1,2-Methide shift

It has been suggested that the effect of selective deuteration on C-13 spectra of symmetrical carbenium ions can be used to distinguish between rapidly equilibrating structures (ie. involving hydride and methide shifts) and delocalized structures (l3). 

Such isomerizations are sometimes desired and sometimes are the cause of or explanation for unwanted structures. In the cationic polymerization forming poly(l-butene), nine different structural units have been found. Classical 1,2-hydride and 1,2-methide shifts, hydride transfer, and proton elimination account for these structures. 

When 3-methyl[3-13C]pentane (17 ) is isomerized to 2-methylpentane, the label distribution shows that the isomerization cannot be explained by a simple 1,2-methide shift 30% of the 2-methylpentane has the 13C label in a position that can be best explained by the ethyl shift (Scheme 5.12). The recovered 3-methylpentane (96%) also shows a very large degree of internal shift [Eq. (5.45)]. 

The exact mechanism of M3 + isomerization has not been established yet. It may involve a 1,3-methide shift, or reaction of the unsaturated exodimer with monomeric cation [Eq. (10)]. The unsaturated dimer is formed by deprotonation of the dimeric cation. 

The isomerized structure dominates even at - 100° C, but accounts for only 70% of the repeat units at - 130° C. Similar but more complicated structures are formed in 4-methyl-1-butene polymerizations by competing hydride and methide shifts [298]. Other monomers whose propagating carbenium ions isomerize include 5-methyl-l-hexene, 4,4-dimethyl-1-pen-tene and some terpenes [299]. 

The red color observed at later stages in polymerization of p-me-thoxystyrenes is probably due to formation of a charge transfer complex with two adjacent aromatic rings as shown in Eq. (9). The internal tertiary carbocation is sterically not accessible and is unreactive. It may be formed by a 1,3-methide shift or by reaction of unsaturated oligomers with growing species [13]. In the latter case, transfer by elimination occurs before termination. 

The 1,2-methide shift as in the 2,3,3-trimethyl-2-butyl cation [5] takes place as shown in (6) with a very small barrier, AG = 3,5 0.1 kcal mol- (Saunders and Kates, 1978), via a corner protonated transition state or intermediate [6] ... 

Recently Saunders and Kates (1978) have been successful in measuring the rates of degenerate 1,2-hydride and 1,2-methide shifts of several simple tertiary alkyl cations employing high field (67.9 MHz) C-nmr spectroscopy. From band broadening in the fast exchange limit the free energies of activation (AC ) were determined to be 3.1 0.1 kcal mol at —138°C for [10] and 3.5 0.1 kcal mol at —136°C for [5]. 

Exchange of a- and p-ring hydrogens in [70] in HF-SbFj at —30°C to +21°C has been reported by Brouwer (1968). A mechanism (53) involving 1,2-hydride and subsequent 1,2-methide shifts, similar to that of the t-amyl... 

Brouwer and van Doom (1970) studied the rearrangements of penta- and hexa-methylcyclopentenyl cations and the degenerate processes (173)-(175) were observed. The remarkable observation that the last of these reactions (175), involving both hydride- and methide-shifts, has a higher rate than the other two was explained by a larger steric crowding of the methyl groups in [270] compared with the intermediate [271]. 

A new member of the series of arenonium ions undergoing degenerate rearrangements, the 9,9,10-trimethylpyrenium ion [321], has been studied by Borodkin et al. (1974a) using H-nmr spectroscopy. The rate of rearrangement via 1,2-methide shifts was very close to that of [307] and this was attributed... 

A study of the rate of degenerate rearrangements of arenium ions as a function of the C-chemical shifts (electron deficiency) of their carbocation centre has been made (Borodkin et al., 1974b). Linear correlations [e.g. (21 la)] oflogA for 1,2-methide shifts in some 9,9,10-trimethyl phenanthrenium ions, [307], [322 R = CH3, Br and CF3], [321] and [279 R = CH3] and 8i3c+ were found at different temperatures. A linear correlation (211b) was... 

Bushmelev et al. (1979a) treated an apparent paradox discovered in the study of degenerate 1,2-methide shifts (213) in the 4,5,9,9,10-pentamethyl-phenthrenium ion [326]. It is well known from calculations and stereochemical studies that the 4,5-disubstituted dihydrophenanthrene skeleton is twisted, with two nonequivalent C(9)-CHs groups, and has a high inversion barrier... 

The 2° R2CH formed undergoes a methide shift ( iCHj) to the more stable 3° R3C+. 

Methane, bromination mechanism, 60 Methide shift, 94 Methylene in synthesis, 67 Methyl salicylate, 440 Michael addition, 385, 459 Microscopic reversibility, 98 Migratory aptitude, 293 Molecular orbital, 14 Molecularity, 40 Molecules, geometry of, 18 polar, 27... 

Propylene, 1-butene, and higher 1-alkenes yield only oligomers (DP no higher than 10-20) with highly irregular structures due to various combinations of 1,2-hydride and 1,2-methide shifts, proton transfer and elimination, and chain transfer. For example, protonation and ethylation of ethylene are rapidly followed by energetically favorable isomerization ... 

Problem 8.21 Write equations to show the different structural units that may result from intramolecular hydride and methide shifts involving only the end unit. Which of the resulting repeating units would be the most abundant ... 

Five different end units (XXI — XXV) may arise from 1,2-hydride and methide shifts. The first-formed carbocation (XXI) undergoes hydride shifts to form carbocations (XXII), (XXni), and (XXIV) (XXHI) rearranges to (XXV) by a methide shift ... 

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