Metal hydride thiols

The D2/H+ exchange reaction requires a heterolytic cleavage of the D2 molecule into D+ and D species and, vice versa, of H2 into H+ and H-. When such a H2 heterolysis takes place at [MS] sites, plausible intermediates are metal hydride thiol species forming according to Eq. 46. 

The starting material is always the chalcogenol and, consequently, is more used for thiols than selenols and tellurols. There are several types of reactions depending if the starting materials are metal hydrides (hydrogen elimination), complexes with M C (alkane elimination), M-N (transamination), or M-O (hydrolysis) bonds. 

If one or more of the hydrogen atoms of a non-metal hydride are replaced formally with another group, R—e.g., alkyl residues—then derived compounds of the type R-XHn-i, R-XHn-2-R, etc., are obtained. In this way, alcohols (R-OH) and ethers (R-O-R) are derived from water (H2O) primary amines (R-NH2), secondary amines (R-NH-R) and tertiary amines (R-N-R R") amines are obtained from ammonia (NH3) and thiols (R-SH) and thioethers (R-S-R ) arise from hydrogen sulfide (H2S). Polar groups such as -OH and -NH2 are found as substituents in many organic compounds. As such groups are much more reactive than the hydrocarbon structures to which they are attached, they are referred to as functional groups. 

As the rate of cyclization becomes slower, the reactivity of the precursor becomes more important. To ensure that the radical generation step does not break the chain, it is important to use the most reactive precursor available. For very slow cyclizations, the advice is simple use iodides whenever possible. The purity of the precursor is also critical for slow cyclizations because tin hydride can sometimes react with impurities to generate hydrogen atom sources that are much more reactive than itself. Any impurities that might generate thiols or selenols may cause undue amounts of reduction (thus, the purity of phenyl sulfides and selenides is especially important). Metal impurities, which may form transition metal hydrides, can be devastating, even for fast cyclizations.41 Empirically, it seems that breaking of the chain is less of... 

In search of model systems for iron hydrogenases, Sellmann et al. (67) investigated the interaction of I e(hdt)2 2 with H+, H2, and H . Formation of H2 was observed in the reaction with H+. The reaction mechanism was proposed to follow a step-wise protonation, forming a thiol-hydride complex and H2 is proposed to form via heterolytic elimination from the metal hydride species (Scheme 6). Theoretical calculations suggest that concerted H2 elimination from a dithiol species is thermally forbidden (67). 

All the metal hydrides are reducing agents and readily are decomposed by water, dilute acids, alcohols, thiols, primary and secondary amines, and other protic compounds, with... 

Ketones can be reduced to secondary alcohols by H and a catalyst or a complex metal hydride or by hydrogen transfer reagents. Deoxygenation with a strong base at 150°C (302°F) can form olefin. Ketones react with thiols to form thioacetals. Reaction with anhydrous hydrazine may be explosive. 

Reductions of organic compounds with polymer-supported reagents have included reports of the reduction of disulfides to thiols, of the use of alumina-supported materials for reduction of alkenes and ketones, and of the various reductions brought about by polymer-supported metal hydrides. The reductions of disulfides to thiols, mainly concerned with biologically related materials, will be discussed later (Chapter 15). 

However, lithinm aluminum hydride or zinc metal and HCl (5) are required as reducing agents to reduce the thiocyanate to the thiol. These reducing agents are stoichiometric reagents and aren t environmentally acceptable at this time because of their hazardous properties and waste disposal problems on a large manufacturing scale. 

---