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Synthesis, Coupling, and Condensation Reactions of 1,2‐Diborylated Benzenes: An Experimental and Quantum‐Chemical Study
Author(s) -
Seven Ömer,
Qu ZhengWang,
Zhu Hui,
Bolte Michael,
Lerner HansWolfram,
Holthausen Max C.,
Wagner Matthias
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201201547
Subject(s) - chemistry , borane , hydride , hydroboration , benzene , boron , crystal structure , crystallography , coupling reaction , solid state , stereochemistry , medicinal chemistry , organic chemistry , catalysis , metal
Abstract 1,2‐Bis(pinacolboryl)benzene (1,2‐C 6 H 4 (Bpin) 2 , 2 ) was synthesized in preparatively useful yields from 1,2‐C 6 H 4 Br 2 , i PrOBpin, and Mg turnings in the presence of 1,2‐C 2 H 4 Br 2 as an entrainer. Compound 2 is a versatile starting material for the synthesis of (un)symmetrically substituted benzenes (i.e., 1,2‐C 6 H 4 (Ar 1 )(Ar 2 )) through sequential Suzuki–Miyaura coupling reactions. Alternatively, it can be transformed into bis‐borate Li 2 [1,2‐C 6 H 4 (BH 3 ) 2 ] ( 3 ) through reduction with Li[AlH 4 ]. In the crystal lattice, the diethyl ether solvate 3⋅ OEt 2 establishes a columnar structure that is reinforced by an intricate network of B(μ‐H)Li interactions. Hydride‐abstraction from compound 3 with Me 3 SiCl leads to the transient ditopic borane 1,2‐C 6 H 4 (BH 2 ) 2 , which can either be used in situ for subsequent hydroboration reactions or trapped as its stable NMe 2 Et diadduct ( 6 ). In SMe 2 solution, the putative diadduct 1,2‐C 6 H 4 (BH 2 ⋅ SMe 2 ) 2 is not long‐term stable but rather undergoes a condensation reaction to give 9,10‐dihydro‐9,10‐diboraanthracene, HB(μ‐C 6 H 4 ) 2 BH, and BH 3 . 9,10‐Dihydro‐9,10‐diboraanthracene was isolated from the reaction mixture as its SMe 2 monoadduct ( 7 ), which dimerizes in the solid state through two BHB bridges (( 7 ) 2 , elucidated by X‐ray crystallography). In contrast, hydride‐abstraction from compound 3 in THF or CH 2 Cl 2 provides the unique exo ‐adduct H 2 B(μ‐H) 2 B(μ‐C 6 H 4 ) 2 B(μ‐H) 2 BH 2 ( 8 , elucidated by X‐ray crystallography). Quantum‐chemical calculations on various conceivable isomers of [1,2‐C 6 H 4 (BH 2 ) 2 ] 2 revealed that compound 8 was the most stable of these species. Moreover, the calculations confirmed the experimental findings that the NMe 2 Et diadduct of 1,2‐C 6 H 4 (BH 2 ) 2 is significantly more stable than the corresponding SMe 2 complex and that the latter complex is not able to compete successfully with borane‐dimerization and ‐condensation. The reaction cascade in SMe 2 , which proceeds from 1,2‐C 6 H 4 (BH 2 ) 2 to the observed adducts of HB(μ‐C 6 H 4 ) 2 BH, has been elucidated in detail and the important role of BCB‐bridged intermediates has been firmly established.