Intermediates in metal-catalyzed

In most cases, the two types of mechanisms, the bond shift and cyclic mechanisms, are not exclusive but parallel pathways. With increasing molecular weight, the contribution of the cyclic mechanism increases and may become dominant. The pure selective mechanism on iridium is a unique exception. Hydrogenolysis, however, is the characteristic transformation on this metal. The nature of possible surface intermediates in metal-catalyzed alkane reactions, the role of electronic and geometric effects in their formation, and the relation of isomerization and hydrogenolysis have been reviewed.163... 

Hydrogenative ring opening of cycloalkanes is also a well-studied area.16 252 253 289-292 Mainly cyclopropanes and cyclopentanes were studied, since three- and five-membered adsorbed carbocyclic species are believed to be intermediates in metal-catalyzed isomerization of alkanes (see Section 4.3.1). Ring-opening reactivity of different ring systems decreases in the order cyclopropane > cyclobutane > cyclopentane > cyclohexane.251 Cyclopropane and its substituted derivatives usually react below 100°C. 

Metallacycloalkanes are proven key intermediates in metal-catalyzed cycloadditions and cycloreversions of alkenes. The relationship of some iron metallacyclopentane derivatives with bis(olefin) complexes has been investigated theoretically. Scheme 1 shows a general route from bisalkene... 

Figure 3. Possible intermediates in metal-catalyzed oxygenation reactions.

W. Midclitz, S. G. Bott, J. G. Bentsen, and S. J. Lippard. Characteri2ation of a novel i -peroxide tetrairon vmit of possible relevance to intermediates in metal-catalyzed oxidations of water to dioxygen. /. Am. Chem. Soc., Ill 372 (1989). 

Conclusive evidence for palladacyclopentanes and palladacyclopentadienes as potentially stable intermediates has been put forward, and they have a close analogy to such metallacycles with other metals.f" They frequently are intermediates in metal-catalyzed or -mediated formal [2+2 + 2] cyclotrimerizations of alkynes and strained alkenes. It is plausible to assume that palladacyclopropanes or -cyclopropenes also play a role in such formal [2+2 + 2] processes catalyzed by palladium(O), for example, in the type of Pd-catalyzed cyclotrimerization of 3,3-dimethylcyclopropene reported by Binger et al. (see also Sect. IV.2.4), for which a palladacyclopentane as intermediate was already isolated. Moreover, dimethyl 3,3-dimethylcyclopropenedicarboxyIate 8 has recently been shown to form the tricyclic palladacyclopentane derivative 10, the structure... 

The scheme depicted is not presented as an exact mechanistic representation, but rather to illustrate several basic points. First, one may observe that oxygen is a unique oxidant compared to other oxygen donors - the oxygen donors being in principle reduced relative to molecular oxygen. In fact, even the active oxidizing intermediate in metal-catalyzed autooxidation pathways is the reduced peroxo intermediate (Scheme 9.7, reaction d). In addition, only one oxygen atom of... 

Figure 35. Some common reaction intermediates in metal-catalyzed oxidations.

The third area is the synthesis and characterization of aryldiazenido complexes of transition metals. In 1964 King and Bisnette isolated the first metal complex with an aryldiazenido ligand. The interest of organometallic chemists was concentrated mainly on the isolation and characterization of stable aryldiazenido complexes and not on potential metastable intermediates involved in metal-catalyzed dediazonia-tions. The situation is different, however, for metal complexes with alkyl-diazenido ligands. Complexes with aryl- and alkyldiazenido ligands are the subject of Chapter 10 in the forthcoming second book (Zollinger, 1995). 

Among electron-rich chiral phosphines, chiral phospholanes have emerged to be one of the most efficient classes of ligands in metal catalyzed enantioselective reactions. We have developed a novel family of bisphospholane ligand namely, catASium M, from laboratory to commercial scale. Trimethylsilylphospholane 1 was employed as a key intermediate to provide access to a large variety of ligands. 

