Aryl boryl complexes

Cross-coupling to form carbon heteroatom bonds occurs by oxidative addition of an organic halide, generation of an aryl- or vinylpalladium amido, alkoxo, tholato, phosphido, silyl, stannyl, germyl, or boryl complex, and reductive elimination (Scheme 2). The relative rates and thermodynamics of the individual steps and the precise structure of the intermediates depend on the substrate and catalyst. A full discussion of the mechanism for each type of substrate and each catalyst is beyond the scope of this review. However, a series of reviews and primary literature has begun to provide information on the overall catalytic process.18,19,22,23,77,186... 

First reported in 2002, aryl(halo)boryl complexes of iron represent versatile precursors for a number of related ligand systems via boron-centred reactions which proceed with retention of the Fe-B bond [62,88-90]. Complexes bearing differing aryl substituents have been synthesized via the general route shown in Scheme 8, allowing for varying degrees of steric bulk at the boron centre. 

Scheme 8 Synthesis of aryl(halo)boryl complexes 8.44-8.48...

Given the structural and reaction studies carried out on iron amino- and aryl(halo)boryl complexes, research effort has also been directed at analogous aryloxy(halo)boryl systems. The complexes ( 5-C5R5)Fe(CO)2B(OMes)X (R = H, X = Cl, 8.55 R = H, X = Br, 8.56 R = Me, X = Cl, 8.57) have been synthesized from the reaction of the corresponding haloborane with one equivalent of Na[( 5-C5R5)Fe(CO)2] (Fig. 22) [75]. Boryl complexes 8.55 and 8.57 both feature relatively short Fe-B distances [1.977(4) A for each] with the torsion angle (0) for 8.57 [87.6°] being consistent with the possibility for tc overlap between the boron-centred p orbital and the metal-based HOMO-2. 

An extensive array of chiral phosphine ligands has been tested for the asymmetric rhodium-catalyzed hydroboration of aryl-substituted alkenes. It is well known that cationic Rh complexes bearing chelating phosphine ligands (e.g., dppf) result in Markovnikoff addition of HBcat to vinylarenes to afford branched boryl compounds. These can then be oxidized through to the corresponding chiral alcohol (11) (Equation (5)) ... 

The direct borylation of arenes was catalyzed by iridium complexes [61-63]. Iridium complex generated from [lrCl(cod)]2 and 2,2 -bipyridine (bpy) showed the high catalytic activity of the reaction of bis (pinaco la to) diboron (B2Pin2) 138 with benzene 139 to afford phenylborane 140 (Equation 10.36) [61]. Various arenes and heteroarenes are allowed to react with B2Pin2 and pinacolborane (HBpin) in the presence of [lrCl(cod)]2/bipyridne or [lr(OMe)(cod)]2/bipyridine to produce corresponding aryl- and heteroarylboron compounds [62]. The reaction is considered to proceed via the formation of a tris(boryl)iridium(lll) species and its oxidative addition to an aromahc C—H bond. 

C-H borylation is a widely used methodology for the synthesis of organoboronates [63-65]. Most of the applications have been presented for the synthesis of aryl-boronates. However, functionalization of alkenes has also attracted much interest [66, 67]. In most applications, iridium catalysis was used. However, in case of alkenes, borohydride forms as a side product of the C-H borylation, which undergoes hydroboration with alkenes. This side reaction can be avoided using palladium catalysis under oxidative conditions. In a practically useful implementation of this reaction, pincer-complex catalysis (Ig) was appHed (Figure 4.17) [51]. The reaction can be carried out under mild reaction conditions at room temperature using the neat aUcene 34 as solvent. In this reaction, hypervalent iodine 36, the TFA analog of 29, was employed. In the absence of 36, borylation reaction did not occur. 

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