Selectivity in Competitive C_sp²–C_sp³ versus C_sp³–C_sp³ Reductive Eliminations at Pt(IV) Complexes: Experimental and Computational Approaches

Abstract

Three classes of Pt(II) complexes of the types [PtMe 2 (NHC)] (NHC = 1,1′-dimethyl-3,3′-methylenediimidazolin-2,2′-diylidene), [PtMe 2 (N^N)] (N^N = 2,2′-bipyridine, 1,10phenanthroline), and [PtMe(C^N)(SMe 2 )] (C^N = 2-phenylpyridinate, benzo[h]quinolinate), with a systematic variation in the chelating ligand, were chosen to investigate the oxidative addition of Pt(II) complexes with acetyl halides and the ensuing C−C reductive elimination from the corresponding Pt(IV) complexes.In accordance with the nature of the chelating ligand, the following conclusions can be ascertained: (i) in the reaction of [PtMe 2 (NHC)] complex 1a with acetyl halides, the order of the energy barriers (kcal/mol) for C−X bond activation is computed as Cl (24.4) ≈ Br (23.7) ≈ I (23.6), which are in a narrow range, showing no significant halide effect; (ii) while the energy barriers for oxidative addition of acetyl halides to complex 1a are almost the same, the C sp 3 −C sp 2 (CH 3 −COMe) reductive elimination barriers from [PtMe 2 (NHC)(MeCO)X] for X = Cl, Br, I are 23.5, 16.3, and 10.3 kcal/mol, respectively (lower than the acetyl halide oxidative addition to [PtMe 2 (NHC)]), suggesting that, when X = I, the Pt(IV) complex is more prone to undergo reductive elimination in comparison to the related Cl and Br analogues; (iii) the NHC Pt(IV) complexes selectively form C sp 3 −C sp 2 bonds instead of C sp 3 −C sp 3 bonds in reductive elimination; (iv) while NHC Pt(IV) complexes undergo a C−C reductive elimination process, the C−C coupling reaction is quite difficult for [PtCl(COMe)Me 2 (N^N)] complexes because of the high energy barrier found for C−C bond formation versus C−Cl oxidative addition; (v) similarly to NHC Pt(IV) complexes, the cycloplatinated (IV) complexes [PtCl(COMe)Me(C^N)(SMe 2 )] readily undergo C−C bond formation with reductive elimination.

Reductive Cross-Electrophile C(sp2)–C(sp3) Coupling Catalyzed by PtI2: In Situ Structural Determination of the Intermediates by X-ray Absorption Spectroscopy and Multinuclear NMR

“…Similarly, no C­(sp 3 )–C­(sp 3 ) coupling was detected from bis-acetonyl and bis-carboxymethyl platinum­(IV) complexes in acetone solutions of NaI . These experimental data are in line with theoretical predictions for the largest barrier for the C­(sp 3 )–C­(sp 3 ) reductive elimination elementary step in the series C­(sp 3 )–C­(sp 3 ) > C­(sp 2 )–C­(sp 3 ) > C­(sp 2 )–C­(sp 2 ). − Thus, as might be expected, the chemoselectivity of the overall cross-electrophile coupling reaction could be controlled not only by oxidative addition steps but also by the reductive elimination step.…”

Cross-Coupling versus Homo-Coupling at a Pt(IV) Center: Computational and Experimental Approaches

“…Reductive elimination (RE) reactions are part of important catalytic processes. − RE with concomitant C–C cross-coupling plays a key role in the synthesis of organic molecules, including natural products and drugs. − RE from organometallic d 6 -configured octahedral Pt­(IV) and Pd­(IV) complexes with the concomitant formation of alkanes by Csp 3 –Csp 3 cross-coupling has been extensively studied, − while investigations on homo-coupling reactions are scarce. − …”

“…If more than two C ligands are bound to a Pt­(IV) or Pd­(IV) center, the selectivity of the reductive elimination products follows essentially two rules: (i) cis -elimination is largely preferred over trans -elimination , and usually also over radical processes. ,, (ii) If the C–C bond formation is the rate-determining step, the hybridization of the C ligand (alkyl: sp 3 ; aryl, alkenyl or vinyl: sp 2 ; alkynyl: sp) usually governs the C–C bond formation, and a preference of sp 3 –sp 3 > sp 3 –sp 2 > sp 2 –sp 2 > sp 3 –sp > sp 2 –sp > sp–sp is observed . However, steric and further electronic factors such as substituents on the C ligands, the trans -influence of the other ligands, and the polarity of the solvent can affect the reactivity and preferences deviating from this rule. ,,− RE of C–C bonds is common for Pt­(IV) and Pd­(IV) complexes with various ligands including phosphines and diimines ,− ,,− and can be competitive to C–X formation. ,,− …”

Comparative Study of Pd-Mediated Carbon–Carbon, Carbon–Heteroatom, and Heteroatom–Heteroatom Bond Formation/Breakage (C ═ Csp3, Csp2, Csp; X = B, N, O, Si, P, S, Se, Te)

“…Most theoretical studies of the mechanisms of coupling reactions are focused on carbon–carbon bond formation. − However, a significant number of studies on C–X, X–X, and X–Y reductive elimination (X and Y are heteroatoms) have also been reported. The most studied reactions are C–N, ,− C–P, C–O, ,, C–S, ,− C–B, − C–Se, , C–Te, C–Si, − C–F, − and C–Cl, ,, reductive elimination, while several papers on X–X and X–Y reductive elimination have been reported: Hal–Hal, B–B, S–S, Se–Se, Te–Te, Si–Si, , and Si–Sn …”