Self-alkylation

Water, which can be taken to a minimum by the use of molecular sieves, can produce a lactamide either through direct reaction with the aziridinone intermediate, or upon hydrolysis of oxazolidinone self-condensation products, previously obtained also in the presence of a strong non-nucleophilic base (H ) (ref. 17). The recently reported 0-self-alkylation compound H bears the (S,S)-configurations at the unreacted C-Br and newly formed C-0 bonds. The presence of bromine was expedient for the x-ray assessment of configuration at the two chiral centers of 11 which forms in high diastereoisomeric excess (ref. 5). 

With hydrofluoric acid (23,50), and to a lesser degree also with zeolites (14,81, 87-89), a significant fraction of the product stems from self-alkylation, which is sometimes also termed hydrogen transfer. The importance of this mechanism depends on the acid, the alkene, and the reaction temperature. Self-alkylation... 

The crucial step in self-alkylation is decomposition of the butoxy group into a free Brpnsted acid site and isobutylene (proton transfer from the Fbutyl cation to the zeolite). Isobutylene will react with another t-butyl cation to form an isooctyl cation. At the same time, a feed alkene repeats the initiation step to form a secondary alkyl cation, which after accepting a hydride gives the Fbutyl cation and an -alkane. The overall reaction with a linear alkene CnH2n as the feed is summarized in reaction (10) ... 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

Fig. 7. Self-alkylation mechanism, depicted with 2-butene as the feed alkene.

Fig. 8. Concerted alkylation mechanism including alkylation, self-alkylation , cracking, dimerization, and hydride transfer via isobutane and via conjunct polymers.

A similar type of catalyst including a supported noble metal for regeneration was described extensively in a series of patents assigned to UOP (209-214). The catalysts were prepared by the sublimation of metal halides, especially aluminum chloride and boron trifluoride, onto an alumina carrier modified with alkali or rare earth-alkali metal ions. The noble metal was preferably deposited in an eggshell concentration profile. An earlier patent assigned to Texaco (215) describes the use of chlorinated alumina in the isobutane alkylation with higher alkenes, especially hexenes. TMPs were supposed to form via self-alkylation. Fluorinated alumina and silica samples were also tested in isobutane alkylation,... 

Patents assigned to Mobil (217) describe the use of boron trifluoride supported on several porous carriers. BF3 supported on silica was found to exhibit a slightly higher performance with added water in the alkylation of a mixed alkene feed at 273 K. It was also shown that self-alkylation activity was considerably lower than that with HF as catalyst. Another patent (218) describes the use of a pillared layered silicate, MCM-25, promoted with BF3 to give a high-quality alkylate at temperatures of about 273 K. BF3 was also supported on zeolite BEA, with adsorbed water still present (219). This composite catalyst exhibited low butene isomerization activity, which was evident from the inferior results obtained with 1-butene. At low reaction temperatures, the product quality was superior to that of HF alkylate. 

Ethyl fluoride shows more tendency to ionize in antimony pentafluoride, than does methyl fluoride. The solutions in neat antimony pentafluoride are, however, not stable and a rapid formation of t-butyl and t-hexyl cations is observed. This observation indicates self alkylation of ethylene formed in equilibrium of equation (2). 

These ions are presumably so reactive that they are able to alkylate their own anion. The stronger the acid, the better the dissociation and the easier the rearrangement. At present, we are investigating the mechanistic details of this self-alkylation. One can imagine that a stronger nucleophile which comes from outside could be alkylated by the nitrosoimmonium/anion complex ... 

Skraup quinoline synthesis, 443 Smiles rearrangement, phenothiazine, 534 Spiroalkylation, 222, 280 Spirocyclization, conjugate addition, 386 Spiroimidazolone formation, 335 Spiropyrazolopiperidine, 375 Stannylation, alkyne, 15 Stereoselective dehydration, 198 Grignard addition, 198, 199 reduction, 129, 226 hydroxyketone, 400 iminoketone beta, 553 oxazaborohydride, 585 transfer chirality, 321 Stilbene formation, self alkylation, 525 Stobbe condensation, benzophenone, 103... 

These rate differences show that, over platinum on silica-alumina, two cyclization reactions occur simultaneously. One is catalyzed by the platinum metal it is the mechanism observed over platinum on silica. Acid catalyzed self-alkylation is the second reaction. The following steps are involved. 

Sulfonates such as mesylates or tosylates are readily prepared from alcohols under mild conditions, and are therefore attractive alternatives to halides as electrophiles. Although sulfonates often undergo clean displacement by nucleophiles, alternative reaction pathways are accessible to these intermediates, which can lead to unexpected results. If the nucleophile used is strongly basic, metalation instead of displacement of the sulfonate can occur. Some potential reactions of such metalated sulfonates include fragmentation into sulfenes and alcoholates, or into sulfmates and carbonyl compounds, or self-alkylation (Scheme4.15). 

The actual research is focused on finding new types of zeolites or modifying already known ones (5), to selectively catalyze branching isomerization and self alkylation reactions in the gasoline fraction, while minimizing overcracking. Efforts have also been devoted to development of new active and selective matrices. 

Proton transfer to paraffinic carbocations and self-alkylation of light olefins Q + iC4Hio + iC4Hg... 

Dimerization and trimerization of alkylbromocyanoacetates (a self-alkylation reaction) has been used for the synthesis of highly substituted cyclopropanes and dialkyl-( )-2,3-dicyanobutendioates (equation 71). The latter products are potentially very useful for the synthesis of a wide range of heterocyclic compounds. 

The relative reactivities of indole, 2- and N-methylindole, pyrrole, thiophene, and furan have been determined in alkylation by a benzenonium ion coordinated to iron tricarbonyl (73CC540). The effects of methyl substituents in pyrrole were determined in alkylation by 4-(7V,7V-dimethyl-amino)benzaldehyde [76JCS(P2)696]. In neither of these methods, nor in the alkylation of indole by aziridinium tetrafluoroborate [67AG(E)178], nor in self-alkylation of a X5-phosphorinyl tetrafluoroborate [73AG (E)753], is a catalyst required. 

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