Synthesis of 6,6′‐Bis(dimethylamino)‐ and 6,6′‐Dibromo‐Substituted 2,2′‐Diphosphanylbiphenyls and Their Palladium Complexes

Abstract

New 6,6Ј-dibromo-and 6,6Ј-bis(dimethylamino)-substituted 2,2Ј-diphosphanylbiphenyl ligands 11-14 were prepared starting from 2,2Ј-dibromo-4,4Ј-dimethyl-6,6Ј-dinitro-1,1Ј-biphenyl (4). Depending on the phosphane groups [diphenylphosphanyl (11, 13) or diisopropylphosphanyl (12,14)] the palladium dichloride complexes show different coordination symmetry. Whereas the smaller diphenylphosphanyl groups lead to C 2 -symmetric complexes, the respective bis(diisopropyl)phosphanes 12 and 14 form C 1 -symmetric complexes that show fluxional behavior due to the restricted rotation of the isopropyl groups as well as the exchange of atom positions within the C 1 -symmetric conformer. All complexes have[a] TU Chemnitz, 4858 been tested as precatalysts in the Suzuki-Miyaura cross coupling reaction of 2-bromotoluene and phenylboronic acid. The activity of the catalytic system increases with the size of the diphosphanes and the donating ability of the ligand. In contrast to C 2 -symmetric palladium complex 15, platinum complex 19 was found to be C 1 -symmetric in the solid state despite the fact that both complexes have the small bis-(diphenylphosphanyl)-substituted diphosphane ligand 11 in common. NiBr 2 adduct 20 with a similar diphosphane 13 exists as a mixture of distorted square-planar and tetrahedral coordination sphere geometries in equilibrium with each other.Scheme 1. 1,1Ј-Biphenyl-2,2Ј-diyl-bridged diphosphanes discussed below and the numbering for relevant positions.phanes are of interest, as the large bromo substituents in the ortho positions lead to high activation barriers for the rotation around the central C-C bond. [22] Additionally, the dimethylamino groups can increase the electron density of the biphenyl system, [23] act as additional hemilabile donors [24] or form proton sponge systems. [25][26][27] Results and DiscussionLigands based on 2-phosphanyl-and 2,2Ј-diphosphanylbiphenyls are accessible by various methods employing electrophilic, [28] nucleophilic, [29] and radical chemistry [30] as well as transition-metal-catalyzed [31,32] reactions. A particularly effective method has been the treatment of a 2,2Ј-di-www.eurjic.org FULL PAPER lithiated biphenyl with a chlorophosphane. [28,33,34] Dilithiated biphenyl species are easily prepared by halogen/lithium exchange from the 2,2Ј-diiodo-/2,2Ј-dibromobiphenyls. We prepared diiodobiphenyl 6 and dibromobiphenyl 7 by the route depicted in Scheme 2. [35,36] The known diamine derivative 5 can be prepared from inexpensive and commercially available 4-methyl-2-nitroaniline (1). The methyl group in the 4-position blocks the C-4 atom from electrophilic attack by bromine in the first step to derivative 2; consequently, 2 is the only isolable regioisomer. [37] Moreover, the 4-methyl group improves the solubility of all compounds and simplifies workup procedures. The key step in the synthesis is an Ullmann [35,38] homo-coupling of iodo compound 3 to form the central C-C bond in 4. This reaction is highly selective and proceeds smoothly under relatively mild conditions in ...

Syntheses of P –aryl- and P –alkyl substituted 2,2′,6,6′–tetraphosphanyl–1,1′–biphenyls

“…induced "Jahn-Teller distortion" [25,31,33]. Worth to notice is the large coupling (J(P,P) ¼ 140 Hz) between the unbonded P( i Pr) 2 groups in 7a which is in contrast to the free mixed substituted phosphine 1b and 1c in both cases only a small P,P-couplings were found.…”

“…The crude products can be further purified by means of recrystallization from ethanol. All of the dibromo-substituted phosphines 4 were remarkably stable towards oxidation in air [25,31].…”

“…Ethanol was purified by distillation from magnesium and acetonitrile by distillation from calcium hydride. The following compounds: 2,2 0 -dibromo-4,4 0 methyl-6,6 0 -diiodobiphenyl 3 [22e24], diphosphanes 4a-c [20,25] as well as tetraphosphane 1a [20] were synthesized following published procedures. All other chemicals were purchased by commercial suppliers and were used without further purification.…”

Substituted 2,2′‐Bis(2‐oxidobenzylideneamino)‐4,4′‐dimethyl‐1,1′‐biphenyl Complexes of Zinc

“…Schiff bases are easily available via the condensation reaction of primary amines with aldehydes. The synthesis of 2,2′‐bis(salicylideneamino)‐4,4′‐dimethyl‐6,6′‐dibromo‐1,1′‐biphenyl ( 1a , H 2 C 5 ) succeeded in ethanol via condensation of 2,2′‐diamino‐4,4′‐dimethyl‐6,6′‐dibromo‐1,1′‐biphenyl with 2‐hydroxybenzaldehyde as well as 3‐methoxy‐, 4‐methoxy‐, and 5‐methoxy‐2‐hydroxybenzaldehyde according to a protocol which had been reported for the halide‐free congener . Substitution of the salicylidene unit with a methoxy group in 5‐ ( 1b ), 4‐ ( 1c ), or 3‐position ( 1d ) changed neither the procedure for preparation and purification nor the yields of approximately 70 % (Scheme ).…”

“…All commercially available substrates were purchased from Sigma Aldrich, Merck, or Alfa Aesar and used without further purification. Starting 2,2′‐diamino‐4,4′‐dimethyl‐6,6′‐dibromobiphenyl and halide‐free (methoxy‐substituted) 2,2′‐bis(salicylideneamino)‐1,1′‐biphenyls were prepared according to literature procedures.…”