Cations methoxymethyl

The methoxymethyl cation can be obtained as a stable solid, MeOCH SbF Carbocations containing either a, p, or y silicon atom are also stabilized, relative to similar ions without the silicon atom. 

Detailed structural calculations have been carried out for this system. This is because the neutral isomer, C2HsO, which is implicated in the thermochemistry of ethanol, is of interest in pollution control, atmospheric chemistry, and combustion. Also, there is new information available from photoionization experiments with which to compare theoretical calculations. For details of these comparisons, see Curtiss et al.73 In the earlier theoretical studies of Nobes et al.,74 calculations were performed at the MP2 and MP3 levels with basis sets of double plus polarization (6-13G ) with electron correlation. These studies revealed four stable minima for the system protonated acetaldehyde, CHj-C H-OH <-> CH3-CH=0+H the methoxymethyl cation, CH3OCH2 protonated oxirane, (CH2)2OH+ and vinylox-... 

This is clearly illustrated in the example given below for the reaction of the methoxymethyl cation with pivaldehyde. There are four possible reaction channels for the unimolecular dissociation of the initially formed adduct, which is presumed to be of the form of a covalent species resulting from the interaction of the carbocation center of the methoxymethyl cation with the carbonyl oxygen of the aldehyde, 2. For smaller carbonyl compounds this adduct has been demon-... 

Figure 16. Metastable ion cyclotron resonance (MICR) spectra for the unimolecular dissociation of the chemically activated adduct ion derived from association of the methoxymethyl cation with pivaldehyde during a 2-s reaction delay at a pressure of pivaldehyde of 1.0 x 10 torr. The three spectra correspond to values of rf amplitude appropriate to eject transient intermediates with lifetimes longer than (a) 60 ps, (b) 80 ps, and (c) 1 70 ps. A partial pressure of CH4 of 1.0 x 10 torr was also present to thermalize ions. The peak at m/z 125 is a secondary reaction product of m/z 85.

Figure 17, Qualitative potential energy surface for the reaction of methoxymethyl cation with pivaldehyde. Energies in parentheses have units of kcal mol". ...

Methyl-l,3,5-trioxazepanium cation was generated on treatment of 1,3-dioxolane with methoxymethyl cation in S02 <1982MMC1587>. 

Another attractive alternative to protonation is to activate the C=0 bond by alkylation. The simplest activated molecule of this kind is the methoxymethyl cation, CH2OCH( [146-149], which has been the subject of numerous gas phase reactivity studies with a wide range of nucleophiles (H20, NH3, H2S, alcohols,... 

The stability of the carbocation is also increased due to the presence of heteroatom having an unshared pair of electrons, e.g. oxygen, nitrogen or halogen, adjacent to the cationic centre. Such carbocations are stabilized by resonance. The methoxymethyl cation is obtained as a stable solid, MeOCH2 SbF4. ... 

The methoxymethyl cation can be obtained as a stable sohd, MeOCHj SbF, Carbocations containing either a, p, or y silicon atom are also stabilized, relative to similar ions without the silicon atom. In super acid solution, ions such as CX (X = Cl, Br, I) have been prepared. Vinyl-stabilized halonium ions are also known. ... 

Freitas and O Hair133 studied the gas-phase reactivity of aniline and other nucleophiles toward the methoxymethyl cation, CH3 — 0+=CH2, an ambident electrophile, using flowing afterglow mass spectrometry. For aniline, two reaction channels were observed addition followed by elimination of methanol with concomitant [M + CH]+ ion formation and adduct formation, viz. [M + CH3OCH2]+. 

RAJ O FIair, MA Freitas, S Gronert, JAR Schmidt, TD Williams. Concerning the regioselectivity of gas phase reactions of glycine with electrophiles the cases of the dimethylchlorinium ion and the methoxymethyl cation. J Org Chem 60 1990-1998, 1995. 

MA Freitas, RAJ O Hair, S Dua, JH Bowie. The methoxymethyl cation cleaves peptide bonds in the gas phase. Chem Commun 1409—1410,1997. 

Figure 3.1 Stereochemistry of oxocarbenium ions. Top the methoxymethyl cation, showing overlap between an empty p orbital on carbon and a p-type lone pair of electrons on oxygen and also the two mechanisms for isomerisation. Rotation involves breaking the n bond, via a perpendicular transition state, whereas during inversion the ion remains planar and the oxygen atom undergoes a process similar to the inversion of ammonia. Centre the two permitted conformations of a xylofuranosyl cation. Bottom the four permitted conformations of a xylopyranosyl cation.

A linear relationship was observed between the logarithms of the rate constants for addition of water to Ar-C(CH3)=0 -CH3 and addition of to ArCOCHs extrapolation of this correlation to formaldehyde gave an estimated lifetime of the methoxymethyl cation of 1 fs. Thus, the reactions of methoxymethyl derivatives in water were required to be bimole-cular, even though the methoxymethyl cation was a stable, observable species in Magic Acid . The requirement was confirmed (see Section 3.3). 

Radicals can be generated by cathodic reduction from carbocations, protonated C=X bonds, and the reduction of halides or onium salts. The reduction potentials of carbocations range from 1.87 V (vs nhe) for NCCH(4-CN-C6H4)+, 0.97 V for the benzylcation, 0.33 V for the tert-butyl cation, and 0.0 V for the methoxymethyl-cation to -0.88 V for Et2N=CHCH3+ [143],... 

Relatively stabilized carbocations can be used to alkylate the pyrrole and indole rings. Introduction of formyl groups, for example, can be done with MeOCHCl2, via a reaction which likely proceeds through the 1-chloro-l-methoxymethyl cation, (Scheme 94) <72JMC97, 86SC1799). 

Olah has used the methods described above to make the methoxymethyl cation in solution. Although this cation can be drawn either as an oxonium ion or as a primary carbocation, the oxonium ion structure is the more realistic. The proton NMR spectrum of the cation compared with that of the isopropyl cation (this is the best comparison we can make) shows that the protons on the CH2 group resonate at 9.9 ppm instead of at the 13.0 ppm of the true carbocation. 

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