The catalytic functionalization of unreactive C À H bonds remains one of the major challenges in chemistry. [1] Metal ± boryl systems have been shown to promote stoichiometric [2] and photocatalytic [3] conversion of alkane and arene CÀH bonds into CÀB(OR) 2 groups by using B 2 pin 2 (pin OC-Me 2 CMe 2 O). A [Cp*Ir] complex catalyzes [4] the borylation of benzene with pinacolborane (HBpin), with about three turnovers obtained at 150 8C, and a related complex [Cp*Rh(h 6 -C 6 Me 6 )] is an excellent catalyst [5] for aliphatic and aromatic CÀH bond borylation at 150 8C, with B 2 pin 2 being more effective than HBpin. As part of our studies of the reactivity of BÀH bonds with Rh and Ir complexes, [6] we report herein that [RhCl(PiPr 3 ) 2 (N 2 )] (1) [7] is an efficient catalyst precursor for the borylation of aromatic and benzylic CÀH bonds with HBpin, and that high selectivity for benzylic C À H functionalization was observed for the first time with toluene, p-xylene, and mesitylene. The boronate ester products represent an important class of synthetic intermediates. [8] The reaction of 0.2 m HBpin [9] with benzene in the presence of 1.0 mol % of 1 at 140 8C gave PhBpin in 62 % yield (62 turnovers) after 14 h and in 86 % yield after 58 h (Scheme 1). When 0.3 mol % of 1 was used, 67 % (222 turnovers) of PhBpin was obtained after 104 h. The yield improved at lower HBpin concentrations. Thus, with 1.0 m HBpin in benzene, PhBpin was obtained in 20 % yield after 58 h, whereas dilution with hexane (fourfold) improved the yield to 35 % in the same time. This effect can be attributed to degradation of HBpin, which is strongly concentration dependent. [10] In addition to PhBpin, the dehydrodimerization product B 2 pin 2 was formed, especially with higher concentrations of HBpin (ca. 7 % for 1.0 m HBpin in benzene).
Toluene was less active than benzene, indicating that the methyl group deactivates the arene CÀH bonds (Table 1, entry 1). Interestingly, the main product, PhCH 2 Bpin, was not
Experimental Section
Compound 1 was synthesized by using mixture of DMF and water as the solvent, and TREN as a structure-directing agent. A reaction mixture containing GeO 2 (0.413 g, 3.95 mmol), TREN (2.35 g, 16.1 mmol), water (5.01 g, 278 mmol), DMF (8.01 g, 110 mmol) was stirred at room temperature over night. The mixture was heated at 180 8C in a Teflon-lined steel autoclave for seven days. Colorless needlelike crystals were obtained in approximately 70 % yield (based on GeO 2 ). X-ray structure analysis: Needlelike single crystals of FDU-4 (0.20 Â 0.03 Â 0.04 mm) were analyzed at room temperature. Crystallographic data for FDU-4: hexagonal, space group P6 3 cm (No. 185), a 23.941(3), b 23.941(3), c 9.798(2) , V 4863.2( 14) 3 ; Z 6; 1 calcd 2.399 g cm À3 , F(000) 3360, l(Mo Ka ) 0.71073 . A full hemisphere of diffracted intensities was measured with graphite-monochromated Mo ka radiation on a Bruker/Siemens Smart1000-CCD. Of 22 955 reflections collected, 4681 were independent (R int 0.1347). Cell constants were obtained from least-squares refinement in the range 0.98 q 23.348. The Siemens/Bruker program SHELXTL-PC software package was used to solve the structure by direct methods. Refinement was by full-matrix leastsquares analysis with anisotropic thermal parameters for all non-hydrogen atoms. The final residuals were R 1 (F) 0.0606 with I b 2s and wR 2 (F 2 ) 0.1551 with GOF(F) 1.050. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-151489. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK