Letter pubs.acs.org/OrgLett
Iridium-Catalyzed ortho-Selective C−H Silylation of Aromatic Compounds Directed toward the Synthesis of π‑Conjugated Molecules with Lewis Acid−Base Interaction Takayuki Wakaki,† Motomu Kanai,*,†,‡ and Yoichiro Kuninobu*,†,‡ †
Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ERATO, Japan Science and Technology Agency (JST), Kanai Life Science Catalysis Project, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
‡
S Supporting Information *
ABSTRACT: We successfully developed an iridium-catalyzed ortho-selective C−H silylation of aromatic compounds. The reaction exhibited a wide substrate scope, and a variety of π-conjugated molecules were synthesized in good to excellent yields, even in gram scale. Several silyl groups could also be introduced into the products. The experimental results indicated that the regioselectivity could be controlled by a Lewis acid−base interaction between the Lewis acidic silicon atoms of fluorinated hydrosilanes and the Lewis basic nitrogen atoms of aromatic compounds. π-Conjugated molecules with a Lewis acid−base interaction have recently attracted attention due to the special properties of such molecules compared with π-conjugated molecules bearing only covalent bonds (Figure 1).1−4 It is expected that the Lewis acid−
controlled by the Lewis acid−base interaction between a boron atom of hydroboranes and a nitrogen atom of aromatic compounds. The Lewis acidity of a boron atom results from an unoccupied orbital of the boron atom. On the other hand, dispersion of electrons from the silicon atom to the electronwithdrawing group(s) causes the Lewis acidity of the silicon atom, and the silicon atom can thus form a five-coordinated silicon species. Therefore, hydrosilanes with electron-withdrawing group(s) are desirable candidate Lewis acidic reagents for ortho-selective C−H functionalization. We report herein iridium-catalyzed C(sp2)−H silylation of aromatic compounds at the ortho-position, in which the regioselectivity could be controlled by the Lewis acid−base interaction.5−8 In this reaction, fluorinated hydrosilanes were used as silylation reagents. First, we investigated palladium complex [Pd(NCCH3)4](BF4)2 and a Pd(OAc)2 salt as a catalyst using 2-phenylpyridine (1a) and fluorodiphenylsilane (2a) as substrates because the two above-mentioned reactions proceeded using palladium catalysts (Table 1, entries 1 and 2).1c,3g The desired ortho-C−H silylation reaction, however, did not proceed at all. Therefore, we next investigated several other transition metal complexes. Although carbonyl complexes W(CO)6 and Ru3(CO)12 did not produce the desired product (Table 1, entries 3 and 5), a rhenium complex, [ReBr(CO)3(thf)]2, afforded silylated product 3a in 37% yield (Table 1, entry 4). Ruthenium and cobalt complexes, [RuCl(p-cymene)]2 and CoCl(PPh3)3, also did not produce good results (Table 1, entries 6 and 7). In contrast, rhodium and iridium complexes exhibited catalytic activities and gave the desired product 3a in 35−48% yield (Table 1, entries 8−11). To
Figure 1. Several examples of π-conjugated molecules with a Lewis acid−base interaction.
base interaction increases the efficiency of the π-conjugation by constraining the π-conjugated system in a planar fashion, and the electronegativity of Lewis acidic heteroatoms decreases the LUMO level of π-conjugated molecules.3a We recently reported the synthesis of silylated 2-phenylpyridines (silafluorene equivalent) with a Lewis acid−base interaction between the Lewis acidic silicon atoms of fluorinated silyl groups and the Lewis basic nitrogen atoms of 2-phenylpyridines, and the silafluorene equivalent has a fluorescent quantum yield higher than that of the corresponding silafluorene.1c We also reported palladium-catalyzed C−H borylation at the ortho-position of aromatic compounds.3g In this reaction, regioselectivity was © 2015 American Chemical Society
Received: February 19, 2015 Published: March 24, 2015 1758
DOI: 10.1021/acs.orglett.5b00529 Org. Lett. 2015, 17, 1758−1761
Letter
Organic Letters Table 1. Investigation of Several Transition Metal Compounds and Hydrogen Acceptorsa
entry
catalyst
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
[Pd(NCCH3)4](BF4)2 Pd(OAc)2 W(CO)6 [ReBr(CO)3(thf)]2 Ru3(CO)12 [RuCl(p-cymene)]2 CoCl(PPh3)3 RhCl(PPh3)3 [RhCl(cod)]2 [IrCl(cod)]2 Ir(acac)(cod) [RhCl(cod)]2 [IrCl(cod)]2 [IrCl(cod)]2 Ir(acac)(cod)
a
Scheme 1. Reactions between Aromatic Compounds 1 and Hydrosilane 2aa
hydrogen acceptor
yield (%)
3,3-dimethyl-1-butene 3,3-dimethyl-1-butene norbornene norbornene
