Carbenium ions 7-substituted

The Ritter reaction with unsaturated carbenium ions under either silver-assisted solvolysis or photolytic conditions leads to excellent yields of isoquiaolines (173). The ease of preparation of the requited vinyl bromides makes an attractive route to highly substituted isoquiaolines. 

S-Substituted thiiranium ions react with water and alcohols to give trans ring opening (Scheme 72). A report that oxygen nucleophiles attack sulfur as well as carbon has been shown to be incorrect (79ACR282). The intermediate thiiranium ion (57) in the presence of lithium perchlorate readily yields the carbenium ion which undergoes a transannular hydride... 

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 ionization mechanism for nucleophilic substitution proceeds by rate-determining heterolytic dissociation of the reactant to a tricoordinate carbocation (also sometimes referred to as a carbonium ion or carbenium ion f and the leaving group. This dissociation is followed by rapid combination of the highly electrophilic carbocation with a Lewis base (nucleophile) present in the medium. A two-dimensional potential energy diagram representing this process for a neutral reactant and anionic nucleophile is shown in Fig. 

The synthesis of an alkylated aromatic compound 3 by reaction of an aromatic substrate 1 with an alkyl halide 2, catalyzed by a Lewis acid, is called the Friedel-Crafts alkylation This method is closely related to the Friedel-Crafts acylation. Instead of the alkyl halide, an alcohol or alkene can be used as reactant for the aromatic substrate under Friedel-Crafts conditions. The general principle is the intermediate formation of a carbenium ion species, which is capable of reacting as the electrophile in an electrophilic aromatic substitution reaction. 

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. 

In the Lewis acid mediated reaction the developing carbenium ion in C is stabilized by the nearby 7t-electrons of the titanium or aluminum enolate. This generates as the major diastereomer the 3,3a-/r .v-relationship between the substitution at the ring junction and the vinyl group at C-3 via a synclinal transition state. 

The pivotal step in this sequence is an electrophilic substitution on indole. Although the use of l,3-dithian-2-yl carbanions is well documented, it has been shown only recently that 1,3-dithian-2-yl carbenium ions can be used in a Priedel-Crafts type reaction. This was accomplished initially using 2-methoxy-l,3-dithiane [1,3-Dithiane, 2-methoxy-] or 2-metlioxy-l,3-dithiolane [1,3-Dithiolane, 2-methoxy-] and titanium tetrachloride [Titanate(l —), tetrachloro-] as the Lewis acid catalyst.9 2-Substituted lysergic acid derivatives and 3-substituted indoles have been prepared under these conditions, but the method is limited in scope by the difficulties of preparing substituted 2-methoxy-1,3-dithianes. l,3-Dithian-2-yl carbenium ions have also been prepared by protonation of ketene dithioacetals with trifluoroacetic acid,10 but this reaction cannot be used to introduce 1,3-dithiane moieties into indole. 

Carbonyl reactions are extremely important in chemistry and biochemistry, yet they are often given short shrift in textbooks on physical organic chemistry, partly because the subject was historically developed by the study of nucleophilic substitution at saturated carbon, and partly because carbonyl reactions are often more difhcult to study. They are generally reversible under usual conditions and involve complicated multistep mechanisms and general acid/base catalysis. In thinking about carbonyl reactions, 1 find it helpful to consider the carbonyl group as a (very) stabilized carbenium ion, with an O substituent. Then one can immediately draw on everything one has learned about carbenium ion reactivity and see that the reactivity order for carbonyl compounds ... 

Allenyl cations 1 are a stabilized form of vinyl cations1-3 in which the /1-carbon atom of the vinylic structure is part of the substituent which effects the stabilization of the ion via its electron-donating ability. This leads to a resonance hybrid having formally the alkynyl cation structure 2. Allenyl cations should be distinguished from the allenyl substituted carbenium ions 3 formulated as the mesomeric structures of the vinyl cations 4 (dienyl cations) stabilized by an w-vinyl group (equation 1). 

Ionic hydrogenations of C=C bonds generally work well only in cases where a tertiary or aryl-substituted carbenium ion can be formed through protonation of the C=C bond. Alkenes that give a tertiary carbenium ion upon protonation include 1,1-disubstituted, tri-substituted and tetra-substituted alkenes, and each of these are usually hydrogenated by ionic hydrogenation methods in high yields. 

The alkylation reaction is initiated by the activation of the alkene. With liquid acids, the alkene forms the corresponding ester. This reaction follows Markovnikov s rule, so that the acid is added to the most highly substituted carbon atom. With H2S04, mono- and di-alkyl sulfates are produced, and with HF alkyl fluorides are produced. Triflic acid (CF3S020H) behaves in the same way and forms alkyl triflates (24). These esters are stable at low temperatures and low acid/hydrocarbon ratios. With a large excess of acid, the esters may also be stabilized in the form of free carbenium ions and anions (Reaction (1)). 

Gas-phase studies of a-silyl substituted carbenium ions show that these intermediates exist only in a very flat potential well (5, 7, 8, 9 ). They undergo fast 1,2-H or -alkyl shifts, producing the more stable silicenium or p-silyl substituted carbenium ions. 

In the 1960s, after Kennedy and Thomas [25] had established the isomerisation polymerisation of 3-methylbutene-l, this became a popular subject. From Krentsel s group in the USSR and Aso s in Japan there came several claims to have obtained polymers of unconventional structure from various substituted styrenes by CP. They all had in common that an alleged hydride ion shift in the carbenium ion produced a propagating ion different from that which would result from the cationation of the C C of the monomer and therefore a polymer of unconventional structure the full references are in our papers. The monomers concerned are the 2-methyl-, 2-isopropyl-, 4-methyl-, 4-isopropyl-styrenes. The alleged evidence consisted of IR and proton magnetic resonance (PMR) spectra, and the hypothetical reaction scheme which the spectra were claimed to support can be exemplified thus ... 

Explanation The ester polystyryl perchlorate is stabilised by M, but it decomposes slowly to Pn4. In the moderately pure system the [Pn+] are consumed by impurities, mainly water, and only when depletion of M leads to fast decomposition of E are enough Pn+ formed to give colour and conductivity. In the very pure system the scavenging of water, etc., by the ions is completed before all the M has been consumed, so that the Pn+ formed thereafter contribute to the rate. At the end of a typical polymerisation of this type the [Pn+] is ca. 10"7 mol l"1. If [H20] > [HClO4]0, the k1 is unaffected because the rate of reaction of E with H20 in CH2C12 is much smaller than the rate of polymerisation, but the Pn+ and/or the HC104 are hydrated so that no colour or conductivity appears. The visible and conducting ions are not polystyryl carbenium ions, but a cocktail of others in which the substituted indanyl ion is the most abundant [28]. 

If anions R are oxidized in the presence of olefins, additive dimers (24) and substituted monomers (26) are obtained (Scheme 5, Table 8, and Ref. [94]). The products can be rationalized by the following pathway the radical R obtained by a le-oxidation from the anion R adds to the alkene to give the primary adduct (25), which dimerizes to afford the additive dimer (24) with regiospeciflc head-to-head connection of the two olefins, or couples with R to form the additive monomer (26). If the substituent Y in the olefin can stabilize a carbenium ion, (25) is oxidized to the cation (27), which reacts intra- or inter-molecularly with nucleophiles to give (28) or (29). 

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