Pyrazolyl-derived ligands

ASYMMETRIC COPPER AND COPPER(II) CATALYZED REACTIONS 9. Pyrazolyl-Derived Ligands... 

Tolman and co-workers (67) investigated a series of pyrazolyl-derived ligands for this reaction. Initial investigations centered on the use of tris(pyrazolyl) phosphine oxide (95) as a ligand with chirality derived from camphor. Diastereoselectivities with ethyl diazoacetate are poor, slightly favoring the cis isomer, and enantioselectivities are modest, Eq. 50. The BHT esters greatly increase the diastereoselectivity of this process (96 4) at the expense of enantioselectivity (10% ee for trans isomer). 

On a similar note, palladium complexes bearing bis-pyrazolyl-derived ligands with pendant imidazolyl-substituents have been used in ionic liquids, and could be recycled up to six times at an initial catalyst loading of lmol% [76]. 

Poly(pyrazolyl)borato ligands offer a variety of coordination modes according to their degree of substitution. Bis and tris(pyrazolyl)hydro-borato derivatives tend to adopt bidentate and tridentate coordination modes almost exclusively (5), while the tetrakis(pyrazolyl)borato ligand... 

Bis and tris(pyrazolyl)hydroborato ligands are generally prepared as the potassium derivatives by the direct reaction of KBH4 with the appropriate pyrazole (RR pzH), as illustrated in Scheme 1 for the parent system (13). The reaction is extremely general and has been applied to a number of different derivatives, which have included the incorporation of chiral (14, 15) and tethered (16) substituents. However, if the alkyl substituents of the pyrazole are inequivalent (i.e., R + R ), a potential problem may be encountered with the formation of isomeric products, in which either of the R and R substituents may occupy the 3-position of the poly(pyrazolyl)borato ligand. Nevertheless, if the difference in steric demands of R and R is large, there is a strong... 

In contrast to the chemistry of the zinc complexes [TpRR]ZnR, the cadmium alkyl derivatives have not been used to prepare an extensive series of half-sandwich [TpRR]CdX complexes. However, several [Tp JCdX complexes supported by sterically demanding tris(pyrazolyl)hydroborato ligands, e.g., [TpBut]CdCl (98), [TpBut]CdI (91), [Tp lCdl (91), and [TpBut,Me]Cd(Tj2-02N0) (91), have been synthesized by metathesis between CdX2 and either K[TpRR ] or Tl[TpRR ] [Eq. (15)]. 

In addition to tin alkyl derivatives with one poly(pyrazolyl)borato ligand, bis[(poIy)pyrazolylborato] complexes of the types [Tp]2SnR2 and [R2Bp]2SnR2 have also been prepared (123,124). 

Finally, the tris(pyrazolyl)hydroborato ligand system has also provided an interesting example in which a crystallographic site is disordered between a vacancy and a chain of three atoms. Thus, the x-ray structure of the cobalt complex [TpAnt]CoNCS (Ant = 9-anthryl) revealed the presence of the cocrystallized thallium derivative... 

Recently, a somewhat different synthetic approach has been reported. Halcrow et al. (215) synthesized a series of five-coordinate copper(II) complexes comprising a tridentate tris(pyrazolyl)borate ligand and a bidentate phenol derivative. Neutral complexes [Cun(TpPh)(bidentate phenolate)] were synthesized and structurally characterized [Tpph] = hydrido-tris(3-phenylpyrazol-l-yl)borate. The species [Cun(TpPh)(2-hydroxy-5-methyl-3-methylsulfanylbenzaldehydato)] can electro-chemically be converted to the (phenoxyl)copper(II) monocation, which has been characterized in solution by UV-vis spectroscopy. It displays two intense absorption maxima at 907 nm (e = 1.2 x 103 M 1 cm-1), and 1037 (1.1 x 103 M l cm-1), resembling in this respect the radical cofactor in GO (Fig. 7). 

Rhenium(V) 4-tolylimido complexes with the hydrotris(pyrazolyl)borate ligand, HB(pz)3, are formed on heating the 0x0 derivatives [ReOX2 HB(pz)3 ] (X = Cl, I) with 4-toluidine, whereas reactions with amines at ambient temperatures only give 0x0 complexes of the composition [ReO(NHR)Cl HB(pz)3 ]. The [Re(NC6H4-4-Me)X2 HB(pz)3] complexes react with ZnEt2 with formation of imido-ethyl complexes of the composition [Re(NC6H4-4-Me)Cl(Et) HB(pz)3 ]. " ... 

Another zinc fluoride has been prepared incorporating a pyrazolyl borate ligand. Reaction of [KHB(3-R-5-methylpyrazol-l-yl)] (R=/ -tol) with [Zn(OAc)2.2H2OJ leads to the production of [TpR MeZnOAc], which is transformed to the fluoride derivative upon addition of KF [99], This compound has been structurally characterised. Use of [Zn(C104)2] as the starting material in combination with [KHB(3- Bu-5-methylpyrazol-l-yl)], followed by addition of KF leads to [TptBu,MeZnF], Further reaction of this compound at the fluoride anion leads to a variety of species. Abstraction of F by BF3.solv. (solv. = py, Et20) leads to [TpZn(solv.)][BF4], whilst use of [HSiEt3] leads to formation of the hydride species,... 

In some instances complexes were obtained derived from poly(l-pyrazolyl)borate ligands which never existed by themselves. This was the case with halogenation of the 4-position in HB[pz-3,5-(CH3)2]3Mo(CO)2NO to produce derivatives of the type HB[pz-3,5-(CH3)2-4-X]3Mo(NO)X2 and in HB[pz-3,5-(CH3y3Re(CO)3 which, on bromination, yielded the species HB[pz-3,5-(CH3)2-4-Br]3Re(CO)3 Another example is the formation of 20 by the reaction depicted in Eq. (7). 

A recent report has shown that tris(pyrazolyl)borate (Tp) (see Tris(pyrazolyl)borates) complexes of Ni tend to form 5-coordinate, formally 18-electron complexes. The hapticity of the tris(pyrazolyl)borate ligand in these complexes appears to be more sensitive to the steric environment around the Ni center as opposed to the electronic factors (i.e. formal electron count of the complex). Thus, the -allyl derivative of tris (3,5-i-Pr2-pyrazolyl)borate NiR (3) displays a -coordination, whereas the -prenyl derivative (4) adopts a /c -coordination... 

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