Carbonium ions hydride shifts

However, there are several drawbacks to this alkylation reaction. The use of longer alkyl chains than ethyl can be complicated by isomerization of the alkyl group arising from carbonium ion hydride shifts. It is therefore not uncommon for mixtures to be produced. In extreme cases, a completely different alkyl group from that of the starting material can be present in the product. 

One of the most characteristic properties of carbonium ions is their great tendency to undergo rearrangements. These rearrangements include 1,2-alkyl shifts, hydride shifts, cyclopropylcarbinyl rearrangements, Wagner-Meerwein rearrangements, and others. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

Although HCo(CO)4 is a strong acid in aqueous solution and is capable of protonating even weak bases like dimethylformamide, there is no evidence that it protonates olefins in hydrocarbon solvents to form carbonium ion intermediates which might then rearrange by conventional 1,2-hydride shifts followed by proton elimination ... 

The occurrence of some substitution in the deamination of 2-amino-2-deoxy-/3-D-mannopyranosides131 152 (72), and its absence in the reaction of the a-D-pyranoside150 69, must be due to the steric effect of the axial anomeric substituent which (in the a-D-pyran-oside) hinders the approach of the nucleophile (water) to either the C-2 carbonium ion or to C-2 of the diazonium ion. The glucose and glucitol tentatively detected as minor products in the deamination of 72 (R = D-glucose residue and R = D-glucitol residue) presumably arose by way of a hydride shift of H-l to C-2. 2-Deoxy-D-glucono-1,5-lactone (75) was not detected, as it would probably have. o,... 

C6. To account for this Roberts suggested that 6,2- or 6,1-hydride shifts occur in the carbonium ion simultaneously with the rearrangement of Equation 6.39 as shown in Equation 6.40.103... 

Hydride shifts can take place directly, without the intervention of a carbonium ion intermediate, if the geometry of the system is favorable. For example, in the solvolysis of cyclohexyl-2,6-d2 tosylate in 97 percent acetic acid, 1,3-hydride shifts have been reported to account for 33 percent of the product.124 If this is so, it must be because the reaction is made facile by the proximity of the 3-axial hydrogen to the empty p orbital. 

The fact that only trans-1,2- and cis- 1,4-glycols are obtained implies that they cannot actually be formed by the simplified mechanism in Scheme 9. The carbenium ions 99-101 should give a mixture of cis and trans glycols. However, the reaction can be neither entirely concerted, as shown for a 1,5-hydride shift in Equation 6.46, nor involve initial formation of a carbonium ion, as shown in Equation 6.47 The kHlkD isotope effects are too small for C—H bond breaking... 

The alkyl carbonium ions which result from these reversible, relatively unselective hydride abstractions then undergo a series of 1,2- (Wagner-Meerwein) or 1,3- (protonated cyclopropane) rearrangements which eventually result in the formation of the thermodynamically most stable products. The number of different reaction sequences by which one may rationalize the formation of a given products is, of course, necessarily large. A variety of independent pathways are generally available for the interconversion of the isomers of a given species by successive alkyl shifts. 

Reversible, random carbonium ion formation is not required to explain the rearrangements of both 2 and 37 to adamantane in the highly acidic media. Sequential 1,2 alkyl shifts coupled with the well documented 5316,2- and 3,2-hydride shifts of the norbomyl system permit a rearrangement pathway analogous to that discussed earlier as the most likely route for the Lewis acid catalyzed rearrangement of 2 to adamantane. 

Rearrangements of tricyclic systems in concentrated sulfuric acid are often unlike those observed in SbFs-S02 solutions. Not only do intermolecular hydride shifts occur readily with ordinary substrate concentrations, but also the stabilities of the product alcohols control product distributions in sulfuric acid, whereas the stabilities of the carbonium ions are the controlling factors in SbFs-S02 solution. 

Once the carbonium ions are formed, the modes of interaction constitute an important means by which product formation occurs during catalytic cracking. For example, isomerization either by hydride ion shift or by methyl group shift, both of which occur readily. The trend is for stabilization of the carbonium ion by movement of the charged carbon atom toward the center of the molecule, which accounts for the isomerization of a-olefins to internal olefins when carbonium ions are produced. Cyclization can occur by internal addition of a carbonium ion to a double bond which, by continuation of the sequence, can result in aromatization of the cyclic carbonium ion. 

The first reaction involves interaction of a hydrocarbon with the catalyst surface. Hydride abstraction occurs to form a carbonium ion. Abstraction can be of any suitable hydrogen atom but if this results in a primary ion as shown, this will rapidly isomerise by hydrogen shift to the more thermodynamically stable secondary ion. This may be further isomerised by carbon shift to a tertiary ion. This contrasts with free radicals and although isomerisation occurs it is relatively slower. The carbonium ions can also undergo inter-molecular transfer (not shown) when a carbonium ion meets another hydrocarbon molecule. 

