of reactants and reaction conditions, we might be able to push a particular reaction toward one mechanism or the other. Ultimately, the molecules in a system will always react by the lowest energy pathway. Depending on the reactants and reaction conditions, the lowest energy pathway for a substitution reaction may be the SnI reaction or the Sn2 reaction or something in between these two extremes. Table 27.2 summarizes some key ideas from this section and identifies the combinations of reactants and reaction conditions that push a substitution reaction toward either the SnI or Sn2 mechanism. [Pg.1283]

The susceptibility or mixing coefficients, pj and pj , depend upon the position of the substituent (indicated by the index, /) with respect to the reaction (or detector) center, the nature of the measurement at this center, and the conditions of solvent and temperature. It has been held that the p/scale of polar effects has wide general applicability (4), holding for substituents bonded to an sp or sp carbon atom (5) and, perhaps, to other elements (6). The or scale, however, has been thought to be more narrowly defined (7), holding with precision only for systems of analogous pi electronic frameworks (i.e., having a dependence on reaction type and conditions, as well as on position of substitution). [Pg.15]

The attack of the nucleophile on the acceptor-substituted allene usually happens at the central sp-hybridized carbon atom. This holds true also if no nucleophilic addition but a nucleophilic substitution in terms of an SN2 reaction such as 181 — 182 occurs (Scheme 7.30) [245]. The addition of ethanol to the allene 183 is an exception [157]. In this case, the allene not only bears an acceptor but shows also the substructure of a vinyl ether. A change in the regioselectivity of the addition of nucleophilic compounds NuH to allenic esters can be effected by temporary introduction of a triphenylphosphonium group [246]. For instance, the ester 185 yields the phos-phonium salt 186, which may be converted further to the ether 187. Evidently, the triphenylphosphonium group induces an electrophilic character at the terminal carbon atom of 186 and this is used to produce 187, which is formally an abnormal product of the addition of methanol to the allene 185. This method of umpolung is also applicable to nucleophilic addition reactions to allenyl ketones in a modified procedure [246, 247]. [Pg.383]

ETEROAROMATics FURAN AND THIOPHENE. The chemical transformation of thiophene at high pressure has not been studied in detail. However, an infrared [441,445] study has placed the onset of the reaction at 16 GPa when the sample becomes yellow-orange and the C—H stretching modes involving sp carbon atoms are observed. This reaction threshold is lower than in benzene, as expected for the lower stability of thiophene. The infrared spectrum of the recovered sample differs from that of polythiophene, and the spectral characteristics indicate that it is probably amorphous. Also, the thiophene reaction is extremely sensitive to photochemical effects as reported by Shimizu and Matsunami [446]. Thiophene was observed to transform into a dark red material above 8 GPa when irradiated with 50 mW of the 514.5-nm Ar+ laser line. The reaction was not observed without irradiation. This material was hypothesized to be polythiophene because the same coloration is reported for polymeric films prepared by electrochemical methods, but no further characterization was carried out. [Pg.201]

Finally, the polar C-O bonds make the carboxy carbon electrophilic, so carboxylic acids react with nucleophiles. Nucleophilic attack occurs at an sp hybridized carbon atom, so it results in the cleavage of the Jt bond, as well. This reaction is also discussed in Chapter 22. [Pg.699]

Additions to nonactivated olefins and dienes are important reactions in organic synthesis [1]. Although cycloadditions may be used for additions to double bonds, the most common way to achieve such reactions is to activate the olefins with an electrophilic reagent. Electrophilic activation of the olefin or diene followed by a nucleophilic attack at one of the sp carbon atoms leads to a 1,2- or 1,4-addition. More recently, transition metals have been employed for the electrophilic activation of the double bond [2]. In particular, palladium(II) salts are known to activate carbon-carbon double bonds toward nucleophilic attack [3] and this is the basis for the Wacker process for industrial oxidation of ethylene to acetaldehyde [41. In this process, the key step is the nucleophilic attack by water on a (jt-ethylene)palladium complex. [Pg.451]

One of the oldest and still one of the most powerful methods of making cyclic alkynes uses the reaction of metal acetylides with halogen compounds [4]. If the halogen (or a similar leaving group) is situated at an sp -hybridized carbon atom, the reaction can usually be carried out without the help of a catalyst in moderately polar solvents such as THE Recently, it has been found that lithium acetylides give better yields than sodium acetylides in the preparation of cyclic dialkynes (Scheme 8-1) [5]. [Pg.286]


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At carbon

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