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The Pyridyldiisopropylsilyl Group: A Masked Functionality and Directing Group for Monoselective ortho ‐Acyloxylation and ortho ‐Halogenation Reactions of Arenes
Author(s) -
Huang Chunhui,
Chernyak Natalia,
Dudnik Alexander S.,
Gevorgyan Vladimir
Publication year - 2011
Publication title -
advanced synthesis and catalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.541
H-Index - 155
eISSN - 1615-4169
pISSN - 1615-4150
DOI - 10.1002/adsc.201000975
Subject(s) - chemistry , aryl , halogenation , regioselectivity , nucleophile , borylation , electrophile , combinatorial chemistry , pyridine , hydride , main group element , reagent , organic chemistry , catalysis , alkyl , transition metal , hydrogen
A novel, easily removable and modifiable silicon‐tethered pyridyldiisopropylsilyl directing group for CH functionalizations of arenes has been developed. The installation of the pyridyldiisopropylsilyl group can efficiently be achieved via two complementary routes using easily available 2‐(diisopropylsilyl)pyridine ( 5 ). The first strategy features a nucleophilic hydride substitution at the silicon atom in 5 with aryllithium reagents generated in situ from the corresponding aryl bromides or iodides. The second milder route exploits a highly efficient room‐temperature rhodium(I)‐catalyzed cross‐coupling reaction between 5 and aryl iodides. The latter approach can be applied to the preparation of a wide range of pyridyldiisopropylsilyl‐substituted arenes possessing a variety of functional groups, including those incompatible with organometallic reagents. The pyridyldiisopropylsilyl directing group allows for a highly efficient, regioselective palladium(II)‐catalyzed mono‐ ortho ‐acyloxylation and ortho ‐halogenation of various aromatic compounds. Most importantly, the silicon‐tethered directing group in both acyloxylated and halogenated products can easily be removed or efficiently converted into an array of other valuable functionalities. These transformations include protio‐, deuterio‐, halo‐, boro‐, and alkynyldesilylations, as well as a conversion of the directing group into the hydroxy functionality. In addition, the construction of aryl‐aryl bonds via the Hiyama–Denmark cross‐coupling reaction is feasible for the acetoxylated products. Moreover, the ortho ‐halogenated pyridyldiisopropylsilylarenes, bearing both nucleophilic pyridyldiisopropylsilyl and electrophilic aryl halide moieties, represent synthetically attractive 1,2‐ambiphiles. A unique reactivity of these ambiphiles has been demonstrated in efficient syntheses of arylenediyne and benzosilole derivatives, as well as in a facile generation of benzyne. In addition, preliminary mechanistic studies of the acyloxylation and halogenation reactions have been performed. A trinuclear palladacycle intermediate has been isolated from a stoichiometric reaction between diisopropyl(phenyl)pyrid‐2‐ylsilane ( 3a ) and palladium acetate. Furthermore, both CH functionalization reactions exhibited equally high values of the intramolecular primary kinetic isotope effect ( k H / k D =6.7). Based on these observations, a general mechanism involving the formation of a palladacycle via a CH activation process as the rate‐determining step has been proposed.