Preparation, Structures, and Thermal Reactivity of Alkoxycarbonyl(cyano)palladium(II) Complexes trans-Pd(COOR)(CN)(PPh₃)₂ (R = Me, Et, ⁿPr, ⁱPr, ⁿBu, ^tBu, and Bn) as Intermediates of the Palladium-Catalyzed Cyanoesterification of Norbornene Derivatives

Abstract

Alkoxycarbonyl(cyano)palladium(II) complexes trans-Pd(COOR)(CN)(PPh 3 ) 2 (1, R ) Me; 2, R ) Et; 3, R ) n Pr; 4, R ) i Pr; 5, R ) n Bu; 6, R ) t Bu; 7, R ) Bn) are prepared via oxidative addition of the corresponding cyanoformates to Pd(PPh 3 ) 4 in toluene at room temperature or 50 °C and characterized by means of NMR ( 1 H, 13 C{ 1 H}, and 31 P{ 1 H}) and IR spectroscopy as well as elemental analyses. X-ray crystallography of 3, 4, and 6 showed a square-planar coordination around the Pd center that is bonded to alkoxycarbonyl and cyano ligands in the trans configuration. The Pd-CN and Pd-COOR bonds in 1, 3, 4, and 6 are similar to those of the cyanopalladium(II) and alkoxycarbonylpalladium(II) complexes reported to date. On reaction of Pd(PPh 3 ) 4 with phenyl cyanoformate a thermally induced ligand exchange takes place to afford the dicyanopalladium(II) complex, trans-Pd(CN) 2 (PPh 3 ) 2 (8). Complex 2 is able to catalyze the reaction of norbornadiene with ethyl cyanoformate to produce (2R*,3S*)-ethyl 3-cyanobicyclo-[2.2.1]hept-5-ene-2-carboxylate (13) and is recovered after the reaction. This observation supports the identification of alkoxycarbonyl(cyano)palladium(II) complexes as intermediates in the catalytic cycle of cyanoesterification of norbornene derivatives.

The origin of exo-selectivity in methyl cyanoformate addition onto the CC bond of norbornene in Pd-catalyzed cyanoesterification

“…Besides the exo-selectivity, another remarkable feature of the proposed reaction mechanism 1b (Scheme 1) of the Pd(PPh 3 ) 4 (1)-catalyzed cyanoesterification of NBE is the possibility of isolating the oxidative addition product alkoxycarbonyl-(cyano)palladium(II) complex (Pd II (CN)(COOR)(PPh 3 ) 2 , 2). The addition of alkyl cyanoformates (NC-COOR) to complex 1 can take place at room temperature or below 50 °C in toluene.…”

Methyl Cyanoformate (Mander’s Reagent)

“…15 (G) Shimizu and Murakami have effected the selective formation of a-keto esters by the rhodium-catalyzed reaction of ethyl cyanoformate with arylboronic acids. 7 The observed conversion arises through preferential addition of an intermediate rhodium(I) species to the cyano group rather than to the ester carbonyl group.…”

“…14 It is believed that the complex trans-Pd(CN)(CO 2 Me)(Ph 3 P) 2 is an intermediate in the catalytic cycle associated with this cyanoesterification process. 15 (G) Shimizu and Murakami have effected the selective formation of a-keto esters by the rhodium-catalyzed reaction of ethyl cyanoformate with arylboronic acids. 7 The observed conversion arises through preferential addition of an intermediate rhodium(I) species to the cyano group rather than to the ester carbonyl group.…”

New carboalkoxybis(triphenylphosphine)palladium(II) cationic complexes: Synthesis, characterization, reactivity and role in the catalytic hydrocarboalkoxylation of ethene. X-ray structure of trans-[Pd(COOMe)(TsO)(PPh3)2]·2CHCl3

“…Methods of synthesis are based on the reaction of Pd(II) complexes with CO and an alkanol, eventually in the presence of a tertiary amine, or an alkoxide [12][13][14][15][16][17] or on the oxidative addition of chloro or cyano formate or phenylcarbonate to Pd(0) complexes [18][19][20][21][22]. Most of the syntheses reported up to now are relevant to carbomethoxy derivatives.…”

“…Most of the syntheses reported up to now are relevant to carbomethoxy derivatives. The synthesis of neutral Pd(II) com-plexes of the type trans-[Pd(COOR)X(PPh 3 ) 2 ] (X = Cl and CN) with R bulkier than Me (up to cyclohexyl) has been also reported [19][20][21][22][23][24].…”