Group 4 boron-bridged

Group 4 Boron-Bridged flni fl-Metallocene Complexes. 136... 

Even after the boron s electrophilicity has been arrested intermolecularly, intramolecular reorganization of the boraethene may still proceed. A remarkable instance is the structure 60 (by XRD) resulting from the oxidation of 57 by SnCl2. Intermediates in this impressive transformation might well be carbene 58 and boraethene 59. The borons in 59 are still sufficiently electrophilic to cause the mesityl group to bridge (Scheme l).61... 

More recently, imidazole- and pyrazole-type ligands with a phosphorus atom as the bridge have been reported,97,98 as depicted in formulae (46)-(48). Also rings with imidazolines (49)," or bridged by a —C—OH group (SO),100 have been reported. Boron-bridged polypyrazolylborates and related systems are dealt with in Chapter 13.6. 

For zirconia with hydroxyl group of bridge type and anion boron, orbital connected with boron appear in the forbidden gap. Thus, zirconia with anion boron and hydrogen on surface can be acceptor or deep donor. 

The ansa-bridged group 4 titanium derivatives containing boron bridges have been reviewed.394,678,1729 The synthesis of the PhB-bridged homobimetallic titanium compound (Scheme 683) has been described using the dehalosilylation method.1729... 

The catalytic cycle for all these reactions is (i) oxidative addition, (ii) transmetallation and (iii) reductive elimination to regenerate Pd(0). However, the reaction conditions required can vary dramatically for different metals, as can compatibility with functional groups. Boron and silicon reagents require the presence of a base and the transmetaUations take place via anionic intermediates, while tin, zinc and magnesium transmetallations probably go via neutral halogen-bridged species. 

Boron-bridged group-4 ansa-metallocene complexes 01EJI321. 

The Pfaltz group developed a chiral boron-bridged bisoxazoline ligand (5,5)-L36 and tested its Cu complex in the Kharasch-Sosnovsky reaction of cyclic olefins. The enantioselectivity of the reaction (79% ee) was comparable with the previously reported best ones obtained using the BOX ligands. ... 

A boron bridge offers more functionality than silicon or carbon bridges to the chemistry of group 4 ansa-metallocene complexes because it can reversibly bind a variety of Lewis basic moieties that are designed to influence the electronic and stereochemical properties of the complex. By coordinating an anionic Lewis base, the boron can potentially serve as an internal, counteranion to the catalytically active, cationic group 4 metal alkyl, as in the hypothetical zwitterionic complex shown in Figure 5.1. The polymerization activity of the zwitterionic catalyst should benefit from the remote location of the counteranion from the active site, where it will not compete with the alkene monomer for a coordination site on the metal. 

Shapiro, P. J. Boron-bridged group 4 aw a-metallocene complexes. Eur. J. Inorg. Chem. 2001, 321-326. 

Considering each boron-terminal hydrogen group p to supply a pair of electrons for the other bonds in the structure, there are p pairs of electrons available. One pair is necessary for each boron-boron-boron, three-center bond t and for each boron-boron, two-center bond y. One electron (half a pair) is necessary for each boron-hydrogen-boron, bridge bond 5- and for each extra terminal hydrogen x beyond the one already assumed on each boron. 

Boron-Bridged Tri- and Tetra-Nuclear Iron Carbonyls Containing Groups... 

Boron-Bridged Trinuclear Iron Carbonyls Containing RBH3 x Groups... 

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