Alkylation reaction paths

The chemistry of metalated aziridines is far less developed than the chemistry of metalated epoxides, although from what is known [lb], it is obvious that their chemistry is similar. Like metalated epoxides, metalated aziridines can act as classical nucleophiles with a variety of electrophiles to give more highly substituted aziridines (Scheme 5.56, Path A). A small amount is known about how they can act as electrophiles with strong nucleophiles to undergo reductive alkylation (Path B), and undergo C-H insertion reactions (Path C). 

A detailed mechanism of Goldschmidt s process has not been given two reaction paths are possible either proton transfer to the acid with the formation of RC(OH) (in which case the slow step would be an Aac2 Ingold mechanism) or nucleophilic attack of the carbonyl group of the acid on the protonated alcohol. The second mechanism would require an alkyl scission (A l). In more recent studies2501, it has been shown that scission in most cases is of the acyl type and particularly in the examples studied by Goldschmidt. 

The surface reactions of graphite electrodes in many nonaqueous solutions have been investigated intensively,29 30 and the major reaction paths in a variety of alkyl carbonate solutions seem to be quite clear. Both EC and PC decompose on graphite electrodes, polarized cathodically, to form solid surface films with R0C02Li species as major components,31 and ethylene or propylene gases, respectively, as co-products. 

Because the cis-decalin molecule extends its two methine carbon-hydrogen bonds on the same side in contrast to frans-decalin, the carbon-hydrogen bond dissociation of adsorbed decalin would be advantageous to the cis-isomer on the catalyst surface (Figure 13.17). A possible reaction path by octalin to naphthalene in dehydrogeno-aromatization of decalin will be favored to the cis-isomer, since its alkyl intermediate provides the second hydrogen atom from the methine group to the surface active site easily. 

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

We looked briefly at reaction profiles in Section 8.2. Before we look at the reaction profile for the concurrent reactions of hydrolysing a secondary alkyl halide, we will look briefly at the simpler reaction of a primary alkyl halide, which proceeds via a single reaction path. And for additional simplicity, we also assume that the reaction goes to completion. We will look not only at the rate of change of the reactants concentration but also at the rate at which product forms. 

The ammonium catalyst can also influence the reaction path and higher yields of the desired product may result, as the side reactions are eliminated. In some cases, the structure of the quaternary ammonium cation may control the product ratio with potentially tautomeric systems as, for example, with the alkylation of 2-naph-thol under basic conditions. The use of tetramethylammonium bromide leads to predominant C-alkylation at the 1-position, as a result of the strong ion-pair binding of the hard quaternary ammonium cation with the hard oxy anion, whereas with the more bulky tetra-n-butylammonium bromide O-alkylation occurs, as the binding between the cation and the oxygen centre is weaker [11], Similar effects have been observed in the alkylation of methylene ketones [e.g. 12, 13]. The stereochemistry of the Darzen s reaction and of the base-initiated formation of cyclopropanes under two-phase conditions is influenced by the presence or absence of quaternary ammonium salts [e.g. 14], whereas chiral quaternary ammonium salts are capable of influencing the enantioselectivity of several nucleophilic reactions (Chapter 12). 

The next step in complexity are systems in which alkylation competes with hydride transfer to give dimeric alkyl cations which, when hydride abstraction occurs, yield dimeric saturated hydrocarbons (equation 14). This reaction path for cyclic aliphatic alcohols and olefins is often accompanied by some rearrangement (Deno etal., 1964 Pittman, 1964). 

Studies on the variation in the distribution of stereoisonieric products as a function of the accessible independent variables permit deductions concerning the multiplicity of the reaction paths. A classic example is found in Hughes and Ingold s studies into the mechanism of the solvolysis of alkyl halides in aqueous-alcoholic media, an important variable, being the concentration of base or other nucleophilic reagent (56 ). The obvious variable in hydrogenation studies is the pressure of... 

