Transition metal-rhodium-boron

Figure 11.7 Dependence of the hardnesses of some transition metal-rhodium-boron perovskites on their d-electron densities.

Boronic esters have been used in a wide range of transformations. These useful reagents have been transformed into numerous functional groups and are essential reagents for several C-C bond-forming reactions. Transition metal-catalyzed hydroboration of olefins often leads to mixtures of branched and linear products. Several groups have reported asymmetric reductions of vinyl boronic esters [50-52] with chiral rhodium P,P complexes however, the first iridium-catalyzed reduction was reported by Paptchikhine et al (Scheme 10) [53]. 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

Many normal oxides are formed on burning the element in air or oxygen. This is true not only of the non-metals boron, carbon, sulphur and phosphorus, but also for the volatile zinc, cadmium, indium and thallium, the transition metals cobalt and iron, in finely divided condition, and the noble metals osmium, ruthenium and rhodium. With some elements, limiting the supply of oxygen produces the lower oxide (e,g, P40g in place of P4O40 (p. 332)). 

From a synthetic chemistry standpoint, reaction of the metaUated intermediates with electrophiles other than a proton is more attractive. Indeed, one of the most important recent developments in boronic acid chemistry strove from the discoveries that transition metals such as palladium(O), rhodium(I), and copper(I) can oxidatively insert into the B-C bond and undergo further chemistry with organic substrates. These processes are discussed in Sections 1.5.3 and 1.5.4. 

This method has not yet found widespread use for the preparation of allylboronates. In fact, uncatalyzed hydroborations of dienes tend to provide the undesired regioiso-mer with the boron atom on a terminal carbon, i.e., homoallylic boranes. By making use of certain transition metal catalysts, however, Suzuki and co-workers found that (Z)-allylic catecholboronates such as 22 can be obtained in high yield from various substituted butadienes (e.g., isoprene. Equation 11) [44]. Whereas a palladium catalyst is the preferred choice for acyclic dienes, a rhodium catalyst (Rh4(CO)i2) was best for the hydroboration of cyclohexadiene. A suitable mechanism was proposed to explain the high regioselectivity of this process. In all cases, a reaction quench with benzaldehyde afforded the expected homoallylic alcohol product from a tandem hy-droboration/allylation (Section 6.4.1.4). 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

A model proposed for rationalizing the stereochemical outcome is also shown in Scheme 5.112. It seems plausible that the skewed structure 441, typical for transition metal BINAP complexes, has just one open space that is filled by a coordination of rhodium to the carbon - carbon bond of cyclohexenone. As a consequence, the insertion of the phenyl group then occurs from the Sf-face to the enone to give the / -rhodium complex 442 that subsequently tautomerizes to the more stable oxallyl-type enolate [205a]. It seems that a transmetallation at the stage of the enolate - from rhodium to boron enolate, as indicated in cycle B (Scheme 5.111) - doesnot occur in all these rhodium-mediated domino reactions without exception. Thus, Hayashi and coworkers produced evidence in support of a rhodium enolate as an active nucleophile in the aldol step when phenyl-9-BBN 438 was reacted with acyclic enones in the presence of [Rh(OH)-(S)-BINAP]2. In this case, the catalytic cycle is maintained by a transmetallation of the rhodium to the boron aldolate [221]. 

Chelating dienes have recently emerged as a promising class of chiral ligands for transition-metal-catalyzed processes as a consequence of independent concurrent studies by Hayashi [148,149] and by Carreira [150-153]. The first example with a chiral rhodium-diene complex for conjugate addition reactions employed chiral C2-symmetric diene ligand 193 (Equation 36) [148, 149]. Addition of boronic acid 192 to cyclohexenone catalyzed by the putative complex formed in situ from 193 and Rh(I) afforded adduct 194 in 90% yield and 99% ee [148]. 

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