Catalytic o-bond metathesis

This book contains four chapters in which part of the recent development of the use of molecular rare-earth metal compounds in catalysis is covered. To keep the book within the given page limit, not all aspects could be reviewed in detail. For example, the use of molecular rare-earth metal complexes as Lewis acidic catalysts is not discussed in this book. The first two chapters review different catalytic conversions, namely the catalytic o-bond metathesis (Chapter by Reznichenko and Hultzsch) and the polymerization of 1,3-conjugated dienes (Chapter by Zhang et al.). Within these chapters, different catalytic systems and applications are discussed. The final two chapters are more concentrated on recent developments of... 

Some of these intermediates are analogous to those proposed by Chauvin in olefin metathesis ( Chauvin s mechanism ) [36]. They can be transformed into new olefins and new carbene-hydrides. The subsequent step of the catalytic cycle is then hydride reinsertion into the carbene as well as olefin hydrogenation. The final alkane liberation proceeds via a cleavage of the Ta-alkyl compounds by hydrogen, a process already observed in the hydrogenolysis [10] or possibly via a displacement by the entering alkane by o-bond metathesis [11]. Notably, the catalyst has a triple functionality (i) C-H bond activation to produce a metallo-carbene and an olefin, (ii) olefin metathesis and (iii) hydrogenolysis of the metal-alkyl. 

SCHEME 18.4 Mechanism suggested by Don Tilley et al. for the dehydrocoupling of RSiHj using CpCp HfHCl (Cp = T15-C5H5 Cp = Ti -CjMej ) as a catalyst. The catalytic cycle involves two o-bond metathesis reactions that pass through four-center transition states. " ... 

The protonation of organo-rare-earth metal species through a-bond metathesis plays a key role in many catalytic applications described below. The high reactivity of rare-earth metals for insertion of unsaturated carbon-carbon multiple bonds [18], in conjunction with smooth o-bond metathesis, allows to perform catalytic small molecule synthesis. This route is atom efficient, economic, and opens access to nitrogen-, phosphorous-, silicon-, boron-, and other heteroatom-containing molecules. The most important catalytic applications of organo-rare-earth metals involving the o-bond metathesis process will be discussed in this review. 

Catalytic applications of organo-rare-earth metal complexes reported prior to 2002 are summarized in two excellent reviews [19,20] and, therefore, will not be discussed unless being relevant for understanding of key reaction details. A recent comprehensive review on theoretical analyses of organo-rare-earth metal-mediated catalytic reactions is available [17], Although o-bond metathesis plays a pivotal role in many rare-earth metal-catalyzed polymerizations, the discussion of these processes is beyond the scope of this review and the interested reader may consult one of the pertaining reviews [21-24],... 

The mechanism of catalytic hydrosilylation (Fig. 5) is analogous to that of hydrogenation. Key steps are alkene insertion and o-bond metathesis with alkene insertion apparently being the product-determining step [29,34,35]. 

The mechanism and scope of rare-earth metal-catalyzed intramolecular hydrophosphination has been studied in detail by Marks and coworkers [147,178-181]. The hydrophosphination of phosphinoalkenes is believed to proceed through a mechanism analogous to that of hydroamination. The rate-determining alkene insertion into the Ln-P bond is nearly thermoneutral, while the faster protolytic o-bond metathesis step is exothermic (Fig. 22) [179,181]. The experimental observation of a first-order rate dependence on catalyst concentration and zero-order rate dependence on substrate concentration are supportive of this mechanism. A notable feature is a significant product inhibition observed after the first half-life of the reaction. This is apparently caused by a competitive binding of a cyclic phosphine to the metal center that impedes coordination of the phosphinoalkene substrate and, therefore, diminishes catalytic performance [179]. 

Periana et al. have reported a mercury system that catalyzes the partial oxidation of methane to methanol.81 Hg(II) is typically considered to be a soft electrophile and is known to initiate electrophilic substitution of protons from aromatic substrates. The catalytic reaction employs mercuric triflate in sulfuric acid, and a key step in the catalytic cycle is Hg(II)-mediated methane C—H activation. For methane C—H activation by Hg(II), an oxidative addition reaction pathway via the formation of Hg(IV) is unlikely. Thus, an electrophilic substitution pathway has been proposed, although differentiation between proton transfer to an uncoordinated anion versus intramolecular proton transfer to a coordinated anion (i.e., o-bond metathesis) has not been established. Hg(II)-based methane C H activation was confirmed by the observation of H/D exchange between CH4 and D2S04 (Equation 11.9). 

Cyclohexane formation is entropically less favorable than cyclopentane generation, and treatment of 1,6-dienes under the conditions listed in Eq. (47) leads to the production of uncyclized, disilylated products (Eq.49) [35] or silicon bridged dimers [40]. To avoid these problems, phenylmethylsilane can be employed as the chain terminator. Utilizing this more hindered silane slows the o-bond metathesis sufficiently to prevent dimerization (Eq. 50). The trapping step can be retarded even further by the use of diphenylsilane (Eq. 51). Thus, not only can the metal and the ligand array be manipulated to bring about the desired result in the catalytic process, but the properties of the silane reagent itself can also be adjusted to meet the demands of the synthesis at hand. 

Hu and Yu reported an iron/macrocyclic polyamine-catalyzed reaction of arylboronic acids with a large excess of pyrrole or pyridine at 130°C tmder air (Eqs. 26 and 27) [63], based on their previous studies on iron-mediated reactions (initial report using a stoichiometric amount of iron [64]). Pyrrole derivatives were arylated at 2-position in good yield (Eq. 26), but when pyridine was used as a substrate, the catalyst turnover was poor and 2-arylpyridine was obtained together with a small amount of 3-aryl- and 4-arylpyridine (Eq. 27). Because a catalytic amotmt of a radical scavenger did not inhibit the reaction, the authors proposed an oxoiron complex as the active species to activate the ort/io-hydrogen of the heterocycle via o-bond metathesis and also performed a DPT analysis of the mechanism. A related iron-catalyzed reaction of aryl boronic acids with heteroarenes was reported by Singh and Vishwakarma [65]. 

The first mechanism to be widely accepted was proposed by TiUey et al., which is called a o-bond metathesis (Fig. 17) [75]. In this model a metallocene precursor is first transformed to a metallocene hydride that enters the hypothetic catalytic cycle, forming a metal silyl species via an initial a-bond metathetic step with the simultaneous production of hydrogen. The second o-bond metathetic step forms the Si-Si bond and is supposed to regenerate the catalytically active metal hydride. 

Based on this reactivity, the reaction has been proposed to proceed via the insertion of the carbodiimide into the M-N IxMid of a transient amide complex formed from o-bond metathesis of the precatalyst with the aniline. Turnover occurs via a similar o-bond metathesis of the guanidinate complex with a further equivalent of amine, a reaction that is most likely endothermic based upon pA"a considerations but occurs readily under catalytic conditions [100]. The potential for reversible insertion of the carbodiimide into M-N bonds has been probed by crossover experiments and studies on the current systems do not suggest a reversible insertion step (although this is likely to be a function of the stabihty of the metal-amide generated from carbodiimide extrusion). The molecularity or kinetic competence of... 

---