Isolation of Transmetalation Intermediates in the Stille Cross‐Coupling Reaction of Stannanes: Synthesis of Palladacycles, Ligand Substitution, and Insertion Reactions

Abstract

A strategy based on a Stille cross-coupling reaction of organostannanes interrupted at the reductive elimination step has been applied to the synthesis of oxa-and azapalladacycles with the general formula cis-[PdArR(L),] . The synthesis of oxapalladacycles was achieved under mild conditions by reaction of 2-iodo-or 2-bromophenyloxymethylstannanes with [Pd(PPh,),]. The synthesis of an aza analogue was similarly carried out from the corresponding 2-iodoaniline derivative. One of the substituted oxapalladacycles rearranged to release steric strain between the palladium and a chloride substituent on the aryl ring, an isomerization promoted by traces of water. 0 P trametalationIn one case, the arylpalladium(I1) intermediate of oxidative addition was isolated by using a palladium(o) complex with a bidentate diphosphane. A variety of new palladacycles, including complexes with weakly coordinating ligands, were prepared by ligand substitution. Reaction of the palladacycles with dimethyl acetylenedicarboxylate led to the formation of chromenes or dihydroquinolines by insertion followed by reductive elimination.

Intramolecular Transmetalation of Arylpalladium(II) and Arylplatinum(II) Complexes with Silanes and Stannanes

“…The isolated palladacycles III are stable species which do not reductively eliminate due to the high ring strain of the four-membered ring heterocycles IV . This truncated intramolecular Stille coupling process was first demonstrated with iodoarylstannanes 1a,b , which cleanly react with [Pd(PPh 3 ) 4 ] to afford the parent oxapalladacycle 2 (Scheme ) . Interestingly, arylpalladium(II) complex 3 , an intermediate of type II , could be isolated as a stable yellow solid from the oxidative addition of 1a to the palladium(0) complex [Pd(dba)dppf]. 18b, This aryliodopalladium(II) complex underwent smooth transmetalation in the presence of Ag 2 CO 3 to form palladacycle 4 18b.…”

“…We have recently reported that derivatives I (X = Br, I) undergo oxidative addition to [Pd(PPh 3 ) 4 ] to give intermediate complexes II , which suffered a rapid transmetalation to form palladacycles III . The isolated palladacycles III are stable species which do not reductively eliminate due to the high ring strain of the four-membered ring heterocycles IV .…”

Organotin Reagents in Cross‐Coupling Reactions

“…The first and third steps appear to be normally slower than the transmetallation step, which is thus the least understood. An intermediate formed after oxidative addition to palladium(0) was isolated by Echavarren et al [5]. Thus, when 2-iodo-or 2-bromophenyloxymethylstannanes were treated with Pd(dba)dppf (dba, dibenzylideneacetone; dppf, 1,1 -bis(diphenylphosphino)ferrocene), an isolable arylpalladium(II) intermediate was formed by oxidative addition of the haloarene to a palladium(0) species.…”

“…Thus, when 2-iodo-or 2-bromophenyloxymethylstannanes were treated with Pd(dba)dppf (dba, dibenzylideneacetone; dppf, 1,1 -bis(diphenylphosphino)ferrocene), an isolable arylpalladium(II) intermediate was formed by oxidative addition of the haloarene to a palladium(0) species. If (Ph 3 P) 4 Pd or Pd(dba)dppe (dppe, 1,2-bis(diphenylphosphino)ethane) was used, transmetallation 4 , Pd(dba)dppf, Pd(dba)dppe Scheme 6.1 Isolation of a Pd/Sn transmetallation intermediate [5].…”

Die Mechanismen der Stille‐Reaktion

“…Vermutlich behindert der chelatisierende dppf‐Ligand die erwartete cyclische Transmetallierung (Abschnitt 3.6). In Gegenwart von Ag 2 CO 3 fand jedoch eine glatte Transmetallierung zum Palladacyclus 43 statt 190b. Unter diesen Bedingungen führt die Abstrahierung von Iodid durch das Silbersalz zu einem kationischen Acetonitrilkomplex, dessen Transmetallierung über einen offenen Übergangszustand begünstigt ist.…”