Boron betaine

Zirconocene Boron-betaine. An interesting concept that can be applied to the synthesis of single-component metallocene Ziegler catalyst consists to covalently bind an anioiiic counterpart to the cationic framework. The counterion can be attached either to an alkyl group linked to the metal or to one of the Cp ring. 

Most of zirconocene boron-betaines are single-component mediators of olefin polymerization and at least rival conventional catalysts with respect to productivity. 

Acyclic boryloxyalkylphosphines with tricoordinated phosphorus and boron are capable of forming cyclic betaine structures with four-coordinated P and B atoms. The ability to be converted into a more stable four-coordinated state accounts for many chemical transformations of boryloxyalkylphosphines. Diphenylboryloxymethyl(methyl) phenylphosphine (92) readily disproportionates to give 1,3,2,5-dioxabora-taphosphoniarinane (103). [Eq. (60)] (83IZV2541). Similar interaction is... 

Betaines may be considered to be the intermediate products in the displacement of the C—O fragment from the P—C—O—B system, whereas the addition of aldehydes to the P—C—O—B system constitutes the first stage. This reaction is due to the fact that phosphorus and boron atoms can change their coordination reversibly and convert into the tetra-coordinated state. The displacement of one aldehyde by another is carried out in a solvent or in excess aldehyde. In general this reaction is represented by the following scheme [Eq. (107)]. 

The C—O fragments separating phosphorus and boron atoms in dioxa-borataphosphoniarinanes are substituted when treated by aldehydes or nitriles. In this case the displacement of aldehyde fragments occurs through the initial dissociation of betaines into compounds with Uncoordinated phosphorus and boron atoms [Eq. (109)] (86IZV2502, 86IZV2510). 

The disulfide fragment separating phosphorus and boron atoms was not replaced in 181 by chloral even after refluxing in benzene, evidence for high betaine stability. In methylene chloride, 175 reacts with 1,2-naphthoquinone, yielding phosphorane 182 [Eq. (135)]. This result is surprising, as one could have expected the formation of a betaine structure. 

Diketones are produced from nitroalkenes and the lithium enolates of ketones. Equation 132 shows the reaction of the enolate of 2-hexanone with 2-nitropropene in the presence of acetic anhydride. The resulting betaine 409, a greenish-blue liquid, is hydrolysed to the diketone by successive treatment with boron trifluoride and water441. 

Efforts to add Lewis acids to dialkyliminoboranes R BNR were not so successful, aa would be expected, since betaine structures of the type R B=NR—A with an unfavorable linear sextet boron atom would be formed (19, 33). Equation (26) is restricted to iminoboranes XBNR with a 7i-electron donating group X. 

More recently, a novel metal-substituted methylenecyclopropene (triafulvene) derivative was obtained when bis(propyne)zirconocene was treated with one equivalent of tris(pentafluorophenyl)borane, followed by excess of benzonitrile (equation 367)430. The first step involves alkynyl ligand coupling to give the isolable Cp2Zr(//-2,4-hexadiyne)B(C6F5)3 betaine. This undergoes a formal intramolecular nitrile insertion into the Zr—C(sp2) c-bond of the adjacent alkenyl zirconocene unit, leading to the zirconium-boron triafulvene-betaine. X-ray analysis of the triafulvene confirmed the planar... 

Zwitterionic structures with a borate anion covalently attached to a cyclopentadienyl donor can be obtained by a number of synthetic approaches. In the known compounds, the means by which the borate is tethered to the ring has a significant impact on the properties of the zwitterions. In particular, the length of the tether connecting the ring with the borate is crucial for the stability and intramolecular ion-ion contacts. For example tris cyclopentadienyl zirconium betaine complex in which boron atom is directly bonded to Cp can be generated from boron substituted cyclopentadienyl anion (equation 21). ... 

Temme B, Erker G, Frohlich R et al (1994) Heterodimetal-Betaine chemistry catal5rtic and stoichiometric coupling of alkynyl ligands under the joint influence of zirconium and boron centers. Angew Chem Int Ed 33 1480-1482... 

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