Kinetic data are not available for the hydrolysis of diazomethane or other simple diazoalkanes. It was reported that the products of hydrolysis of 1-diazopropane are propanol and isopropanol [217]. This observation supports a mechanism with a carbonium ion intermediate which may undergo rearrangement by a hydride shift. 

The reaction involves formation of carbonium ions and 1.2-hydride shifts. Use of more concentrated acid (8 % oleum) or of 75 % acid decreases the yield of (2) markedly. 

Cycloheptatriene and derivatives thereof donate hydride readily to a variety of carbonium ion acceptors. The position of the end equilibrium depends on the thermodynamics of the exchange. " These reactions are prototypes of a broad area of carbonium ion chemistry wherein carbonium ions equilibrate via intra- and inter-molecular hydride shifts between a donor C—H bond, usually jp hybridized, and a carbonium ion acceptor. This chemistry is often achieved with heterogeneous catalysts and is of great industrial significance it lies outside the emphasis of this review, however. Excellent treatises are available, and a review has appeared on the use of carriers like adamantane to promote hydride transfer in hydrocarbons under strongly acidic conditions. ... 

The skeletal isomerization of tetrabydrodicyclopentadiene into adamantane is an example of a very complex rearrangement diat is commercially carried out over strong Lewis acids with a hydride transfer initiator. The reaction can be catalyzed by rare earth (La, Ce, Y, Nd, Yb) exchanged faujasites (Scheme 1) in a Hj/HCl atmosphere at 25(yX3. Selectivities to adamantane of up to 50% have been reported, when a metal fimction, such as Pt, capable of catalyzing hydrogenation is added [54]. Initially acid catalyzed endo- to exo- isomerization of tetrahydro-dicyclopentadiene takes place and then a series of 1,2 alkyl shifts involving secondary and tertiary carbonium ions leads eventually to adamantane[55]. The possible mechanistic pathways of adamantane formation from tetrahydro-dicyclopentadiene are discussed in detail in ref [56]. 

The existence of a free carbonium ion such as VII in a strongly solvating medium is highly improbable. Only if VII could exist in association with the palladium could decomposition to vinyl acetate be expected to occur with a reasonable degree of frequency, in competition with the reaction with acetate to form ethylidene diacetate. Similar results have been reported in the Wacker acetaldehyde synthesis when D2O is used as the solvent (25). Stern (54) has reported results in which 2-deuteropropylene was used as substrate in the reaction. Based on assumed /J-acetoxyalkylpalladium intermediates, on the absence of an appreciable isotope effect in the proton-loss step, and on the product distribution observed, excellent agreement between calculated (71%) and observed (75%) deuterium retention was obtained. Several problems inherent in this study (54) have been discussed in a recent review (I). Hence, considerable additional effort must be expended before a clear-cut decision can be made between a simple / -hydrogen elimination and a palladium-assisted hydride shift in this reaction. 

Isomerization from the secondary to the more stable tertiary carbonium ion precedes reaction with the next molecule of monomer. A 1,3 polymerization has therefore occurred. Similarly, 4-methyl-l-pentene gives a 1,4 polymer (probably involving two successive 1,2 hydride shifts) that has a structure corresponding to an ethylene-isobutylene copolymer (Reaction 25). 

This is not our first encounter with the transfer of hydride ion to an electron-deficient carbon we saw much the same thing in the 1,2-shifts accompanying the rearrangement of carbonium ions (Sec. 5.22). There, transfer was intramolecular (within a molecule) here, it is intermolecular (between molecules). We shall find hydride transfer playing an important part in the chemistry of carbonyl compounds (Chap. 19). 

As an alternative to the one-step 1,2-hydride shift described in Sec. 5.22, one might instead propose—in view of the reactions we have studied in this chapter—that carbonium ions rearrange by a two-step mechanism, involving the intermediate formation of an alkene ... 

We are already familiar with the facile transfer of hydride from carbon to carbon within a single molecule (hydride shift in reanangements), and between molecules (abstraction by carbonium ion. Sec. 6.16). Later on we shall encounter a set of remarkably versatile reducing agents (hydrides like lithium aluminum hydride LiAlH4, and sodium borohydride, NaBH4) that function by transfer of hydride ion to organic molecules. 

Wagner-Meerwein rearrangements presumably lead to carbonium ions 28 and 29 or 30. The ion 28 may then undergo a hydride shift or formation of bicyclohexanone, 26, in a manner that probably parallels the behaviour of the unsolvated n-propyl cation. (Lee et al., 1965). 

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