Carbon−Carbon Bond-Forming Reductive Elimination from Arylpalladium Complexes Containing Functionalized Alkyl Groups. Influence of Ligand Steric and Electronic Properties on Structure, Stability, and Reactivity

Abstract

A series of arylpalladium alkyl complexes of the formula [(DPPBz)Pd(Ar)(R)] (DPPBz ) 1,2-bis(diphenylphosphino)benzene; R ) methyl, benzyl, enolate, cyanoalkyl, trifluoroalkyl, or malonate) has been prepared to reveal the influence of steric and electronic parameters on structure, stability, and reactivity. Arylpalladium enolate and cyanoalkyl complexes ligated by EtPh 2 P, 1,1′-bis(diisopropylphosphino)ferrocene (D i PrPF), and BINAP were prepared to evaluate the effect of the ancillary ligand. The coordination modes of the enolate and cyanoalkyl complexes were determined by spectroscopic methods, in combination with X-ray crystallography. In the absence of steric effects, the C-bound isomer was favored electronically if the enolate or cyanoalkyl group was located trans to a phosphine, and the O-bound isomer was favored if the enolate was located trans to an aryl group. The thermodynamic stability of the enolate and cyanoalkyl complexes was controlled by the steric properties of the enolate or cyanoalkyl group, and complexes with more substitution at the R-carbon were less stable. Arylpalladium methyl, benzyl, enolate, cyanoalkyl, and trifluoroethyl complexes underwent carbon-carbon bond-forming reductive elimination upon heating. Reductive elimination was faster from complexes with electron-withdrawing substituents on the palladium-bound aryl group and with sterically hindered alkyl groups. The electronic properties of the alkyl group had the largest impact on the rate of reductive elimination: electron-withdrawing groups on the R-carbon retarded the rate of reductive elimination. The rates of reductive elimination correlated with the Taft polar substituent constants of the groups on the carbon alpha to the metal.

Cooperative dual palladium/silver catalyst for direct difluoromethylation of aryl bromides and iodides

“…In the presence of 1.0 equivalent of dppf to trap the Pd 0 product, the difluoromethylated complexes 3a-e formed difluoromethylated arenes in 92-99% yields. Interestingly, these results indicated that the reductive elimination from a difluoromethyl palladium complex is much faster than those of its analogous CF 3 complex [32][33][34][35] . The rates of reductive elimination of difluoromethylarenes from complexes 3a-e at 47°C were monitored by 19 F NMR spectroscopy.…”

Palladium‐Catalyzed 2,2,2‐Trifluoroethylation of Organoboronic Acids and Esters

“…Considering that the reductive elimination may be ratelimiting in the cross-coupling reaction (as reported by Culkin and Hartwig), [9] we first examined various supporting ligands for the palladium catalyst, especially those known to be able to efficiently facilitate reductive elimination (Scheme 2 and Table 1). 4-Biphenylboronic acid (1 a) was used as a model substrate, and the reaction was carried out in dioxane at 80 8C using K 3 PO 4 as a base (Table 1, entries [1][2][3][4][5][6][7][8][9].…”

“…4-Biphenylboronic acid (1 a) was used as a model substrate, and the reaction was carried out in dioxane at 80 8C using K 3 PO 4 as a base (Table 1, entries [1][2][3][4][5][6][7][8][9]. Flexible bidentate phosphine ligands (L1 and L2) and sterically hindered trialkyl phosphine ligands (L3 and L4) were Scheme 1.…”

Aus der Sicht des Synthetikers: Palladiumkomplexe N‐heterocyclischer Carbene als Katalysatoren für Kreuzkupplungen

“…Methode A: 1 oder Chloralkanen; [133,134] 2) wegen des Platzbedarfs der NHCs ist ebenso wie bei sperrigen Phosphanen die reduktive Eliminierung erleichtert; [135,136] 3) wegen der starken Pd-NHC-Bindung und den wenigen möglichen Zerfallswegen bleibt das Metall, selbst wenn nur ein einziger NHC-Ligand koordiniert ist, sicher in einem löslichen, katalytisch aktiven Zustand. Durch die besonderen Komplexierungseigenschaften von NHCs an Pd sind also definierte Komplexe synthetisierbar, die leicht aktiviert werden können und gegenüber in situ hergestellten Komplexen viele Vorteile bieten.…”

“…Schließlich demonstrierten Herrmann et al [97] und Iyer et al, [100] dass NHC-ligierte Palladacyclen ausgezeichnete Präkatalysatoren für die Heck-Mizoroki-Reaktion sind. Dabei war die TON mit dem PhosphanPalladacyclus 137 um eine Größenordnung höher als mit Palladacyclen, die von N-Donoren abgeleitet wurden (135,422). Erwähnenswert ist, dass 422 bei der Arylierung von para-Chloracetophenon (381) ebenfalls eine hohe TON erzielte.…”