Carbenium ion addition

TABLE 5. Comparison of substituent effects on the relative rates of carbenium ion addition to carbon-carbon double bonds... 

One of the rare cases of an intermolecular carbenium ion addition to an alkene without polymer formation occurs in the industrial synthesis of isooctane (Figure 3.54). 

Fig. 3.54. Carbenium ion additions to isobutene as key steps in the cationic polymerization of isobutene. The dashed arrow corresponds to the overall reaction.

Carbenium ion additions to C=C double bonds are of greater preparative importance when they take place intramolecularly as ring closure reactions (see Figure 3.55). 

Chiral rhodium(II) oxazolidinones 5-7 were not as effective as Rh2(MEPY)4 for enantioseleetive intramolecular cyclopropanation, even though the sterie bulk of their chiral ligand attachments (COOMe versus /-Pr or C Ph) are similar. Significantly lower yields and lower enantiomeric excesses resulted from the decomposition of 11 catalyzed by either Rh2(4S-IPOX)4, Rh2(4S-BNOX)4, or Rh2(4R-BNOX)4 (Table 3). In addition, butenolide 12, the product from carbenium ion addition of the rhodium-stabilized carbenoid to the double bond followed by 1,2-hydrogen migration and dissociation of RI12L4 (Scheme II), was of considerable importance in reactions performed with 5-7 but was only a minor constituent ( 1%) from reactions catalyzed by Rh2(5S-MEPY)4. This difference can be attributed to the ability of the carboxylate substituents to stabilize the earboeation form of the intermediate metal carbene. 

Intermolecular additions of carbenium ions to olefins give polymers. Such a reaction is used in industry, for example, in the cationic polymerization of isobutene (Figure 3.43). One of the rare cases of an intermolecular carbenium ion addition to an olefin without polymer formation occurs in the industrial synthesis of isooctane (Figure 3.44). 

Fig. 3.44. Carbenium ion addition to isobutene as a partial step in the Ipatiev synthesis of isooctane. The upper line shows the overall reaction.

The similar order of magnitude of the reactivities of methyl-substituted 1,3-dienes (Table 4) which depended on the number but not on the position of the substituent was strong evidence that allyl cations serve as reaction intermediates in these reactions. The rate decrease with increase in the ring size of the cycloalkadienes was attributed to the increased deviation of the jr-system from planarity. The reactivities of 1,3-dienes deviated markedly from the roughly linear relationship between the rates of proton and carbenium ion additions to alkenes. These deviations were ascribed to abnonnally low reactivity of the conjugated jr-systems. although this interpretation was inconsistent with the similar behavior of alkenes and dienes in the structure-reactivity relationship for hydration . 

This section divides the carbenium ion olefin reactions into four parts (1) Carbenium ion addition to simple alkenes and alkyncs1 (2) Prins reaction and Prins cyclization2 (3) Carbenium ion olefin cyclization3 (4) Transannular carbenium ion olefin reactions of eight-, nine-, ten- and eleven-membered rings4. 

Other reactions, in which carbenium ion addition to an isocyanide is the key step, are the Passerini and Ugi reactions and reactions of similar type. These multicomponent transformations have recently been reviewed The Passerini reaction starts from an isocyanide, a carbonyl compound and a carboxylic acid. 

The activity of solid acid catalysts for polymerization of olefins to products with higher boiling points has long been known. The polymerization proceeds throu the carbenium ion mechanism proton addition to the double bond followed by a carbenium-ion addition to the double bond. 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

S-Substituted thiiranium ions react with secondary amines to give ring-opened products. Nitriles also react with thiiranium ions, probably via an open carbenium ion whose formation is favored by increasing the polarity of the medium by the addition of lithium perchlorate (Scheme 79) (79ACR282). An intramolecular displacement by an amide nitrogen atom on an intermediate thiiranium ion has been invoked (80JA1954). 

The initial step is the coordination of the alkyl halide 2 to the Lewis acid to give a complex 4. The polar complex 4 can react as electrophilic agent. In cases where the group R can form a stable carbenium ion, e.g. a tert-buiyX cation, this may then act as the electrophile instead. The extent of polarization or even cleavage of the R-X bond depends on the structure of R as well as the Lewis acid used. The addition of carbenium ion species to the aromatic reactant, e.g. benzene 1, leads to formation of a cr-complex, e.g. the cyclohexadienyl cation 6, from which the aromatic system is reconstituted by loss of a proton ... 

The Prins reaction often yields stereospecifically the and-addition product this observation is not rationalized by the above mechanism. Investigations of the sulfuric acid-catalyzed reaction of cyclohexene 8 with formaldehyde in acetic acid as solvent suggest that the carbenium ion species 7 is stabilized by a neighboring-group effect as shown in 9. The further reaction then proceeds from the face opposite to the coordinating OH-group " ... 

In an initial step the carbenium ion species 2 has to be generated, for example by protonation of an alcohol 1 at the hydroxyl oxygen under acidic conditions and subsequent loss of water. The carbenium ion 2 can further react in various ways to give a more stable product—e.g. by addition of a nucleophile, or by loss of a proton from an adjacent carbon center the latter pathway results in the formation of an alkene 3. 

Diazomethanc undergoes addition of xanthylium perchlorate to afford dibenz[6,/]oxepin (4).193 The formation of this product can be rationalized by a carbenium ion that undergoes a Wagner Meerwein rearrangement. 

Since the double-bond configuration is established in the final elimination step from a /t-silicon-(or tin-) substituted carbenium ion in a conformation of lowest energy, often high E selectivity is observed. In reactions of allylstannanes, catalyzed by tin(TV) chloride or titanium(IV) chloride, occasionally a metal exchange occurs, followed by the pericyclic addition pathway leading to the iwti-diastereomers17 19. A more detailed discussion is given in Section D.1.3.3.3.5. 

The mechanism of the polymerization contains ionic intermediate steps. The free H+ goes to a carbenium ion and, as shown in route B, rearranges to form tetrapropylene. It is highly likely that this actual tetrapropylene exists only in very small concentrations. The product variety is explained by the rearrangement of the carbenium ion to dodecene isomers according to route C. In addition, short-chain olefins formed by fragmentation (route D). Polymerization proceeds at almost 100% to mono olefins. Aromatics, paraffins, and diolefins exist only in trace amounts. The propylene tetramer is best characterized by its distillation range. 

In addition to the ratio of concentrations olefine/HA, the donor ability, or the nucleophilicity of the anion A- is a deciding factor for the manner in which the reaction continues. This anion is formed simultaneously with the carbenium ion. When the nucleophilicity of the anion is sufficiently high, as in the case of CP, Br-, I-, for instance, the reaction proceeds as an addition by the formation of a covalent bond between A- and the carbenium ion72). 

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

Alkene protonation at pore mouths can exclusively lead to secondary carbenium ions. In addition, the alkene standard protonation enthalpies increase with the number of carbon atoms inside the micropore because charge dispersive effects are supposed to be more effective on carbon atoms inside the micropores. 

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