Protic substrates

The influence of the acidity of the relevant protic substrates on the final reaction products is further demonstrated by two examples shown in Scheme 2. Decamethylsilicocene (1) reacts with HBF4 to yield the compound [Cp SiF]4, whereas in the reaction with PyH+BF4 (Py=pyridine) the oxidative addition product Cp 2Si(H)F is formed. Reaction of 1 with two equivalents of F3CS03H leads to an ionic species,... 

Differences in the reactivity of 1 and its heavier homologues towards protic substrates have already been described [7], and become evident also from the reaction with catechol whereas oxidative addition and subsequent H2 elimination is observed with 1, a substitution process takes place with decamethylstannocene (see Scheme 3) [4]. 

The mechanism to be dicussed resembles that given by Weber [9] for the reaction of dimethylsilylene with protic substrates. Zwitterionic compounds, formed by interaction of the lone pair of the substrate with the vacant p orbital at silicon, are regarded as intermediates. 

Scheme 6. Insertion reactions of 1 with protic substrates.

The analogous coordination behavior of hydridotris(pyrazolyl)borate and tetrakis(pyrazolyl)-borate has been demonstrated by numerous reactions. Reduction of the rhenium(VII) complex [Re03 B(pz)4] with PPh3 in THF gives a reactive dimeric intermediate which reacted with monodentate and bidentate protic substrates to yield products with tridentate B(pz)4 ligands... 

The chiral guanidine s role as a strong Brpnsted base for the reactions of protic substrates has been proposed. In 1999, Corey developed a C -symmetric chiral guanidine catalyst to promote the asymmetric Strecker reaction [117]. The addition of HCN to imines was promoted high yields and high enantioselectivities for both electron-withdrawing and electron-donating aromatic imines (Scheme 64). 

The reaction of 82 with protic substrates HX can follow different pathways, as depicted in Scheme 23. In most of the cases studied so far, simple oxidative addition takes place leading to compounds of the type (Me5C5)2Si(H)X190. The reaction of 82 with tetrafluoroboric acid leads to elimination of pentamethylcyclopentadiene and to the formation of a cyclotetrasilane derivative, via several intermediates with divalent silicon from which the dimeric pentamethylcyclopentadienyl(fluoro)silylene was proved by Si NMR spectroscopy191,192. In contrast, an HF-oxidation product is formed in the reaction of 82... 

HX = HF,HCtHBr,EtC02H,F3CC02H,F3CS03H,C6H50H>p-MeC6H4SH>R2C = N0H SCHEME 23. Reaction of (Most s)2Si (82) with protic substrates... 

An uncharged nucleophile may also play the role of a Bronsted base if interaction with a protic substrate can increase its nucleophilicity [Eqs. (41) and (42)]. This pathway, formulated by Allen and Modena (137), seems to be important in many solvolytic reactions (7). There is evidence that more than one solvent molecule participates in the solvolysis of chlorosilanes (7,137,244). One functions as the nucleophile attacking silicon, while the others, forming hydrogen bonding complexes, enhance the nucleophilic reactivity. 

The strong catalytic effect of amines in silylation with silyl hydrides of some protic substrates, like alcohols, phenols, and carboxylic acids, is well known (for review, see Ref. 316). In this case, the amine is not likely to ionize the silicon-leaving group bond. General base catalysis has been... 

The propensity of 02 to remove protons from substrates accounts for its reactivity with acidic reductants and their overall oxidation. Thus, combination of 02 - with protic substrates (a-tocopherol, hydroquinone, 3,5-di-t-butylcatechol, L(- -)-ascorbic acid) yields products that are consistent with an apparent one-electron oxidation of the substrate and the production of HOOH. However, the results of electrochemical studies provide clear evidence that these substrates are not oxidized in aprotic media by direct one-electron transfer to 02. The primary step involves abstraction of a proton from the substrate by 02 to give substrate anion and the disproportionation products of HOO- (HOOH and O2). In turn, the substrate anion is oxidized by O2 in a multistep process to yield oxidation products and HOOH. Thus, by continuously purging the O2 that results from the... 

Pentavalent phosphorus compounds with a suitable leaving group, listed below, serve effectively as phosphorylating agents for some protic substrates such as alcohols, phenols, primary and secondary amines, thiols, phosphates, etc. 

Our strategy to synthesize a compound containing the MesCsSi cation is based on observations from the chemistry of the heavier group 14 element metallocenes. Reaction of the decamethylmetallocenes (Me5Cs)2El with several protic substrates led to the isolation of compounds containing the corresponding MesCsEr cations (Scheme 1) [8]. 

In contrast, the reaction of decamethylsilicocene with protic substrates follows three different pathways (Schemes 2 and 3) [6]. Oxidative addition is observed with the hydrogen halides HX, and the protonated silicocene is formed in the reaction with two equivalents of trifluorosulfonic acid. In the reaction with HBF4, the wanted MesCsSr cation might be present together with the BF4" anion as a highly reactive intermediate which easily eliminates BF3 under formation of the polymer (Me5C5SiF) in a final step. 

We would like to note that a related Lewis acid/base interaction with hydrides may also suffice the piupose of the assessment of basicity. The thermodynamic analyses of the hydride adduct formation with protic substrate is usually accomplished by spectroscopic investigations of their equilibria of formation (eq 7). 

Figure 10. Oxygenation of protic substrates mediated by Cu-dependent oxygenases.

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