Cycloheptatrienyl Cyclopentadienyl Titanium Phosphane Ligands in Palladium‐Catalyzed Suzuki–Miyaura Cross‐Coupling Reactions

Abstract

The reaction of the cycloheptatrienyl cyclopentadienyl (Cht-Cp) titanium sandwich complexes [(η 7 -C 7 H 7 )Ti(η 5 -C 5 R 5 )] (R = H, Me) with nBuLi followed by treatment with the chlorophosphanes ClPR′ 2 (R′ = Cy, tBu) afforded the Cht-substituted phosphanes [(η 7 -C 7 H 6 PR′ 2 )Ti(η 5 -C 5 R 5 )] (R = H, Me; R′ = Cy, tBu). Together with their previously reported Cp-substituted analogues [(η 7 -C 7 H 7 )Ti(η 5 -C 5 H 4 PR 2 )] (R = Cy, tBu), these complexes were employed as ancillary phosphane ligands in palladium-catalyzed Suzuki-Miyaura coupling between sterically encumbered aryl bromides and aryl boronic acids. The unusually short reaction times indicated rapid generation of catalytically active Pd 0 species, and the reaction of Pd(OAc) 2 with an excess [a] 5588 of the phosphorus ligands afforded linear bis-phosphane palladium(0) complexes. These reactions gave no indication of phosphane oxide formation, and it is supposed that the Cht-Cp titanium sandwich moiety serves as an integrated reducing agent. Accordingly, the reaction of [(η 7 -C 7 H 7 )Ti(η 5 -C 5 M 5 )] with Pd(OAc) 2 or Ag(OAc) resulted in a three-electron redox reaction with formation of the half-sandwich complex [(η 5 -C 5 Me 5 )-Ti(OAc) 3 ] together with 7,7′-bi-1,3,5-cycloheptatriene (ditropyl) and elemental Pd or Ag metal. On reaction with silver(I) trifluoromethanesulfonate, a four-electron redox process furnished the tropylium titanate complex [C 7 H 7 tant examples and have been well studied as ligands in transition-metal-catalyzed reactions, for example, with palladium [38][39][40] or nickel. [41] Chiral ferrocenyl phosphanes V exhibit another possible substitution pattern of the ferrocene backbone and are useful in various catalytic reactions. [42][43][44][45][46][47][48][49][50] Eur.

Allyl Palladium Complexes of Cycloheptatrienyl‐Cyclopentadienyl Phosphane Ligands in Buchwald‐Hartwig Amination Reactions

“…The solid‐state structures of these complexes show Ti–Ct Cht and Ti–Ct Cp (Ct = centroid) distances of around 1.5 Å and 2.0 Å, respectively. These values lie in the range of other heterobimetallic complexes with troticenyl phosphane ligands . Regarding the Ct Cht –Ti–Ct Cp angle, complex 7 t Bu 1Me shows the highest deviation from 180° with 166.043(14)°.…”

“…The protons of the phenyl ring of 7 t Bu 1Ph can be assigned in the aromatic region. In the 13 C{ 1 H} NMR spectrum of 6 t Bu , the doublet of the α‐carbon atom is very broad, whereas this signal cannot be assigned in the 13 C{ 1 H} NMR spectra of 7 t Bu and its derivatives because of peak broadening . The seven‐membered ring gives rise to a characteristic singlet for 6 t Bu and 6 Cy at around 88 ppm, whereas the signal of the five‐membered ring of 7 Cy , 7 t Bu and its derivatives can be assigned at a characteristic chemical shift of about 100 ppm.…”

“…Given the known ability of the ligands 3 R and 4 R (R = Cy, t Bu) to act as ancillary ligands in Suzuki–Miyaura coupling reactions,, we were interested in the catalytic activity of the new complexes 6 R and 7 R (R = t Bu, Cy) in Buchwald‐Hartwig amination reactions. For these investigations, the coupling of 4‐bromotoluene and N ‐methylaniline was chosen as a model reaction (Scheme ).…”

“…Recently, we introduced phosphane ligands with cyclohexyl and tert ‐butyl moieties based on [(η 7 ‐C 7 H 7 )Ti(η 5 ‐C 5 H 5 )] ( 1 , troticene) and its pentamethylated derivative [(η 7 ‐C 7 H 7 )Ti(η 5 ‐C 5 Me 5 )] ( 2 , pentamethyltroticene) . This functionalization was described earlier in our group for the phenyl derivatives, and these complexes were investigated as ancillary ligands in bimetallic complexes with, e.g., platinum(II), gold(I), rhodium(I) or iridium(I) , .…”

“…The reductive behavior of the seven‐membered ring was further investigated by the reaction of 1 and 2 towards palladium or silver acetate, and also silver triflate. In these reactions elemental palladium or silver was formed and the formation of ditropyl or a tropylium cation was observed , …”