Arylboron compounds have intriguing properties and are important building blocks for chemical synthesis. A family of Ir catalysts now enables the direct synthesis of arylboron compounds from aromatic hydrocarbons and boranes under "solventless" conditions. The Ir catalysts are highly selective for C-H activation and do not interfere with subsequent in situ transformations, including Pd-mediated cross-couplings with aryl halides. By virtue of their favorable activities and exceptional selectivities, these Ir catalysts impart the synthetic versatility of arylboron reagents to C-H bonds in aromatic and heteroaromatic hydrocarbons.

[reaction: see text]. A protocol for performing Rh catalyzed aromatic borylations in cyclohexane has been devised. Borylation at the 5-position of several 1,3-substituted aromatic species ranging from electron-rich (1,3-(NMe(2))(2)C(6)H(4)) to electron-deficient (1,3-(CF(3))(2)C(6)H(4)) yields the corresponding aryl boronate esters. Veratrole was selectively borylated at the 4-position, thus extending regioselectivity to 1,2-substituted benzenes. Selective borylation at the 3-position of an N-protected pyrrole has also been demonstrated, providing a valuable reagent for cross-coupling reactions in a single step.

A mixture of endo- and exo-Cp2TaH(CH2CHMe) (1a) and catecholborane (HBCat, Cat = 1,2-O2C6H4) reacted to give endo-Cp2TaH2(BCat) (2a) and n-PrBCat as the major products. Small quantities of exo-2a are also formed during the reaction. When the reaction was monitored by 1H NMR, the resonances for endo-1a were diminished relative to exo-1a, and eventually all of the olefin complex was consumed. The analogous reaction employing DBCat led to deuterium incorporation at the α-methylene position of n-PrBCat and the deuteride positions of 2a. The alkylborane and deuteride resonances in the 2H NMR spectrum integrated in a 40:60 ratio. 1H NMR spectra indicate the α-methylene integration in n-PrBCat-d 0 - 1 is depleted by 50% of its normal value. A mechanism involving borane attack on a propylidene hydride intermediate is invoked to account for the labeling results. A mixture of endo- and exo-Cp2NbH(CH2CHMe) (1b) reacts with HBCat to generate n-PrBCat, propane, propylene, Cp2NbH2BCat (2b), and Cp2NbH(BCat)2 (3). The Markovnikov hydroboration product, i-PrBCat was not detected. Cp2NbH(BCat)2 was isolated as lemon-yellow crystals in 21% yield by fractional crystallization from toluene. 1H NMR indicates inequivalent boryl environments in compound 3, and two distinct boron resonances at δ 65 (Δν 1/2 = 250 Hz) and δ 60 (Δν 1/2 = 210 Hz) were resolved in the 11B NMR spectrum (C6D6, 60 °C). 1H{11B} spectra and isotopic labeling experiments indicated coupling between the niobium hydride and the 11B resonance at δ 60. Reaction of 1b with DBCat gave 2b-d 2, 3-d, propane-d 0 - 2 and n-PrBCat-d 0 - 2. The deuteride resonance in 3-d is shifted to higher field by 180 ppb relative to the hydride shift in compound 3. The chemical shift of the hydride resonance in compound 3 was temperature independent between −80 and 25 °C (THF-d 8).Compound 3 was crystallized as a yellow acetone solvate, and its molecular structure was determined. The Nb center lies on a C 2 axis, and the chemically inequivalent boryl groups are symmetry related. An Nb−B distance of 2.29(1) Å was found for compound 3, and the hydride position could not be reliably located. At low temperature the reaction between exo- and endo-1b with HBCat generates a persistent intermediate, 4, as the major Cp-containing component. 1H NMR spectra indicated two new hydride resonances reaction δ −4.40 and δ −6.00, and 1H{11B} spectra demonstrated that the resonance at δ −6.00 is coupled to boron. A NOESY spectrum revealed a cross-peak between the two hydride positions of intermediate 4. Generation of 4-d 1 from DBCat and exo- and endo-1b proved that the hydride resonance at δ −6.00 arises from the borane. The deuteride resonance in 4-d is shifted to higher field by 210 ppb relative to the hydride shift in compound 4. A modest temperature dependence for the hydride chemical shifts was observed between −50 and 50 °C (toluene-d 8). Intermediate 4 isomerizes to endo-2b, and reacts with CO (100 psi at 25 °C) to give the carbonyl compound, Cp2NbH(CO), and HBCat. Small quantities of intermediate...