Cleaving transition-metal catalyzed

Organobismuth compounds would have high potential as reagents for transition-metal catalyzed carbon-carbon bond forming reactions, because bismuth-carbon bonds are weak and can be easily cleaved by transition metals. The mean bond... 

The transition metal-catalyzed coupling reaction that forms and cleaves the bonds of two organic molecules occurs by a sequence of oxidative addition-transmetalation (alkylation)-reductive elimination (Fig. 1) [1,7d, 32d-f,41]. 

Enantiomerically pure phosphines are frequently employed as ligands in transition-metal-catalyzed asymmetric reactions. For this reason, various methods have been designed for their preparation.1 Many of them involve the use of borane adducts of trivalent phosphorus compounds,2 in which the borane moiety mainly acts as a protecting group. These Lewis adducts are easily prepared and stable to air, and several methods have been designed to cleave them. 

Allylic malonate 100 completely isomerizes to the thermodynamically favored linear isomer 101 on treatment with a palladium catalyst [119]. Formation of a stabilized carbanion and Jt-(allyl)palladium species facilitates the C-C bond cleavage. Analogous isomerization is also catalyzed by a nickel complex [120]. These results demonstrate that the transition metal-catalyzed nucleophilic substitution of an allylic substrate with a carbon nucleophile is reversible, if the cleaved nucleophile is sufficiently stabilized. 

Proton donors have been shown to cleave the silicon-hydrogen bond of hydrosilanes in the presence of various catalysts (77, 219) (eq. [50]). The stereochemistry of this transition-metal-catalyzed reaction has been studied in the reaction of chiral hydrosilanes. 

The utilization of ring strain is an effective strategy for transition metal-catalyzed C—C bond cleavage. A variety of late transition metal complexes have been found to cleave the C—C bond of biphenylene to give insertion complexes and relieve ring-strain [84]. These metal complexes participate in various insertion reactions with small unsaturated molecules, such as CO, olefins, and alkynes, to give functionalized products (Scheme 11.5) [82]. 

Alkene metathesis is a transition-metal-catalyzed reaction in which alkene bonds are cleaved and redistributed to form new alkenes [1-3]. The reaction proceeds through the formal [2 + 2] cycloaddition of an alkene and a metal alkylidene to yield a metallocyclobutane intermediate (Scheme 1). The productive retrocydoad-dition of this intermediate generates a new metal alkylidene and a new alkene product. These processes are generally reversible, and the reaction is under thermodynamic control. 

More often in organometallic chemistry, the catalytic reaction occurs by a mechanism that is completely different from the mechanism of the uncatalyzed process. In this case, the reaction typically occurs by more steps, but the activation energy of each of the individual steps is lower than the activation energy of the imcatalyzed process. The overall barrier is then lower than that of the uncatalyzed reaction. A comparison of the uncatalyzed and catalyzed hydroboration of alkenes with a dialkoxyborane (ROl BH, such as cat-echolborane (see Chapter 16), illustrates this scenario. Qualitative reaction coordinates for tihe uncatalyzed and rhodium-catalyzed process are shown in Figure 14.4. In the absence of a catalyst, the B-H bond adds across the alkene through a concerted four-center transition state, albeit at elevated temperatures in neat alkene. hi contrast, late transition metal-catalyzed hydroborations first cleave the B-H bond by oxidative addition. Coordination... 

Over the last few decades, the olefin metathesis reaction has become a very important reaction in organic synthesis and polymer synthesis. It involves the transition metal-catalyzed redistribution of carbon-carbon double bonds. It can be understood as a reaction, in which the a- and n-bonds of the C=C units are cleaved, and double bonds are reformed with the alkylidene groups exchanged (Scheme 20.1). 

Such reactions are known to occur at about 2S0°C for monoaminosilanes reacted with a second type of amine [IS]. Amines incorporated in a polysilazane network require higher temperatures for the transamination because of steric hindrance and the necessity to cleave-rebuild simultaneously two Si-N bonds. Transition metal catalysts can reduce the temperature requirements. In early studies [16] of the transition metal-catalyzed polymmzation of silazanes, we observed the catalytic cleavage of Si-N bonds at temperatures below 1S0°C and the ability to metathesize silicon nitrogen bonds. The transamination reactivity occurs at temperatures below that of the carbonization and carbidization of the organic groups. 

Transition Metal-catalyzed Reactions. Chlorodimethylvinyl-silane has been used in the synthesis of silyl-containing Heck reaction precursors.24 Heck reaction of aryl or alkenyl iodides with dimethylvinylsilylpyridine (36) using Pd2(dba)3 and tri-2-furylphosphine (TFP) produced the coupled alkene products 37 (R = Ph, 2-py, 2-thiophene, and others) in high yields and with exclusive E selectivity due to the pyridine directing group (eq 17). The pyridine moiety was also employed as a phase tag, which enabled easy purification via acid-base extraction. The silicon linker was subsequently cleaved by H2O2 oxidation. ... 

