C—Zr bond cleavage

C-Zr Bond Formation + RZrCp2Ln Interconversion + C-Zr Bond Cleavage... 

Aside from two-center (Patterns 1 and 2) and three-center (Patterns 3, 4, 11, and 12) processes, most of the processes shown in Scheme 1.3 are four-center processes involving either addition (Patterns 5—10) or 0-bond metathesis (Pattern 13). In this context, it should be noted that addition is simply a four-center metathesis in which one molecule happens to be multiply-bonded. In addition to these metathetical processes, there is yet another fundamentally important four-center metathetical process termed migratory insertion and deinsertion (Patterns 14 and 15). It should be clear from Patterns 14 and 15 shown in Scheme 1.3 that distinction between insertion and deinsertion is only a relative and semantic issue. In the current discussion, a process involving cleavage of the C—Zr bond is termed migratory insertion, while the reverse process is termed migratory deinsertion. 

Boronates are more stable since they are stabilized owing to the donor effect of oxygen lone-pairs to the empty orbital of the boron. The two different carbon-metal bonds afford particnlar reactivity. For example, addition of propargylbromide on (94) in presence of a catalytic amount of copper cyanide nndergoes a carbon-carbon bond formation with exclusive cleavage of the C-Zr bond. The subsequent borylallene, by treatment with a ,/3-unsaturated aldehydes, affords two trienes, depending on the reaction conditions (Scheme 20). 

The selective cleavage of C-Zr bond of the 1,1-bimetalloalkanes with bromine results in a convenient method for the preparation of a-bromoboranes. Consequently, addition of bromine in dichloromethane in situ at -35 C results in the discharge of color and formation of white precipitate within 1 h of zir-conocene dihalide. The removal of CHjClj, followed by extraction with hexane of reaction mixture provides the a-bromoborane in high yield (Table 19.1) [6]. 

Cleavage of Zr—C a bonds occurs readily on treatment with H20 or dilute acids, while the Zr—Cp bond usually survives mild protonolysis conditions. The use of D20 or DC1/D20 permits the replacement of Zr with D. Deuterolysis provides a generally reliable method for establishing the presence of Zr—C bonds. Protonolysis or deuterolysis of Zr—Csp bonds proceeds with retention of configuration [97]. In the hydrozirconation of terminal alkynes, deuterium can be introduced at any of the three positions in the vinyl group in a completely regio- and stereoselective manner, as shown in Scheme 1.18. Although relatively little is known about the mechanistic details, the experimental results appear to be consistent with concerted c-bond metathesis (Pattern 13) between C—Zr and H— X bonds. 

Other bond cleavage (0-Si [176], 0-Sn [177], 0-C [178], 0-P [179], O-Ti [176b, 180], O-Zr [181], S-C [182], Si-Si [183]) can be readily set up in radical cations for synthetic use and in several instances kinetic data are known. Rapid O-C bond cleavage might even occur in the antioxidant action of 7,8-diacetoxy-4-methylcoumarin [184]. 

DDQ, TCNQ, etc., yield cyclopropanes stereochemical studies show that the reductive elimination is either concerted with the oxidation step or that C—C coupling following the first Ti—C bond cleavage is faster than C—C bond rotations (4/). It is currently believed that these reactions proceed via outer-sphere one-electron oxidation from a metal-ligand bond. In the case of Zr, this is followed by loss of an R radical and trapping... 

Equation (8). Insertion of acetonitrile on rf-phosphabenzyne-zirconocene A selective cleavage of the C2-Zr bond was described when 100 c is submitted to react with triphenylphosphine sulfide as sulfur donor. The transient formation of a poorly soluble intermediate complex was observed with release of free triphenylphosphine. Upon hydrolysis with HC1, the thiol 102 is formed (Eq. 9) ... 

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