Methyl, migration

FIGURE 5 7 The first formed carbocation from 3 3 dimethyl 2 butanol is secondary and rearranges to a more stable tertiary carbocation by a methyl migration The major portion of the alkene products is formed by way of the tertiary carbocation... 

FIGURE 5 8 Methyl migration in 1 2 2 tnmethylpropyl cation Structure (a) is the initial second ary carbocation structure (b) is the transition state for methyl migration and structure (c) is the final tertiary carbocation... 

Transalkylation is also catalyzed by acids, but requires more severe conditions than isomerization. As shown below, the methyl migration is intermolecular and ultimately produces a mixture of aromatic compounds ranging from benzene to hexamethylbenzene. The overall equiHbrium constants for all possible methylbenzenes have been deterrnined experimentally and calculated theoretically (Fig. 2 and Table 3). 

Migration to the developing electron sextet at nitrogen is not restricted to hydrogen. In (79) there is methyl migration with formation of methylamine and acetone in the acid-catalyzed decomposition of (80), phenyl migration leads to aniline and acetaldehyde. 

Acid hydrolysis of (161) yields acetone and methylamine by N—N cleavage and methyl migration. 

The second part of lanosterol biosynthesis is catalyzed by oxidosqualene lanosterol cyclase and occurs as shown in Figure 27.14. Squalene is folded by the enzyme into a conformation that aligns the various double bonds for undergoing a cascade of successive intramolecular electrophilic additions, followed by a series of hydride and methyl migrations. Except for the initial epoxide protonation/cyclization, the process is probably stepwise and appears to involve discrete carbocation intermediates that are stabilized by electrostatic interactions with electron-rich aromatic amino acids in the enzyme. 

If, however, the 7-position is blocked by a methyl group, then a more complex reaction ensues involving ring contraction, methyl migration and loss of the nitrogen function to give 4,6.6-trimethyl-2,3,5-triphenylcyclohexa-2,4-dienone (12% mp 198 C) and other unidentified... 

As indicated in this Scheme, triflate 184 presumably ionizes to vinyl cation 185, which can eliminate a proton and give acetylene 187 or react with solvent to give pinacolone 188 it can also undergo a methyl migration to give the tertiary... 

Carbenes can also be stabilized by migration of alkyl or aryl groups. 2-Methyl-2-phenyl-1-diazopropane provides a case in which products of both phenyl and methyl migration, as well as intramolecular insertion, are observed. 

Quinone methides 156 and 157 were generated on irradiation of the cis and trans forms of 155, respectively (Eqs. 1.42 and 1.43).99 100 The mechanism is believed to involve a radical ring opening of the cyclopropane ring, followed by hydrogen or methyl migration. 

Analogous behavior was followed by the phenyl-substituted silene 156. The initially formed silene 157 underwent 1,3-methyl migration to give the silene 158, which then dimerized in a head-to-tail manner to yield three different stereoisomeric dimers 159, two of which were characterized by crystal structures. Again, the exchange of trimethylsilyl and trimethylsi-loxy groups at the ends of the Si=C bond occurred, followed by 1,3-methyl silicon-to-silicon rearrangements. The steps are summarized in Eq. (54). 

Wolff rearrangements were also observed when most of the same acylsi-lyldiazoalkanes were photolyzed in acetone instead of benzene.21 The ketenes 185 resulting from a 1,3-methyl migration of the silene were detected in addition to the expected ene product 186 derived from the reaction of the silene with acetone (or other enolizable ketones) (Eq. 58). When R = Ad, only the cyclic siloxatene 187 was formed under the same... 

Above 323 K, the surface hydride catalyzes the hydrogenolysis of neopentane, isobutane, and propane, whereas ethane does not undergo any significant hydrogenolysis. The first step of the reaction is the activation of the C—H bond, whereas the next step is the activation of the C—C bond of the alkyl groups via (l-methyl migration steps. 

Casey et al. have studied the decarbonylation reactions of [cis-(OC)4M(MeCO)(PhCO)], in which M is Mn or Re (16,17). These complexes lose a carbonyl ligand to form five-coordinate intermediates of the type [(OC)3M(MeCO)(PhCO)]. Reversible methyl migration proceeds much more rapidly than does phenyl migration. In the course of these studies, a phosphine substituted rhena-/3-diketonate complex, [fac-(OC)3(Et3P)Re(MeCO) (PhCO)], was prepared. 

Methyl migration to the electrophilic methylene carbon of a cationic Ir(III) complex (66) to form an iridium ethyl complex has been reported (93) ... 

The methyl-rhodium complex (2) was detected, at low concentration in neat CH3I solution, by a combination of FTIR and NMR spectroscopy.6 The rate of the reaction (2) —> (3) was measured between 5°C and 35°C. An Arrhenius plot yielded activation parameters of A7/= 63 k.lmol 1 and AS= — 59 JmoC1 K 1 for the methyl migration step. 

Complex (5) undergoes methyl migration after oxidative addition of CH3I to afford the acyl complex (6) containing two Rh—O bonds. Heating (6) in the presence of CO results in the reductive elimination of Acl, which upon hydrolysis is transformed to AcOH.12... 

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