Direct Synthesis of Aryl Halosilanes through Iridium(I)‐Catalyzed Aromatic CH Silylation by Disilanes

Abstract

Aryl halosilanes and their hypervalent derivatives are versatile reagents for carbon-carbon [1][2] and carbon-heteroatom [3] bond formation in modern organic synthesis. Although they have been prepared by arylation of halosilanes with aryl magnesium or aryl lithium reagents, [4] and by Pd-catalyzed cross-coupling of halogenated disilanes with aryl electrophiles, [5] direct silylation of arenes through C À H bond activation would provide a more attractive route from the viewpoints of economy, efficiency, and environmental benignity. Indeed, aromatic CÀH silylation by disilanes [6] or hydrosilanes [6a, 7] catalyzed by a transition-metal complex has been developed by several research groups. However, the application of this protocol has been limited to the synthesis of aryl triorganosilanes. On the other hand, we recently found that Ir I complexes generated from 1/2[{IrCl(cod)} 2 ] (cod = 1,5-cyclooctadiene) or 1/2[{Ir(OMe)(cod)} 2 ] and 2,2'-bipyridine (bpy) or 4,4'-di-tert-butyl-2,2'-bipyridine (dtbpy) are excellent catalysts for aromatic C À H borylation by bis(pinacolato)diboron.[8] These results prompted us to extend the methodology to the aromatic C À H silylation of arenes (2) by 1,2-ditert-butyl-1,1,2,2-tetrafluorodisilane (tBuF 2 Si) 2 (1) in the presence of a 1/2[{Ir(OMe)(cod)} 2 ]-dtbpy catalyst. This process enables, for the first time, the direct synthesis of aryl halosilanes (3) through aromatic C À H bond activation (Scheme 1).Our initial investigation focused on the effects of substituents on the disilanes on the reaction. The reactions were carried out at 80 8C for 16 h in a resealable Schlenk tube by using disilanes (1.0 mmol), benzene (60 mmol), [{Ir(OMe)-(cod)} 2 ] (0.015 mmol), and dtbpy (0.03 mmol). Among the disilanes examined, 1 exhibited the highest reactivity to produce the corresponding phenylsilane in 28 % yield with 28 % conversion of 1. The use of tetrafluorodisilanes is critical as (Me 2 FSi) 2 gave the corresponding arylsilane in 9 % yield, and (tBuCl 2 Si) 2 or (Me 3 Si) 2 led to no reaction. The structures of alkyl substituents also had significant effects on silylation. For example, (sBuF 2 Si) 2 and (nBuF 2 Si) 2 formed no silylated product.The reaction of o-xylene with 1 was carried out at 120 8C to optimize catalyst precursors and ligands. Of the precursors and ligands examined, the combination of 1/2[{Ir(OMe)-(cod)} 2 ] and dtbpy was found to be the best catalyst to provide isomerically pure 4-silyl-1,2-dimethylbenzene in 99 % yield based on the molar amount of 1. Although the substrate has weaker benzylic CÀH bonds, [9] the reaction selectively occurred at the aromatic C À H bonds. ] as a catalyst precursor were markedly affected by steric and electronic properties of bipyridine ligands. Bpy, 4,4'-di-Me-bpy, 5,5'-di-Me-bpy, and even 6,6'-di-Me-bpy displayed moderate reactivity (49-55 % yields), while 3,3'-diMe-bpy showed little activity (8 % yield) owing to the importance of a parallel arrangement of two pyridine rings. Electronic effects of bpy derivatives were perplexing...

Catalytic Reductive ortho-C–H Silylation of Phenols with Traceless, Versatile Acetal Directing Groups and Synthetic Applications of Dioxasilines

“…Furthermore, the removal of alkyl masking groups in the presence of silanes is not trivial. Miyaura 18 and Hartwig 19 have reported Rh- and Ir-catalyzed steric-controlled meta- or para- silylation of anisoles, respectively (Scheme 1.a2). Whereas Miyaura’s Ir-catalyzed silylation required 60-fold excess of anisole substrates 5 , the method developed by Hartwig showed broad substrate scope and high site-selectivity, yet the removal of a hydroxyl masking group in the presence of silanes is again questionable.…”

Diverse sp3 C−H functionalization through alcohol β-sulfonyloxylation

“…To overcome such limitations, perhaps a more practical approach is to first install a latent FG site selectively through C−H activation and then branch out to other FGs through reliable chemistry. For instance, C−H borylation [18][19][20][21][22] , silylation 18,23,24 and fluorination 25 of arenes serve as seminal examples of diverse functionalizations of an sp 2 C−H bond through the transformations of aromatic boranes, silanes and fluorides. In contrast, for unactivated (for example, non-benzylic, -allylic, -α to a heteroatom or -strained) sp 3 C−H bonds, although the corresponding C−H borylation and silylation have been demonstrated elegantly 18,[26][27][28][29][30][31] , the diversification becomes more challenging because of the significantly reduced reactivity of alkyl boranes and silanes towards electrophiles 32 .…”

Multiple CH Activations To Construct Biologically Active Molecules in a Process Completely Free of Organohalogen and Organometallic Components

“…[13] We found that the arylation between 1 a and o-xylene (2 a) took place smoothly in the presence of Pd(OAc) 2 (10 mol %) as a catalyst with Cu(OTf) 2 (20 mol %) as an oxidative cocatalyst [Eq. [14] To further verify our prediction, the more sterically hindered p-xylene was subjected to this transformation, and ortho arylation was observed in very low yield, with most of the starting material 1 a being recovered. Gratifyingly, dioxygen (O 2 , 1 atm) could be applied as the terminal oxidant to complete this transformation.…”

“…In the proposed mechanism (Scheme 2), the selectivity in this arylation was most likely controlled by the acetamino directing group in the first CÀH activation [13] and by steric effects in the second CÀH activation. [14] To further verify our prediction, the more sterically hindered p-xylene was subjected to this transformation, and ortho arylation was observed in very low yield, with most of the starting material 1 a being recovered.…”