Metal hydride deprotonation

Strong bases such as alkali metal hydrides deprotonate B5H9 at —78 °C in ether solution to the octahydropentaborate(l—) which not only is a strong base but also an important reagent for the synthesis of derivatives [52]. The anion itself is not... 

KTB and KTA are superior to alkaU metal hydrides for deprotonation reactions because of the good solubiUties, and because no hydrogen is produced or oil residue left upon reaction. Furthermore, reactions of KTA and KTB can be performed in hydrocarbon solvents as sometimes requited for mild and nonpolar reaction conditions. Potassium alkoxides are used in large quantities for addition, esterification, transesterification, isomerization, and alkoxylation reactions. 

In this context we postulated that the shift reaction might proceed catalytically according to a hypothetical cycle such as Scheme I. There are four key steps in Scheme I a) nucleophilic attack of hydroxide or water on coordinated CO to give a hydroxycarbonyl complex, b) decarboxylation to give the metal hydride, c) reductive elimination of H2 from the hydride and d) coordination of new CO. In addition, there are several potentially crucial protonation/deprotonation equilibria involving metal hydrides or the hydroxycarbonyl. The mechanistic details have been worked out (but only incompletely) for a couple of the alkaline solution WGSR homogeneous catalysts. In these cases,... 

The synthesis of alkali metal organophosphides and arsenides is usually most conveniently achieved by the direct metalation of a primary or secondary phosphine/arsine with a strong deprotonating agent such as an alkyllithium or an alkali metal hydride ... 

An entry into the chemistry of mdo-decaborates is provided by utdo-B oH 4. As already mentioned, decaborane BioHi4 is easily deprotonated even in aqueous medium to yellow B10Hb and with strong bases, such as alkali metal hydrides, even further to the colorless anion B10H122-. This is summarized in the reversible reactions shown in Eq. (59). 

One must always keep in mind that in aqueous solutions the transition metal hydride catalysts may participate in further (or side) reactions in addition to being involved in the main catalytic cycle. H and P NMR studies established that in acidic solutions [RhCl(TPPMS)3] gave cis-fac-and ci5-7 er-[RhClH2(TPPMS)3] [86,88], while in neutral and basic solutions these were transformed to [RhHX(TPPMS)3] (X = H2O or Cl ) [86]. Simultaneous pH-potentiometiic titrations revealed, that deprotonation of the dihydride becomes significant only above pH 7, so this reaction of the catalyst plays no important role in the pH effects depicted on Figs. 3.2.a and 3.2.b. 

Aryldiazene complexes can be formed from arenediazonium ions and metal hydrides, and subsequent deprotonation leads to aryldiazenido complexes. Mechanistic details of ArN insertion into Rh—H and Pt—H bonds have been recently studied (equations 124-126).453 454... 

Conjugated dienes were thus selectively obtained by hydrovinylation of alkynes catalyzed by a cationic ruthenium alkylidene complex [43] (Eq. 31). This reaction is thought to be promoted by the ruthenium hydride species resulting from the deprotonation of the <5-methyl group of the metallic precursor, followed by the sequential insertion of alkyne and ethylene into the metal-hydride and metal-vinyl bonds. 

M-H (metal hydride) Sodium hydride Potassium hydride Strong, not nucleophilic base Deprotonation of weak organic acids with acidities as high as pKa 25... 

As might be expected, the 17-electron metal-hydride cation radicals typically undergo proton-transfer reactions [30, 42-44], The 17-electron radical that results from the deprotonation is usually oxidized, presumably via solvent coordination, to... 

Stepwise deprotonation of methylenebis(triorgano)phosphonium salts with bases yields carbodiphosphoranes via intermediate semi-ylide salts, which may also be accessible by alkylation or phosphinol-ation of corresponding alkylidenephosphoranes (equation 118). " It depends on the starting phosphonium salt and the base whether the intermediate ylide salts can be isolated or not. Suitable bases are sodium amide, alkali metal hydrides, alkylidenetrialkylphosphoranes, potassium and lithium orga-nyls. For the synthesis of hexaphenylcarbodiphosphorane improved methods have been reported by which this compound may be generate without isolation of the ylide salt and on a large scale. 

The deprotonation of M-H to give M and H+ is one quite common dissociation reaction in which the pair of electrons of the bond goes to the metal. The metal doesn t change its total electron count, and its oxidation state decreases by 2. It s strange to think of a deprotonation as causing a reduction of the metal. The conundrum arises because of the oddities of the language that is used to describe metal complexes. The oxidation state of a complex with a M-H bond is calculated as if the bond were polarized toward H (i.e., M+ 11 ). Thus, when metal hydrides are deprotonated, it seems as if the metal is gaining electrons (from the hydride) that it did not have previously. 

MH4- Complex metal hydrides LiAlH4 LiAlH(OR)3 NaBH4 NaBHjCN The more ionic MH bond is more reactive Substitutions Additions. Deprotonates acidic Hs on heteroatoms NaH and KHas MH bases... 

Basic system both imines and amines are basic compounds this can lead to deprotonation of the metal hydrides formed as intermediates in the catalytic cycle and finally cause a decomposition of the active catalyst. 

The reverse reaction, formation of H2 from 2H+ and 2e, involves protonation of the 2Fe center to form a metal hydride. The most basic site for initial protonation in the enzyme active sites may be the electrons in the M-M bonds, which can readily be reversibly protonated to form hydride-bridged species.43 Electrophilic [M-HJ+ systems often prefer to form p-H complexes rather than M-H on deprotonation of t]2-H2 by external bases ... 

If protonation of a metal hydride by an inter or intramolecular acid (HA) produces a transient dihydrogen complex, rotation about the H-H bond followed by deprotonation can permute the protons of MH and HA. (A dihydrogen complex has been observed during many exchange processes between metal hydrides and acids.) This is of interest because it shows that fast exchange between a metal and an acid prior to metal protonation by the same acid implies a dihydrogen intermediate. 

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