Palladium-Catalyzed C−P Bond Formation: Mechanistic Studies on the Ligand Substitution and the Reductive Elimination. An Intramolecular Catalysis by the Acetate Group in Pd^IIComplexes

Abstract

Ligand substitution and reductive elimination of the palladium-catalyzed C−P bond forming cross-coupling were investigated in depth. It was found that for PhPdII(PPh3)2X (X = I, Br, Cl) complexes, a step commonly referred to as ligand substitution commenced with coordination of an H-phosphonate diester, followed by its deprotonation to form an equilibrium mixture of penta- and tetracoordinate palladiumphosphonate intermediates, from which reductive elimination of the product (diethyl phenylphosphonate) occurred. For the acetate counterpart, PhPdII(PPh3)2(OAc), the incorporation of a phosphonate moiety to the complex was preceded by a rate-determining removal of the supporting phosphine ligand, facilitated by an intramolecular catalysis by the acetate group. Both the reaction steps, i.e., formation of palladiumphosphonate intermediates and reductive elimination, were significantly faster for the acetate versus halides containing PdII complexes investigated. Similar observations were found to be true also for bidentate ligand complexes [(dppp)PdII(Ph)X]; however, in this instance, a single palladiumphosphonate intermediate, (dppp)PdII(Ph)(PO(OEt)2), could be observed by 31P NMR spectroscopy. The synthetic and kinetic studies on the cross-coupling reaction of diethyl H-phosphonate with phenyl halides permitted us to elucidate a crucial catalytic role of an acetate group in PdII complexes and to propose two distinctive catalytic cycles, which complemented traditional Pd0/PdII schemes, for the palladium-mediated C−P bond formation.

Pd-Catalyzed P-Arylation of Triarylantimony Dicarboxylates with Dialkyl H-Phosphites without a Base: Synthesis of Arylphosphonates

“…First, we performed the reactions of 9a (1.2 eq) with a variety of antimony reagents (1a-6) to compare the reactivity using 5 mol% of Pd(PPh 3 ) 4 as catalyst in 1,4-dioxane at 60°C under argon atmosphere (entries [1][2][3][4][5][6][7][8]. In comparison with these results, triphenylantimony dicarboxylates (1a, 2) and pentavalent bismuth(V) compounds (7,8) afforded the expected diethyl phenylphosphonate (10) in good to high yields along with homo-coupling product (11) and reductive products (12,13). Among these, triphenylantimony diacetate (1a) appeared to be the best substrate for this reaction in terms of the yield (77%) of the cross-coupling product (10) and reaction time (6 h).…”

“…We also examined a similar reaction using other catalytic systems such as Pd(dba) 2 , Pd(dba) 2 -dppf, PdCl 2 , Pd(OAc) 2 , and PdCl 2 (PPh 3 ) 2 . They were inferior to Pd(PPh 3 ) 4 in the terms of the reaction time and yield of the cross-coupling product (entries [12][13][14][15][16]. The screening of solvent showed that the reaction proceeded effectively in dioxane (94%), 1,2-dichloroethane (DCE) (78%), 1,2-dimethoxyethane (DME) (75%) and tetrahydrofuran (THF) (71%) whereas toluene, CH 3 CN, N-methylpyrrolidone (NMP) and EtOH gave inferior results (entries [17][18][19][20][21][22][23].…”

“…The Pd-catalyzed P-arylation of aryl halides with dialkyl H-phosphites is promoted by acetate ions, and ArPdOAc (C) is the key intermediate for reaction mechanism. 12,13,19) The ligand exchange of ArPdOAc (C) with dialkyl H-phosphite (9) gives ArPdP(O)(OR) 2 intermediate D, which undergo reductive elimination to form the coupling product and regenerate the Pd(0) species (A).…”

“…as an internal standard) and 13 C-NMR (CDCl 3 : δ: 77.00 as an internal standard) spectra were recorded on a JEOL JNM-ECA400 (400 MHz and 100 MHz) spectrometers in CDCl 3 unless otherwise stated. Mass spectra (MS) were obtained on a JEOL JMP-DX300 instrument (70 eV, 300 µA).…”

“…[7][8][9] With regard to the synthesis of arylphosphonates, the Pd-catalyzed cross-coupling reactions of aryl halides with dialkyl H-phosphites (i.e., Hirao P-arylation) have proved to be a powerful method. [10][11][12][13][14][15][16][17][18][19][20][21] The general procedure of these P-arylations involves a Pd-phosphine complex as the catalyst with the use of a stoichiometric or an excess amount of a base. Recently, different aryl substances were investigated for their reactivity with phosphorus agents.…”