Hydride route

War research forced us to explore new synthetic routes and we discovered the alkali metal hydride route to diborane. This solved the synthetic problem. At the same time we discovered sodium borohydride and developed simple synthetic methods for its preparation and manufacture. 

In Scheme 1 is represented an idealized picture of the two possibilities for the hydrogenation of alkenes by metal complexes not containing an M—11 bond. One possibility involves initial coordination of the alkene followed by activation of H2 (alkene route). The other (more general) possibility is the hydride route, which involves initial reaction with H2 followed by coordination of the alkene. The second general mechanism, usually adopted by catalysts containing an M—H bond, is shown in Scheme 2. 

From a mechanistic point of view, two very general pathways can be envisaged for hydrogen transfer direct hydrogenation transfer, consisting of a concerted process that involves a six-membered cyclic transition state in which both the hydrogen donor and the acceptor are coordinated to the metal (1 in Scheme 22) and a hydridic route (2 in Scheme 22).116... 

In this latter hydridic route for hydrogen transfer from alcohols to ketones, two additional possibilities can be considered one involving a metal hydride arising purely from a C—11 (path 2a), and another in which it may originate from both the O—11 and C—I I (path 2b) in this case any of the hydrides on the metal may add to the carbonyl carbon. 

The dimeric complex [RhClL2 L>, present in varying amounts according to the conditions, is also an effective catalyst via a similar hydride route involving complex 1. An originally proposed (80) dimeric tetrahydride was not detected. Detailed crystal structures of both the red and orange forms of RhCl(PPh3)3 have appeared (81). 

Details of the various steps which will depend on the substrates and donors involved, are usually not well understood. Prior coordination of the donor followed by that of the substrate, equivalent to a hydride route (Section II,A), is also possible (494, 496). Formation of intermediate dihydrides from a donor (e.g., from an alcohol via oxidative addition to give a hydrido-alkoxide, and then /8-hydrogen transfer) has also been invoked (491, 492, 496, 499, 500) in mechanistic terms, the hydrogenations then become equivalent to using molecular hydrogen for the reductions. The /3-hydrogen transfer step is usually considered rate-determining (494, 496). 

The hydride route involves the initial reaction with hydrogen followed by coordination of the substrate the well-known Wilkinson catalyst [RhCl(PPh3)3] is a representative example. A second possible route is the alkene (or unsaturated) route which involves an initial coordination of the substrate followed by reaction with hydrogen. The cationic catalyst derived from [Rh(NBD)(DIPHOS)]+ (NBD = 2,5-norbornadiene DIPHOS = l,2-bis(diphenyl)phosphinoethane) is a well-known example. The above-mentioned rhodium catalysts will be discussed, in the detail, in the following sections. 

We calculate only a 0.7 kcal/mol difference between R-DIHY-B and S-DIHY-B, with nearly identical migratory insertion barriers, so we would predict only modest enantioselectivity if a DuPHOS-ligated catalyst reacted along the hydride route. However, we are not aware of any evidence that a solvated Rh-DuPHOS catalyst reacts with hydrogen to form dihydrides. 

Although the isolation and reactivity of acyl complexes strongly support the hydride mechanism, the other mechanism cannot be excluded. For example H20, the acid or molecular hydrogen, which can act as a hydride source, can promote the Pd - C splitting of the Pd-alkylcarboalkoxy intermediate in the alkoxy cycle as well. More convincing for the hydride route is the fact that the acid, which does not promote the formation of a Pd-OCH3+ species, has a promoting effect on the catalysis and can activate a Pd(0) complex, otherwise inactive, whilst a base, which not only promotes the formation of this species, but also deprotonates a Pd - H+ species to Pd(0), suppresses the catalysis. 

The kinetic findings can be rationalized by assuming that these catalytic hydrogenations involve a heterolytic activation of H2 and proceed on the hydride route (Scheme 3.4). 

In principle, the mechanism of homogeneous hydrogenation, in the chiral as well as in the achiral case, can follow two pathways (Figure 9.5). These involve either dihydrogen addition, followed by olefin association ( hydride route , as described in detail for Wilkinson s catalyst, vide supra) or initial association of the olefin to the rhodium center, which is then followed by dihydrogen addition ( unsaturate route ). As a rule of thumb, the hydride route is typical for neutral, Wilkinson-type catalysts whereas the catalytic mechanism for cationic complexes containing diphosphine chelate ligands seems to be dominated by the unsaturate route [1]. 

Figure 9.5 The mechanism of homogeneous hydrogenation unsaturate route versus hydride route .

Catalytic cycles operating via the initial formation of a hydridometal complex followed by coordination of the unsaturated compound (S) are termed the hydride route23 (Scheme 11.7). In contrast, the unsaturate route23 involves prior binding of the organic compound (Scheme 11.7). In many cases the hydride route was proved to be more efficient. 

Of the numerous proposals, the metal hydride route with a-hydride abstraction (p elimination) [Eq. (12.18)] has gained substantial experimental support 7... 

Scheme 4.6 Hydridic route established for transfer hydrogenation with Rh-, Ru- and Ir-catalysts.

Recent mechanistic studies on transition metal-catalysed hydrogen transfer reactions have been reviewed. Experimental and theoretical studies showed that hydrogen transfer reactions proceed through different pathways. For transition metals, hydridic routes are the most common. Within the hydridic family there are two main groups the monohydride and dihydride routes. Experimentally, it was found that whereas rhodium and iridium catalysts favour the monohydride route, the mechanism for ruthenium catalysts proceeds by either pathway, depending on the ligands. A direct hydrogen transfer mechanism has been proposed for Meerwein-Ponndorf-Verley (MPV) reductions.352... 

Aus Tab. 3 Nr. 1 erkennt man, daft bei der hydride route die Hydrierung des Katalysators KaLx eine wesentliche Rolle spielt. Falls sich das Gleichgewicht nach Schema (8) nicht sehr schnell einstellt... 

Hydridkomplex KaLxH2 22,29) jm Gleichgewicht vorhanden ist, so mufi das Verhaltnis von rHj zu r noch grofier sein. Fiir die Beispiele der Tabelle 4 kann so-mit die hydride route nicht der geschwindigkeitsbestimmende Schritt bei der Hydrierung sein. 

Fiir den Katalysator RhH(CO)(P0)3)3 (Tabelle 2, Nr. 3 und 4) wurde die hydride route nicht diskutiert, da die Bildung des Hydridkomplexes RhH3(CO)(PQ3)3... 

There are two possible ways to approach the M(H)n(alkene) intermediate in a hydrogenation reaction the hydride route <2), implies an initial reaction with hydrogen followed by coordination of the substrate, while the unsaturated... 

---