Methine hydrogen, exchange

Stadherr and Angelici459 studied the effects of various metal ions on the base-catalyzed racemization and deuterium exchange at the methine hydrogen of N,N-bis(carboxymethyl)-D-phenylgly-cine (144). The metal complexes underwent racemization at rates up to 5 x 103 times greater than for the free ligand. 

The tot-butyl cation (1) reforms isobutane via hydride abstraction from isobutane according to Bartlett et al.,834 Nenitzescu et al.,835 and Schmerling836 involving the tertiary C—H bond only through intermediate structure 463, and thus not exchanging the methine hydrogen. 

I.6. Higher Alkonium Ions The acid-induced H-D exchange of isobutane (49) in deuterosulfuric acid was studied by Otvos and coworkers. All nine methyl hydrogens were readily exchanged, but not the methine hydrogen. The mechanism for this observation is considered to involve the initial formation of trivalent zerz-butyl cation (50), probably in an oxidative... 

The tert-butyl cation (50) reforms isobutene, according to Bartlett et al. Nenitzescu et al., ° and Schmerling, by hydride abstraction from isobutane involving the tertiary C-H bond only, through intermediate structure 52, and thus no methine hydrogens would exchange with the deuterosulfuric acid. 

In contrast, Olah et al showed that in superacidic media, for example, DSOsF-SbFs or DI SbK, and at low temperatures, initially only the methine hydrogen is exchanged (with the acid), indicating that no deprotonation-protonation equilibria involving species such as 50 and 51 are involved in the strong superacidic medium. The reaction proceeds through the intermediary of the 53 protonated isobutane [Eq. (5.11)] ... 

As mentioned earlier in Chapter 5 (see Section 5.4.I.6.), the H-D exchange in isobutane in superacid media is fundamentally different from the H-D exchange observed by Otvos et al. in weaker acids such as D2SO4, who found the eventual exchange of all the nine methyl hydrogens with deuterium but no exchange of the methine hydrogen was observed (Scheme 6.4). 

This conclusion suggested that the sulfone 8J (Fig. 8.23) (which has been shown [28] to have the conformation as drawn) would be expected to be well-arranged for the exchange of the a-sulfonyl methine hydrogen it was found that the exchange in 8J occurs almost 200 times faster than that in 75 (on a per hydrogen basis) [28]. 

The nonexistence of alkane-alkene equilibrium in superacid medium has been elegantly demonstrated by the behavior of isobutane in deuterated superacid medium (DSOsF-SbFs or DF-SbFs). Isobutane at low temperature undergoes hydrogen-deuterium exchange only at the methine position through the involvement of a three-center bound pentacoordinate carbonium ion (Scheme 17). 

Metalation of allylic methylene and methine groups. This base selectively metalates the allylic position of cyclohexene. This hydrogen-metal exchange has been used in a high-yield synthesis of 2-(3-cyclohexenyl)ethanol (1). The reaction is carried out by stirring cyclohexene with a suspension of the base in... 

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