Phenylchromium(III) Chemistry Revisited 100 Years afterFranz Hein(Part III): From (Ar)_3–nCrCl_n(L)_x(Ar = Ph,p-F-C₆H₄, C₆D₅;n= 0, 1, 2; L = thf, dme, tmeda, 12C4) to Bis(π-arene)chromium Complexes

Abstract

One century ago, Franz Hein started the highly regarded arylchromium chemistry with his initial publication in January 1919. The formation of bis(π-arene)chromium compounds according to Hein's protocols has been performed with pure substrates of the type (Ar) 3−n CrCl n (L) x (n = 0, 1, 2; Ar = Ph, C 6 D 5 , p-F-C 6 H 4 , L = neutral Lewis bases with different base strength and denticity) as model compounds at magnesium halide-free conditions. Three main synthesis phases for the σ−π transfer of the phenyl groups have been recognized and simulated: Initial phase (excess of MgPh 2 ): Ph 3 Cr(thf) 3 •0.25 dx•(3-thf 3 , dx = 1,4-dioxane) eliminates tetrahydrofuran (THF) reversibly in C 6 D 6 , yielding bluish-green [Ph 3 Cr(thf) 2 ] (3thf 2 ) that reacts above 5 °C irreversibly to a black-brown solution of biphenyl, benzene, and "{(η 6 -Ph 2 )CrI(μ-η 6 -Ph)} 2 " (intermediate A). Ph 2 Mg reduces A to "(η 6 -Ph 2 )Cr 0 (η 6 -PhMgPh)" (intermediate B). Middle phase: Subsequent reactions of A: (i) Reduction to [(η 6 -C 6 H 6 )(η 6 -Ph 2 )Cr 0 ] (π-3) by THF, (ii) disproportionation to [(η 6 -Ph 2 ) 2 Cr 0 ] (π-4) and "[(η 6 -C 6 H 5 ) 2 Cr II ] n " that is reduced to "(η 6 -C 6 H 5 MgPh) 2 Cr 0 " by MgPh 2 , (iii) reductive coupling yielding [{(η 6 -Ph 2 )Cr} 2 (μ-η 6 :η 6 -Ph 2 )] ((π-3) 2 ), or (iv) decomposition to biphenyl and chromium. Labeling experiments with (D 5 C 6 ) 3 Cr(thf) 3 •0.25 dx ([D 15 ]3-thf 3 ) in C 6 D 6 and [fac-(p-F-C 6 H 4 ) 3 Cr(thf) 3 ] (3 F -thf 3 ) allow one to determine the origin of the hydrogen atom and the identification of the end products. Final phase: Deficit of Mg(Ar) 2 hinders formation of bis(π-arene) chromium complexes. "Ph 2 CrCl(thf) x " forms an equilibrium in THF solution with 3-thf 3 and PhCrCl 2 (thf) x (1-Cl). During heating of a solution of 3-thf 3 , CrCl 3 (thf) 3 , and H 2 O (3:1:1) in THF o-metalation of a phenyl group leads to formation of mixed-valent tetranuclear [Ph 4 Cr 4 (μ 4 -OH)(μ-Cl) 2 (μ 3 -Cl)(μ 3 -C 6 H 4 )(thf) 4 ] (4-Cr 4 ) besides biphenyl and benzene. The analogous reaction of [D 15 ]3-thf 3 , CrCl 3 (thf) 3 , and H 2 O yields [D 5 ]benzene, [D 10 ]biphenyl, and heptanuclear oxo-centered [Cr 7 II (μ-Cl) 10 (μ 4 -O) 2 (thf) 6 ] (0-Cr 7 ). Complexes of the type [(Ar)CrCl 2 (thf) n ] (Ar = Ph, Mes, p-F-C 6 H 4 ) show homolytic Cr−C bond cleavage, leading to chromium(II) chloride and biaryls.

Chromium-Catalyzed syn-Selective Ring-Opening Aryl-/Alkylation of 7-Oxabenzonorbornadiene Derivatives with Grignard Reagents

“…No decomposition of 10 was observed in THF at 30 °C over 24 h (see eq S4 in the Supporting Information), whereas the treatment of 10 with 10 equiv of phenylmagnesium bromide ( 2d ) at 30 °C for 24 h resulted in a significant color change from deep red to black with the formation of 9d (1.0 equiv with respect to 10 ), indicating the generation of diphenylchromate(I) species in the reaction mixture (Scheme a). Addition of MgBr 2 (thf) 3 also promoted reductive elimination of 9d from 10 , probably due to the interaction of bromide to the chromium center: in fact, elimination of bromide salts was indispensable for the isolation of 10 using MgPh 2 (1,4-dioxane) . When 1.0 equiv of 1a was added to the mixture of 10 and 10 equiv of PhMgBr in THF, 3ad was obtained in 96% yield within 10 min (Scheme b), consistent with the result shown in Scheme b. , Based on these observations, we propose that a tetraarylchromate(III) species was initially generated in the presence of more than 4.0 equiv of the Grignard reagents and its spontaneous reductive elimination resulted in the formation of a diarylchromate(I) species as a catalytically active species.…”

“…In sharp contrast, the reaction in the presence of 3.0 equiv of 2a with respect to CrCl 3 (thf) 3 at −20 °C, a typical condition for generating CrAr 3 (thf) 3 , resulted in no formation of 3aa . According to the requirement of at least 4.0 equiv of 2a for the ring-opening arylation of 1a , tetraphenylchromate­(III) species 10 was alternatively synthesized and applied to the ring-opening reaction. No decomposition of 10 was observed in THF at 30 °C over 24 h (see eq S4 in the Supporting Information), whereas the treatment of 10 with 10 equiv of phenylmagnesium bromide ( 2d ) at 30 °C for 24 h resulted in a significant color change from deep red to black with the formation of 9d (1.0 equiv with respect to 10 ), indicating the generation of diphenylchromate­(I) species in the reaction mixture (Scheme a).…”