Transition metal-catalyzed reactions proceed through multiple elementary steps in general and, consequently, the mechanisms are often complicated, especially when backbone structures are reconstructed through a sequence of cleavage and formation of C-C bonds. A step-by-step understanding of elementary steps would be valuable to understand such catalytic transformations. This chapter focuses on elementary steps during which carbon-carbon a-bonds are cleaved by means of organometallic complexes. 

Similar to VCPs, transition-metal-catalyzed reactions of methylenecyclopropanes (MCPs) have been the subject of intensive investigation since the 1970s. Both oxidative addition and P-carbon elimination achieved cleavage of the C-C bond in MCPs. Two C-C bond cleavage patterns can operate with MCPs either the proximal or the distal bond is cleaved (Figure 2.1). This makes the chemistry of MCPs a fertile area of research. Since there are excellent reviews on transition-metal-catalyzed reactions of MCPs [88], only the advances of the last decade are summarized here. 

In 1996, when transition-metal-catalyzed carbon-carbon bond cleavage was just emerging as a current topic in organic chemistry, Kondo and Mitsudo reported the first transition-metal-catalyzed retro-allylation of homoallylic alcohol to simply cleave off the allylic moiety with a ruthenium complex [7]. After 10 years of silence, transition-metal-catalyzed retro-allylation has been rapidly developing since 2006. Nowadays, catalytic retro-allylation can be regio- and stereoselective, hence being a useful tool for modern organic synthesis [8]. 

Now that many facile controlled/living radical polymerization systems have been developed for a wide range of monomers, many researchers have adopted them as a tool for preparing well-defined stmcture polymers not only in polymer chemistry ° but also in the biochemical, medical, and optoelectronic fields. Among the various radical polymerization systems, the transition metal-catalyzed atom transfer process is one of the most promising processes in terms of controllability, facility, and versatility. In this reaction, one polymer chain forms per molecule of organic halide as an initiator, while a catalytic amount of the metal complex serves as an activator, which would homolytically cleave the carbon-halogen terminus (Scheme 1). 

The oxidative cleavage of C=C bond is a common type of reaction encountered in organic synthesis and has played a historical role in the structural elucidation of organic compounds. There are two main conventional methods to oxidatively cleave a C=C bond (1) via ozonol-ysis and (2) via oxidation with high-valent transition-metal oxidizing reagents. A more recent method developed is via the osmium oxide catalyzed periodate oxidative cleavage of alkenes. All these methods can occur under aqueous conditions. 

Transition metals such as iron can catalyze oxidation reactions in aqueous solution, which are known to cause modification of amino acid side chains and damage to polypeptide backbones (see Chapter 1, Section 1.1 Halliwell and Gutteridge, 1984 Kim et al., 1985 Tabor and Richardson, 1987). These reactions can oxidize thiols, create aldehydes and other carbonyls on certain amino acids, and even cleave peptide bonds. The purposeful use of metal-catalyzed oxidation in the study of protein interactions has been done to map interaction surfaces or identify which regions of biomolecules are in contact during specific affinity binding events. 

In this reaction, one polymer chain forms per molecule of the organic halide (initiator), while the metal complex serves as a catalyst or as an activator, which catalytically activates, or homolytically cleaves, the carbon—halogen terminal. Therefore, the initiating systems for the metal-catalyzed living radical polymerization consist of an initiator and a metal catalyst. The effective metal complexes include various late transition metals such as ruthenium, copper, iron, nickel, etc., while the initiators are haloesters, (haloalkyl)benzenes, sulfonyl halides, etc. (see below). They can control the polymerizations of various monomers including methacrylates, acrylates, styrenes, etc., most of which are radically polymerizable conjugated monomers. More detailed discussion will be found in the following sections of this paper for the scope and criteria of these components (initiators, metal catalysts, monomers, etc.). 

The palladium-catalyzed acylation of siloxycyclopropane furnishes a 1,4-di-carbonyl compound. A C-C bond of the three-membered ring is cleaved by an electrophilic attack of a palladium(II) species [103]. An analogous electrophilic ring opening of siloxycyclopropane was induced by various so-called ligand free transition metals such as Ag+ and Cu2+ [104]. 

The bond between the carbon atoms a and (3 to a C-C double bond can be broken by a transition metal with formation of a Jt-allyl intermediate providing the driving force. Whereas stoichiometric reactions of this sort are yet to appear, jt-(allyl)metal intermediates are occasionally involved in catalytic C-C bond cleaving reactions. The nickel catalyzed skeletal rearrangement of 1,4-dienes involves the formation of an olefin coordinated Jt-(allyl)nickel complex (99) [118]. 

---