Carbon−Hydrogen Bond Oxidative Addition of Partially Fluorinated Aromatics to a Ni(PⁱPr₃)₂Synthon: The Influence of Steric Bulk on the Thermodynamics and Kinetics of C−H Bond Activation

Abstract

The reaction of (P i Pr 3 ) 2 NiCl 2 with the anthracene adduct (THF) 3 Mg(η 2 -C 14 H 10 ) in THF provides the anthracene adduct (P i Pr 3 ) 2 Ni(η 2 -C 14 H 10 ). In aromatic solvents (benzene, toluene, mesitylene) a thermal equilibrium exists between the bis(phosphine)nickel(0) anthracene adduct, (P i Pr 3 ) 2 Ni(η 2 -C 14 H 10 ), and the monophosphine solvent adduct, (P i Pr 3 )Ni(η 6 -solvent). The reaction of (P i Pr 3 ) 2 Ni(η 2 -C 14 H 10 ) with 1,2,4,5-C 6 F 4 H 2 affords the C-H activation product trans-(P i Pr 3 ) 2 NiH(2,3,5,6-C 6 F 4 H). The thermodynamic C-F activation product is not obtained even after hours of heating at 100 °C. Similar reactions with 1,2,3,5-C 6 F 4 H 2 and pentafluorobenzene produce the desired C-H activation products, trans-(P i Pr 3 ) 2 NiH-(2,3,4,6-C 6 F 4 H) and trans-(P i Pr 3 ) 2 NiH(C 6 F 5 ), respectively, in >95% yield. The reaction with 1,2,3,4tetrafluorobenzene did not produce an observable C-H activation product. Unlike previously reported analogous C-H activation products with Ni(PEt 3 ) 2 synthons, the bulkier Ni(P i Pr 3 ) 2 moiety did not provide observable mononuclear or dinuclear η 2 -fluoroarene adducts. Solutions of Ni(COD) 2 with 2 equiv of triisopropylphosphine and 1,2,4,5-C 6 F 4 H 2 reacted to give the 1,5-cyclooctadiene insertion and rearrangement product, (η 3 -C 8 H 13 )Ni(P i Pr 3 )(2,3,5,6-C 6 F 4 H). The same reaction with 1,2,3,5-and 1,2,3,4-C 6 F 4 H 2 afforded analogous compounds, which demonstrates that C-H bond activation is kinetically accessible at room temperature with 1,2,3,4-tetrafluorobenzene despite the presence of a single ortho-fluorine substituent adjacent to the site of activation. The room-temperature reactions of the C-H activation products (P i Pr 3 ) 2 NiH(Ar F ) (Ar F = 2,3,5,6-C 6 F 4 H; C 6 F 5 ) with 3-hexyne provided a mixture of the alkyne adduct (P i Pr 3 ) 2 Ni(η 2 -EtCtCEt), with the liberation of Ar F H, and the insertion product (P i Pr 3 ) 2 Ni(CEtdCHEt)(Ar F ), even in the presence of excess fluorinated aromatic Ar F H. The reaction of (P i Pr 3 ) 2 Ni(η 2 -EtCtCEt) with 1,2,4,5-C 6 F 4 H 2 resulted in no reaction at room temperature, but heating at 50 °C provided the insertion product (P i Pr 3 ) 2 Ni(CEtdCHEt)(Ar F ) as the initial product, followed by the product of reductive elimination, 1,2,4,5-C 6 F 4 H-CEtdCHEt. In contrast, the reaction of (PEt 3 ) 2 Ni(η 2 -EtCtCEt) with 1,2,4,5-C 6 F 4 H 2 at 80 °C slowly produced (PEt 3 ) 2 Ni(CEtdCHEt)(2,3,5,6-C 6 F 4 H), but very little 1,2,4,5-C 6 F 4 H-CEtdCHEt.

Mechanistic insight into carbon–fluorine cleavage with a ( Pr3P)2Ni source: Characterization of ( Pr3P)2NiC6F5 as a significant Ni(I) byproduct in the activation of C6F6

“…As previously noted in the literature, the reaction of 1 with 1,2,4,5-C 6 F 4 H 2 proved to be the most successful in synthesizing an isolable C-H bond activation product, trans-( i Pr 3 P) 2 NiH(2,3,5,6-C 6 F 4 H) (8), with no observable impurities and was remarkably stable up to 100°C in toluene [14]. Solutions of 8 could stand for weeks without any spectroscopic evidence of isomerization to the thermodynamic C-F bond activation product.…”

“…10°degrees larger than in 2 [14]. The Ni(1)-C(1) bond length in 3 is 1.973(2) Å, which is 0.085(5) Å longer in comparison to compound 2.…”

“…It has been shown that the reaction of the anthracene adduct 1 with C 6 F 5 H provides the C-H activated complex trans-( i Pr 3 P) 2 NiH(C 6 F 5 ) (5) in high spectroscopic yields, although it is not isolable [14]. Analysis of the crude reaction mixture showed there were less than 5% of other impurities that could be identified as the C-F activation products: the para-F activation compound trans-( i Pr 3 P) 2 NiF(2,3,5,6-C 6 F 4 H) (6), and the ortho-F activation product trans-( i Pr 3 P) 2 NiF(2,3,4,5-C 6 F 4 H) (7) in a ratio of 6:1.…”

“…A toluene solution of 1 and 1,2,3-C 6 F 3 H 3 on an NMR scale reacts slowly to produce the C-F activation products, trans-( i Pr 3 P) 2-NiF(2,3-C 6 F 2 H 3 ) (14) and trans-( i Pr 3 P) 2 NiF(2,6-C 6 F 2 H 3 ) (15) in low yields. At the end of one week, the reaction had still not gone to completion; however a dark precipitate of nickel metal was present in the reaction tube.…”

“…In previous work [14] we have shown that the anthracene adduct ( i Pr 3 P) 2 Ni(g 2 -C 14 H 10 ) (1) can successfully C-H activate partially fluorinated aromatics in high yields with occasional impurities attributed to C-F activation compounds. In general these nickel-hydrides are stable at room temperature for days.…”