The Effect of the Trimethylsilyl Group on Electrophilic Cyclopalladation; A Study of Caryl–Si versus Caryl–H Selective Bond Activation with 2,6-(Me2NCH2)2C6H3R (R=H or SiMe3)

Abstract

The site selectivity of electrophilic palladation has been studied by using two bis(aminomethyl)-substituted benzenes 1,3-(Me 2 NCH 2) 2 C 6 H 4 (6) and 2,6-(Me 2 NCH 2) 2 C 6 H 3 (SiMe 3) (7) and Li 2 [PdCl 4 ] or Pd(OAc) 2 in solution in MeOH or CH 2 Cl 2. The major product of direct palladation of 6 in both solvents is the polymeric cyclopalladated organometallic complex [1,5-{PdCl} 2-2,4-(Me 2-NCH 2) 2 C 6 H 2 ] n , which was characterized as its dinuclear pyridine derivative [1,5-{PdCl(C 5 H 5 N)} 2-2,4-(Me 2 NCH 2) 2 C 6 H 2 ] (9). The effect of the trimethylsilyl group present at the 1-position in 7 leads to inversion of the site selectivity in comparison to that observed for 6, and to activation of the C ± Si bond when MeOH is used as solvent; the major product of this direct palladation is the known monomeric cyclopalladated complex [{PdCl}2,6-(Me 2 NCH 2) 2 C 6 H 3 ] (8). However, using CH 2 Cl 2 instead of MeOH in the palladation reaction of 7 leads to the major product arising from C ± H rather than C ± Si bond activation.

Cyclometallated nitrogen heterocycles

“…1): 3,6-bis-(4-alkyloxyphenyl)pyridazines (1), 3,6-bis(4-alkyloxyphenyl) [1,2,4,5]tetrazines (2), 2,5-bis-(4-alkyloxyphenyl)pyrazines (3) and 1,4-bis-(5-alkyloxypyrimidin-2-yl)benzenes (4). A number of different alkyloxy groups were used, and there were no differences in the reactivity of the homologues.…”

“…Tetrazines have the potential to be mono, di, tri and tetra-dentate, although by comparison with similar coordination complexes of 3,6-bis(2-pyrimidyl)- [1,2,4,5]tetrazine the formation of either tri-or tetranuclear compounds is highly unlikely [23]. While the mesomorphic properties of 3,6-disubstituted tetrazines have been studied [24,25] and the use of tetrazines as guest Á/host dyes for use in guest Á/host liquid crystal displays has previously been described [26,27], the mesomorphic properties of the tetrazines discussed here does not seem to have been reported in the literature before, we include full details here in the supporting information.…”

“…Conventionally, cyclometallation has involved one ligating group holding a metal centre close to a C Á/H bond and the subsequent closure of the ring via the formation of a carbon to metal bond [1]. Whilst some have adapted this reaction to activate bonds other than C Á/H (for example C Á/Si [2] or C Á/C bonds [3]), we have been investigating bringing about double cyclometallations with a single metal (generating Ĉ N C tridentate complexes) [4,5]. Others have followed Trofimenko's pioneering work of thirty years ago [6,7] using two ligating groups to hold two metal centres close to a single benzene ring, thus forming two metal to carbon bonds on the same aromatic ring.…”

Mononuclear versus dinuclear palladacycles derived from 1,3-bis(N,N-dimethylaminomethyl)benzene: Structures and catalytic activity

“…Palladacycle 2 was synthesized from [1,5-{PdCl(C 5 H 5 N)} 2 -2,4-(Me 2 NCH 2 ) 2 C 6 H 2 ], which was prepared according to literature [16], by the reaction with silver triflate in dichloromethane at room temperature with a yield of 80% [17].…”

“…Its 1 H NMR spectrum shows the characteristics of 1,2,4,5-tetrasubstituted benzene ring with two singlet peaks at 6.57 (H 3 ) and 4.56 (H 6 ) ppm, respectively, which are consistent with those in its chloride precursor [16] and the closely related [1,5- proton on the phenyl backbone. The great up-field shift of H 6 can be attributed to the double shielding effects from two pyridine rings, which are positioned trans to the amine moieties.…”