Unsaturated route

The cyclooctene dimer [IrCl(C8H]4)2] can selectively hydrogenate cy-clooctene in mixtures with hex-l-ene, and an unsaturate route [Eq. 1(b)] via a monomeric olefin complex was demonstrated (181). The pentamethylcyclopentadienyl dimer was mentioned at the end of Section II, B, 2. 

Itaconic acid was hydrogenated rapidly to a 20% ee, and kinetic and spectrophotometric studies on this system were interpreted in terms of a mechanism involving a standard unsaturate route [cf. Eq. (5)]. The actual catalyst was thought to be HRh(DIOP)(DIOP ), where DIOP represents a monodentate DIOP with one dangling —CH2PPh2 moiety (273, 275). Rhodium(I) carbonyls (276) and ruthenium(II) complexes (90, 275) (Section III,B) containing monodentate DIOP have been isolated. 

The cluster catalyzes hydrogenation (20°C and 3 atm) of dialkyl- and diarylacetylenes to the c/s-olefins via unsaturate routes, likely involving Ni4(CNR)6(RC=CR) and Ni4(CNR)4(RC==CR)3 (391, 392). The acetylenes in the latter complex bridge three nickel centers, and increase of the acetylenic carbon-carbon bond distance is considered to enhance reduction by hydrogen (392, 393). 

Successive hydrogen transfers within 60, followed by coordination of olefin and then H2 (an unsaturate route), constitute the catalytic cycle, while isomerization is effected through HFe(CO)3(7r-allyl) formed from 59. Loss of H2 from 60 was also considered to be photoinduced, and several hydrides, including neutral and cationic dihydrides of iridium(III) (385, 450, 451), ruthenium(II) (452) and a bis(7j-cyclopentadienyltungsten) dihydride (453), have been shown to undergo such reductive elimination of hydrogen. Photoassisted oxidative addition of H2 has also been dem-... 

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. 

Time-proven concepts for the reaction mechanisms of homogeneous hydrogenations follow two approaches which, according to Halperrfs step-wise analysis of hydrogenations using Wilkinsorfs catalysr [25] and the cationic catalyst DI-PHOS [26], respectively, can be grouped into the so-called dihydride or unsaturate routes [27] (Fig. 12.9). 

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 .

P is an optically active tertiary phosphine, likely will resemble the RhCl(PPh3)3 system (23). However, even in this exhaustively studied system, both hydride and/or unsaturate routes are feasible (23, 24) by varying conditions, the choice of route could affect stereoselectivity. Most asymmetric hydrogenations have used prochiral olefinic acid substrates, and these systems have not been thoroughly studied even with nonchiral catalysts. 

The mechanism shown in Scheme 7, called the unsaturate route, is characterized by initial substrate coordination to metal followed by oxidative addition of hydrogen however, depending on the catalyst, substrate, and reaction conditions, the order of the individual steps can be... 

Wie Tabelle 2 zeigt, wurde als Mechanismus der homogenen katalytischen Hydrierung mit RhH(CO)(PQ3)3 und IrX(CO)L2 die,.unsaturate route wahrscheinlich ge-... 

Nachdem die quantitativen Untersuchungen zum Reaktionsmechanismus gezeigt haben, daft die Katalyse der Hydrierung zum uberwiegenden Teil iiber die unsaturate route verlauft, war es von Interesse, wie die Dissoziationskonstante K2 nach Schema (10)... 

Vom Mechanismus her gesehen verlauft die Hydrierung zum iiberwiegenden Teil iiber die unsaturate route". 

This celebrated mechanism follows what is called the unsaturated route . This means that in the catalytic cycle the substrate is bonded first, before the oxidative addition of hydrogen occurs. However, this order may be reversed. If the oxidative addition of hydrogen precedes the coordination of the olefin, the reaction is said to occur by the hydride route . Actually, the hydride route is the mechanism of the... 

Reactions in which an unsaturated ligand is removed by complete hydrogenation to yield catalytically active species are related to the hydrogenolysis process. Common examples are provided by Rh and Ir systems, equation (t). Depending on L, the product solvated species can subsequently perform hydrogenation via the hydride or unsaturated routes (see 14.3.3.1, 14.3.4.5). 

In general for soluble hydrogenation catalysts, activation of the organic substrate involves its coordination to the metal center. If a mono- or dihydride species is formed prior to substrate activation, the catalytic process is said to operate via a hydride route [equation (a)] if substrate binding precedes activation of Hj, the catalysis operated via an unsaturated route [equation (b)]. ... 

Scheme 1, which shows reaction pathways available for hydrogenation of alkenes using dihydride catalysts , has been developed largely from studies on Rh catalysts. The steps define the hydride route, and Kg,k2 the unsaturated route via oxidative addition of H2 to the metal-alkene complex. The common key dihydride-alkene intermediate 1 gives the saturate product wifii regeneration of catalyst M via two successive hydrogen atom transfer steps k. The and equilibria are usually established... 

Scheme 1, which shows reaction pathways available for hydrogenation of alkenes using dihydride catalysts , has been developed largely from studies on Rh catalysts. The define the hydride route, and K k2 the unsaturated route via oxidative... 

Table 1. Catalysts That Operate via Dihydride Intermediates within Unsaturated Routes for Hydrogenation of Alkenic Substrates...

Taking the facile reaction of [RhCl(TPPMS)3] with H2 (Eq. 8), the kinetic data suggest a mechanism analogous to the familiar one of alkene hydrogenation with [RhCl(PPh3)3] [42], Whether the reductions of maleic, fumaric and crotonic acids proceed via the unsaturate route or the hydride route was not established, but this would not effect the overall kinetics. 

RuCl3.aq and RhCl3.aq were among the first catalysts of hydrogenation in aqueous solution [59, 60]. Spectrophotometric experiments revealed the formation of an Ru(II)-alkene complex prior to the heterolytic activation of H2 ( unsaturated route of hydrogenation). 

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