Multidonor Ligands. 3. Cyclopalladation of py{SCH₂C(O)R}-2 (py-2 = 2-pyridyl, R = Ph, Me, OMe) through a C(sp³)−H Bond Activation

Abstract

[PdCl2(NCPh)2] reacts with R−carbonylmethylenethiopyridines [py{SCH2C(O)R}-2] to give, depending on the substituent R and the reaction conditions, or the coordination complexes [Pd{py{SCH2C(O)R}-2}2Cl2] (R = Ph, Me, OMe). The latter can be doubly deprotonated by a base (Na2CO3 or NaH) to give dicyclometalated complexes These transmetalate one ligand to [PdCl2(NCPh)2] to give which in turn reacts with (Me4N)Cl or with monodentate ligands L or with MeCN in the presence of AgClO4 to give anionic neutral [R = Ph, L = PPh3, py{SCH2C(O)Ph}-2, tBuNC, NH2CH(Me)Ph, pyridine (py), tetrahydrothiophene (tht); R = Me, L = PPh3, py, tht], or cationic (R = Ph, Me, OMe) complexes, respectively. The latter react with PPh3 or with bidentate ligands to give complexes or [L = PPh3, L2 = N,N,N‘,N‘-tetramethylethylenediamine (tmeda) or 2,2‘-bipyridine (bipy)]. The crystal structures of and have been determined.

Stepwise cyclopalladation of 2-phenacylthiopyridine to give C,C,N-pincer complexes

“…Scheme 1 shows the results, previously reported by us, 13 on the reactivity of [PdCl 2 (NCPh) 2 ] toward various 2-R-carbonylmethylenethiopyridines pyl-SCH 2 C(O)R ( pyl = 2-pyridyl, R = Ph, Me, OMe) including dehydrohalogenation reactions, spontaneous or induced by the use of a base such as Na 2 CO 3 or NaH, to give palladacyclic complexes of the type B or C, respectively. However, further deprotonation of the methine proton or of the R group did not occur even when an excess of these bases was used.…”

“…With the purpose of further studying dehydrohalogenation reactions on complexes [Pd{C,N-pyl-SCHC(O)R}(μ-Cl)] 2 (D in Scheme 1) to give C,C,N-pincer complexes, we attempted an alternative synthesis of 1•Cl (D, R = Ph) because the previous methods were rather cumbersome ([PdCl 2 (NCPh) 13 Thus, a double deprotonation of [Hpyl-SCH 2 C(O)Ph]Cl ( pyl-= 2-pyridyl), 26 with Pd(OAc) 2 (1 : 1, refluxing in acetone for 3 h), gave 92% yield of 1•Cl in a single step. Similarly, its homologous [Pd{C,N-pyl-SCHC(O)Ph}-(μ-Br)] 2 (1•Br) was obtained in 83% yield from [pylH-SCH 2 C(O)-Ph]Br 26 and Pd(OAc) 2 (1 : 1, refluxing in acetone for 3 h).…”

“…This is confirmed by the shift of the ν(CO) frequency towards the low energy region in all C,N-cyclopalladated complexes compared to that in N-coordinated [Pd]-pyl-SCH 2 C(O)Ph complexes. 13 In fact, we have failed other attempts to deprotonate the methine group (see below). In addition, an intermediate similar to I involving the methine proton would probably be less favored from a steric point of view.…”

“…7 Cyclopalladation reactions involving a C(sp 2 )-H activation are by far the best known while those involving a C(sp 3 )-H bond are in the minority, 2,[8][9][10][11][12] those concerning a prochiral C(sp 3 ) atom being particularly rare. 11,12 We have previously reported the cyclopalladation 13 and cycloauration 14 of 2-R-carbonylmethylenethiopyridines pyl-SCH 2 C(O)R ( pyl = 2-pyridyl, R = Ph, Me, OMe). By reacting these ligands with [PdCl 2 (NCPh) 2 ], or the coordination complexes [Pd{pyl-SCH 2 -C(O)R} 2 Cl 2 ] (A, Scheme 1) with Na 2 CO 3 or NaH, chiral complexes with one (B) or two (C) five-membered C,N-metallacycles were prepared (Scheme 1).…”

“…Transmetallation reactions between C and [PdCl 2 (NCPh) 2 ] or deprotonation of B with Na 2 CO 3 allowed the syntheses of dinuclear complexes D. Reactions of complex B or D (R = Ph) with excess Na 2 CO 3 , which we carried out in an attempt to prepare pincer complexes resulting from the additional cyclometalation of the phenyl group, were unsuccessful. This was not surprising because, although double C-H activation processes leading to various types of doubly cyclopalladated compounds are well known, 15 cyclometalation reactions involving two C-H bond activations per metal atom are very rare, not only for palladium 8,9,13 but also for any metal. 16 However, in this paper we report that using AgOAc as a base it is possible to prepare, from complex D (R = Ph), pincer complexes bearing a dianionic C(sp 2 ),C(sp 3 ),Ndonor ligand.…”