One of the earliest and most important discoveries in metal-catalyzed asymmetric synthesis is Sharpless s Ti-catalyzed epoxidation of allylic alcohols. A mere mention of all the total syntheses that have used this technology would require a separate review article Here, we select Trost s masterful total synthesis of solamin (100, Scheme 14), for its beautiful and multiple use of Sharpless s asymmetric epoxidation.1161 Optically pure epoxy alcohol 95 is converted to both epoxy iodide 96 and diol 97 The latter two intermediates are then united to give 98, which is oxidized and converted to dihydrofuran 99 by a Ramberg-Backlund transformation. The Re catalyzed butenolide annulation that is used to afford the requisite unsaturated lactone only adds to the efficiency of this beautiful total synthesis. 

This chapter is concerned with reactions that introduce or replace substituent groups on aromatic rings. The most important group of reactions is electrophilic aromatic substitution. The mechanism of electrophile aromatic substitution has been studied in great detail, and much information is available about structure-reactivity relationships. There are also important reactions which occur by nucleophilic substitution, including reactions of diazonium ion intermediates and metal-catalyzed substitution. The mechanistic aspects of these reactions were discussed in Chapter 10 of Part A. In this chapter, the synthetic aspects of aromatic substitution will be emphasized. 

Pichler and Schulz were the first to suggest that the initial step in metal-catalyzed CO/H2 reactions might be the migration of catalyst-bound hydride to CO to yield a formyl intermediate [Eq. (1)] (3). The alkyl ligand in many isolable metal carbonyl alkyls can similarly be induced to migrate to coordinated CO (4. ). Thus, Eq. (1) has some precedent in homogeneous chemistry. Other suggestions for the involvement of formyl intermediates have come from Wender (6) and Henrici-Olive and Olive (7). 

The rapid development of the chemistry of transition metal complexes containing terminal carbene (A) or carbyne (B) ligands (7) has been followed more recently by much research centered on bridged methylene compounds (C) (2). The importance of /t-methylidyne complexes, whether in recently established binuclear examples (D), the well-known trinuclear derivatives (E), or the unusual complexes (F), has also become apparent. All are based on one-carbon (C,) fragments, and considerable interest is centered on their possible significance as models for intermediates in surface-catalyzed reactions between carbon monoxide and hydrogen (Fischer -Tropsch reactions) and related processes. These topics have been extensively ... 

The modified Chalk-Harrod mechanism was suggested by several researchers on the basis of identification of intermediates in hydrosilations catalyzed by metals such as Fe, Co, Rh, and Pd. Alternative mechanisms were also proposed by several researchers on the basis of their studies with their own catalytic systems such as Seitz-Wrighton mechanism... 

The use of C-H bonds is obviously one of the simplest and most straightforward methods in organic synthesis. From the synthetic point of view, the alkylation, alkenylation, arylation, and silylation of C-H bonds are regarded as practical tools since these reactions exhibit high selectivity, high efficiency, and are widely applicable, all of which are essential for practical organic synthesis. The hydroacylation of olefins provides unsymmetrical ketones, which are highly versatile synthetic intermediates. Transition-metal-catalyzed aldol and Michael addition reactions of active methylene compounds are now widely used for enantioselective and di-astereoselective C-C bond formation reactions under neutral conditions. 

Palladium forms a more extensive series of 7r-allyl complexes than any other transition metal. In many cases these molecules are thermally stable, air-stable crystalline sohds. In addition to stoichiometric 7r-allyl compounds, many palladium 7r-allyls are prepared under catalytic conditions as intermediates in palladium-catalyzed organic synthetic reactions. 

Key questions regarding the reactivity of silylene complexes concern their potential role in metal-catalyzed transformations. For the participation of intermediate silylene complexes in a catalytic cycle, low-energy chemical pathways must exist for the conversion of simple silanes to silylene ligands via activation processes at the metal center. Most probably, a key step in such silylene-forming processes would be the a-migration of a group from silicon to the metal. In search of such a reaction, we prepared the methyl silyl complex shown in Eq. 3. This complex is quite... 

M(C0)5X , and H2 in the presence of a general base to provide anionic metal hydrides. This process was shown to be first-order in both metal complex and dihydrogen and was not inhibited by addition of carbon monoxide. Consistent with the rds in catalysis being formation of the metal hydride intermediate, the metal catalyzed reaction of RX/CO2/H2 to provide HCOOR is not inhibited by CO. The well-established formation of metalloformate, M(C0)s02CH", from M(C0)5H and CO2 is followed by a less facile process involving the reaction of the metalloformate with RX. This latter reaction is first-order in both metal complex and alkyl halide and is inhibited by carbon monoxide. 

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