Until now, for most of the systems described here it has been accepted that alkane activation occurred through oxidative addition to the 14-electron intermediate complexes. Yet, Belli and Jensen [26] showed, for the first time, evidence for an alternative reaction path for the catalytic dehydrogenation of COA with complex [lrClH2(P Pr3)2] (22) which invoked an Ir(V) species. Catalytic and labeling experiments led these authors to propose an active mechanism (Scheme 13.12), on the basis of which they concluded that the dehydrogenation of COA by compound 22 did not involve an intermediate 14-electron complex [17-21], but rather the association of COA to an intermediate alkyl-hydride complex (Scheme 13.12). 

Primary and secondary alkyl phosphines autoxidize rapidly. In fact lower members may ignite on exposure to air. However, reaction paths appear to involve both P—H bond cleavage and participation of phos-phoranyl radicals (discussed below). 

Similar double deprotonation and alkylation of the -substituted diastereomeric homologs of complex 15, i.e., 22 and 23, occurs with a high degree of stereocontrol and each homolog follows the reaction path that minimizes steric interactions of the /1-substituent with the metal ligands27,70. 

Serratosa [64c] reported that the data in the literature suggest that two reaction paths are operative in the reaction of propargyl bromide with alkyl-magnesium bromide. These reactions are dependent on temperature, solvent, and the structure of the Grignard reagent (Eqs. 49, 50). 

Catalysis over Typical Zeolites - In the alkylation of naphthalene, a-alkylation occurs in the initial stage because -positions are more reactive than P-positions. However, the (3/a ratio in the product mixture increases with the increase of reaction temperature and time on stream. Figure 8 shows the three reaction paths for producing diisopropylnaphthalene (DIPN) isomers. The reactions are (1) alkylation, (2) isomerization, and (3) transalkylation. Isomerization and transalkylation accompany the rearrangement of isopropyl groups. The zeolite type and reaction conditions, e.g., temperature and time on stream, usually determine the type of reaction path.4... 

Since the second solvent pair fall within the poor hydrogen bonding group of solvents, increased basicity of the organic base in these solvents would be consistent with the observed behavior. Based on the model compound studies, indications are that the base-catalyzed imidization process may involve a two-step mechanism, Jee Scheme 23. The first step corresponds to the complete or partial proton abstraction from the amide group with the formation of an iminolate anion. Since this iminolate anion has two possible tautomers, the reaction can proceed in a split reaction path to either an isoimide- or imide-type intermediate. Although isoimide model reactions indicate an extremely fast isomerization to the imide under the conditions employed for base-catalysis, all indications to date are that it is not an intermediate in the base-catalyzed imidization of amic alkyl esters. 

Irradiation of octyl nitrite has already been described Kabasakalian et al. have also studied the behavior of its lower homologs.24 It was observed that primary alkyl nitrites with more than four carbon atoms in a chain produced essentially identical yields (37 to 45%) of nitroso dimer formed by the Barton reaction until the minimum straight-chain length of four carbon atoms was reached. Butyl nitrite underwent internal hydrogen abstraction in poor yield. This is the result of a more difficult abstraction of a primary hydrogen as compared to a secondary hydrogen atom available in compounds with longer chains. Reaction paths 2 and 4 predominated to afford 1-butanol and butanal as the major products. 

The Negative-Temperature-Coefficient Region The equilibrium constant for the reaction R + O2 ROO (R64) is strongly temperature dependent, and as the temperature increases, the equilibrium shifts in favor of R + O2. The shift in equilibrium is the primary reason for the existence of the region where the conversion decreases with an increase in temperature (i.e., where there is a negative temperature coefficient). Above about 650 K, the alkyl peroxy radical becomes less thermally stable, and alternative reaction paths for ROO begin to compete with the isomerization reaction (R65). A new product channel opens up for the R + O2 reaction... 

Fig. 1. A possible reaction path and energy profile for the uncatalysed one-alkyl mercury-for-...

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