Catalytic reactions alkene precursors

A modification of an earlier procedure for debromination of v/c-dibromides in the presence of catalytic amounts of diorganotellurides has allowed the synthesis of terminal alkenes and cis- and frani-l,2-disubstituted alkenes from appropriate precursors the relative substrate reactivities suggest that, as for the stoichiometric reaction, the catalytic reaction involves intermediate bromonium ion formation. The Te(IV) dibromides formed in the debrominative elimination are reduced back to the catalysts by either sodium ascorbate or the thiol glutathione. 

Although the catalytic reactions described above involve mononuclear Rh and Rh complexes, dinuclear Rh compoimds have also been studied as catalyst precursors in oxygenation reactions. The system [Rh2(p.-OAc)4]/ f-BuOOH is effective in the oxidation of cyclic alkenes such as cyclopentene, cyclohexene and cycloheptene, mainly to o, /i-unsaturated ketones and allylic acetates, but with poor yields (Eq. 4) [30,31]. 

Figure 6.13 Examples of photoinduced catalytic reactions (a) alkene hydrogenation by the [Fe(CO)3(alkene)f photocatalyst and (b) the double bond migration by the [Fe(CO)3(1-pentene)] photocatalyst both catalysts are generated in photosubstitution and photodissociation reactions of the [Fe(CO)5] precursor [28]...

Abstract Ruthenium holds a prominent position among the efficient transition metals involved in catalytic processes. Molecular ruthenium catalysts are able to perform unique transformations based on a variety of reaction mechanisms. They arise from easy to make complexes with versatile catalytic properties, and are ideal precursors for the performance of successive chemical transformations and catalytic reactions. This review provides examples of catalytic cascade reactions and sequential transformations initiated by ruthenium precursors present from the outset of the reaction and involving a common mechanism, such as in alkene metathesis, or in which the compound formed during the first step is used as a substrate for the second ruthenium-catalyzed reaction. Multimetallic sequential catalytic transformations promoted by ruthenium complexes first, and then by another metal precursor will also be illustrated. 

Co2(CO)8 is a catalyst precursor in the hydroformylation (oxo reaction) of alkenes to give aldehydes [5], The active catalytic species generated in situ is proposed to be HCofCO),. In industry, production of butanal is performed by this homogeneous catalytic reaction. PBuj is added to increase a n/iso ratio of the aldehydes (eq (2)). 

Since the discovery of Wilkinson s complex RhCKPPh ) in 1966 [39], many useful catalytic reactions have been reported which are catalyzed by this and related complexes. In particular, RhCl(PPh is a very useful catalyst precursor for hydrogenation of variety of alkenes. The chemical reaciivitv of RhCl(PPh3)3 is summarized in Scheme 11.4 [40]. 

The transition-metal NPs dispersed in imidazolium ILs are active catalysts for the hydrogenation of alkenes, arenes and ketones (Table 6.3). Moreover, Pd(0) NPs are active catalyst precursors for C—C coupling reactions, serving as reservoirs of mononuclear catalytically active species. In most cases, the catalytic reactions are typically multiphase systems in which the NPs dispersed in the ILs form the denser phase and the substrate and product remain in the upper-phase. In these cases the ionic catalytic solution is easily recovered by simple decantation and can be reused several times without any significant loss in catalytic activity. 

This is illustrated in the mechanism of the Mizoroki-Heck reaction depicted in Scheme 1.22. Indeed, three main factors contribute to slow down the fast oxidative addition of Phi (i) the anion AcO delivered by the precursor Pd(OAc)2, which stabilizes Pd L2 as the less reactive Pd°L2(OAc) (ii) the base (NEts) which indirectly stabilizes Pd L2(OAc) by preventing its decomposition by protons to the more reactive bent Pd L2 (iii) the alhene by complexation of Pd°L2(OAc) to form the nonreactive ( -CH2=CHR)Pd°L2(OAc). On the other hand, the slow carbopalladation is accelerated by the base and by the acetate ions which generate ArPd(OAc)L2, which in turn is more reactive than the postulated ArPdIL2. The base, the alkene and the acetate ions play, then, the same dual role in Mizoroki-Heck reactions deceleration of the oxidative addition and acceleration of the slow carbopalladation step. Whenever the oxidative addition is fast (e.g. with aryl iodides or activated aryl bromides), this dual effect favours the efficiency of the catalytic reaction by bringing the rate of the oxidative addition closer to the rate of the carbopalladation [Im, 34]. 

Metal carbonyl dusters (e.g. of Rh) can be used as precursors to form catalysts for reactions involving CO, induding the water gas shift reaction, alkene hy o-formylation, and CO hydrogenation. Although the catalysts exhibit some unusual selectivities, such as in the hydrogenation of CO to give non Schulz-Flory distributions in their hydrocarbon products, they are not highly active relative to some of the more conventional catalysts. The spedes in the zeolites that are formed from the cluster precursors and which are the actual catalytically active spedes have not yet been eluddated. 

A simple approach for the formation of 2-substituted 3,4-dihydro-2H-pyrans, which are useful precursors for natural products such as optically active carbohydrates, is the catalytic enantioselective cycloaddition reaction of a,/ -unsaturated carbonyl compounds with electron-rich alkenes. This is an inverse electron-demand cycloaddition reaction which is controlled by a dominant interaction between the LUMO of the 1-oxa-1,3-butadiene and the HOMO of the alkene (Scheme 4.2, right). This is usually a concerted non-synchronous reaction with retention of the configuration of the die-nophile and results in normally high regioselectivity, which in the presence of Lewis acids is improved and, furthermore, also increases the reaction rate. 

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