Proton insertion into oxides

Schober T, Schilling W, Wenzl H. Defect model of proton insertion into oxides. Solid State Ionics. 1996 86-88 653-8. 

Pd, or Ni (Scheme 5-3). First, P-H oxidative addition of PH3 or hydroxymethyl-substituted derivatives gives a phosphido hydride complex. P-C bond formation was then suggested to occur in two possible pathways. In one, formaldehyde insertion into the M-H bond gives a hydroxymethyl complex, which undergoes P-C reductive elimination to give the product. Alternatively, nucleophilic attack of the phosphido group on formaldehyde gives a zwitterionic species, followed by proton transfer to form the O-H bond [7]. 

Acrylonitrile or methyl acrylate readily inserts into allylnickel bonds (example 34, Table HI). A trans double bond is formed by loss of a proton. Insertion of acetylene followed by oxidative elimination with allyl halides gives cis double bonds (example 32, Table III). Insertion of methyl propiolate, followed by proton uptake, leads to a trans double bond (example 33, Table III). Norbomene has been shown to insert stereoselectively cis.exo into an allylnickel bond (example 35, Table III). 

In aerobic oxidations of alcohols a third pathway is possible with late transition metal ions, particularly those of Group VIII elements. The key step involves dehydrogenation of the alcohol, via -hydride elimination from the metal alkoxide to form a metal hydride (see Fig. 4.57). This constitutes a commonly employed method for the synthesis of such metal hydrides. The reaction is often base-catalyzed which explains the use of bases as cocatalysts in these systems. In the catalytic cycle the hydridometal species is reoxidized by 02, possibly via insertion into the M-H bond and formation of H202. Alternatively, an al-koxymetal species can afford a proton and the reduced form of the catalyst, either directly or via the intermediacy of a hydridometal species (see Fig. 4.57). Examples of metal ions that operate via this pathway are Pd(II), Ru(III) and Rh(III). We note the close similarity of the -hydride elimination step in this pathway to the analogous step in the oxometal pathway (see Fig. 4.56). Some metals, e.g. ruthenium, can operate via both pathways and it is often difficult to distinguish between the two. 

The fact that the pKa of the various oxidation states of quinones differ dramatically has been exploited in a recent design of artificial systems that mimic the light-driven transmembrane proton transport characteristic of natural photosynthesis [218]. Triad artificial reaction centers structurally related to 41 were vectorially inserted into the phospholipid bilayer of a liposome (vesicle) such that the majority of the quinone moieties are near the external surface of the membrane, and the majority of the carotenoids extend inward, toward the interior surface. The membrane... 

Since the relative rate of formation of dimethyhnethylcarboxonium ion from isobutane is considerably faster than that of rert-butyl cation from isobutane in the absence of ozone under the same conditions,it is, however, likely that protonated ozone inserts into the C-H bond of the alkane to form a pentacoordinate peroxonium ion 70 that decomposes to a very reactive tert-alkyloxenium ion (71) that subsequently undergoes Baeyer-Villiger oxidation [Eq. (6.52)]. 

In sum, the course of heteroatom oxidation appears to be sensitive to the oxidation potential of the heteroatom, the acidity of hydrogens on the adjacent carbon, and steric factors. The bulk of the evidence suggests that oxidation of the nitrogen in amines generally involves electron abstraction followed primarily by A-dealkylation if a labile proton is present, or nitrogen oxidation if it is not. As the nitrogen oxidation potential increases, there is a shift toward direct insertion into the C-H bond, as is thought to occur in the A-dealkylation of amides. 

Substituted tricyclopentanols (alcohols iclated to 9) have served as precursors of pyramidal cations by protonation and deltydration [37], but the sequence proposed above requires conversion of the ketone to a caibene and the insertion of the caibene center into a CH bond (a distal CH insertion into a proxiinal CH would form a highly strained bridgehead alkene), a known process [38], A possible sequence for the conversion of 9 to the caibene is preparation of the lydrazone and oxidation of this to the diazo compound [39], followed by catalytic or photochemical expulsion of nitrogen [40] ... 

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