Cleavage of polar bond

Oxidative addition of organic compounds having carhon-X bond (X = heteroatom) to a low valent transition metal complex such as Pd(0) or Rh(I) with cleavage of the C-X bond often yields reactive organotransition metal complexes. Cleavage of non-polar bonds will be discussed in Chapter 2, while cleavage of polar bonds will be dealt with in Chapter 3. 

The enzyme catalyzes the cleavage of the bond between carbon 1 of residue [4] and the oxygen atom of the glycosidic linkage of residue [5]. Two amino acid side chains in the region of this bond can serve as proton donors or acceptors Asp 52 and Glu 35, each of which is about 0.3 nm from the bond. Asp 52 is in a polar environment and is ionized at the pH optimum of lysozyme (pH 5), whereas Glu 35 is in a nonpolar region and is not ionized. The proposed catalytic mechanism is given in Fig. 8-3. 

Selenium forms weaker a-bonds than sulfur and many reactions which involve the cleavage of such bonds are faster than for analogous sulfur compounds and proceed under milder reaction conditions. The sj -elimination of selen-oxides was discovered in 1970 [9] and had a major impact on organoselenium chemistry. This reaction is about three orders of magnitude more rapid than the elimination of the corresponding less polar and less basic sulfoxides. Sigmatropic rearrangements proceed at markedly lower temperatures. These reactions are discussed in detail in Chap. 8 by Y. Nishibayashi and S. Uemura. 

The combination of hard oxophilic early transition metals and soft nucleophilic late transition metals with opposite functionalities, provided they do not inhibit one another, is a priori ideal for promoting cooperative effect. A proof of concept can be found in the stoichiometric reactivity of early—late heterobimetallic complexes featuring a metal-metal bond [76]. It has been shown that such complexes are good candidates to realize the heterolytic cleavage of a bond in polar and apolar substrates. An illustrative example by Bergman et al. is the reaction of the Zr-lr complex 20 with carbon dioxide which leads to the rupture of the metal—metal bond (Scheme 18) [77]. The CO2 insertion occurs in the expected fashion with the CO2 bridging the two metals, the carbon atom coordinated to iridium, and the oxygen atom on the zirconium center. 

These disproportionation reactions are examples of photoinduced electron transfer reactions. Early mechanistic considerations were based on the concept that polar solvent molecules induced a dipole in the metal-metal bond, " and that excitation of this dipolar species led to the heterolytic cleavage of this bond. Subsequently, however, it has been shown that these reactions occur in nonpolar solvents, thereby reducing credence that solvent polarity is the only contributing factor that affects the mechanism, and establishing the importance of the incoming ligand. ... 

The term Al contributes to Af during the decomposition of peroxide compounds with the cleavage of two bonds when the polar transition state is formed... 

Burck S, Gudat D, Nieger M (2009) Cleavage of polarized P—P bonds in n-heteroeyelic diphosphines in reactions with metal olefin complexes. Organometallies 28 1447... 

With electrons flowing from ethylene to zirconium the Zr—CH3 bond weakens the carbons of ethylene become positively polarized and the methyl group migrates from zirconium to one of the carbons of ethylene Cleavage of the Zr—CH3 bond is accom panied by formation of a ct bond between zirconium and one of the carbons of ethylene m Step 3 The product of this step is a chain extended form of the active catalyst ready to accept another ethylene ligand and repeat the chain extending steps... 

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