C–H Nickelation of Naphthyl Phosphinites: Electronic and Steric Limitations, Regioselectivity, and Tandem C–P Functionalization

Reactions of cyclonickelated complexes with hydroxylamines and TEMPO˙: isolation of new TEMPOH adducts of Ni(II) and their reactivities with nucleophiles and oxidants

This contribution describes a study on the reactivities of the complexes [{κP,κC-(i-Pr)2PO-Ar}Ni(μ-Br)]2, 1a-d (Ar: C6H4, a; 3-Cl-C6H3, b; 3-OMe-C6H3, c; 4-OMe-napthalenyl, d), with hydroxylamines in the presence of TEMPO˙ (TEMPO˙ = (2,2,6,6-tetramethylpiperidinyl-1-yl)oxyl). The results of this study showed that treating 1a-d with a mixture of Et2NOH and TEMPO˙ did not afford the desired oxidation-induced functionalization of the Ni-aryl moiety in 1a-d, giving instead the corresponding κO-TEMPOH adducts [{κP,κC-(i-Pr)2PO-Ar}Ni(Br)(κO-TEMPOH)], 3a-d (TEMPOH = N-hydroxy-2,2,6,6-tetramethylpiperidine). The TEMPOH moiety in these zwitterionic compounds 3 can be displaced by a large excess of acetonitrile (MeCN), 10 equiv. of morpholine, or 1-2 equivalents of imidazole. Although these reactions have given the authenticated products [{κP,κC-(i-Pr)2PO-C6H4}Ni(Br)(NCMe)], 4a, [{κP,κC-(i-Pr)2PO-C6H4}Ni(Br)(morpholine)], 5a, and [{κP,κC-(i-Pr)2PO-C6H4}Ni(imidazole)2]Br, 6a, a few other products were also detected by NMR, indicating that the observed reactivities are far more complex than simple substitution of the TEMPOH moiety. Similarly, treating 3a with AgOC(O)CF3 results in the isolation of [{κP,κC-(i-Pr)2PO-C6H4}Ni{OC(O)CF3}(κO-TEMPOH)], 7a, arising from the substitution of the bromo ligand, whereas spectroscopic evidence suggests further reactivity, possibly including displacement of TEMPOH and oxidative decomposition.

Oxidative Addition of a Phosphinite P-O Bond at Nickel

Oxidative addition is an essential elementary reaction in organometallic chemistry and catalysis. While a diverse array of oxidative addition reactions has been reported to date, examples of P-O bond activation are surprisingly rare. Herein, we report the ligand-templated oxidative addition of a phosphinite P-O bond in the diphosphinito aniline compound HN(2-OPⁱPr₂-3,5-^tBu-C₆H₂)₂ [H(P₂ONO)] at Ni⁰ to form (PONO)Ni(HPⁱPr₂) after proton rearrangement. Notably, the P-O cleavage occurs selectively over an amine N-H bond activation. Additionally, the ligand cannibalization is reversible, as addition of XPR₂ (X = Cl, Br; R = ⁱPr, Cy) to (PONO)Ni(HPⁱPr₂) readily produces either symmetric or unsymmetric (P₂ONO)NiX species and free HPⁱPr₂. Finally, the mechanisms of both the initial P-O bond cleavage and its subsequent reconstruction are investigated to provide further insight into how to target P-O bond activation.

Reactivities of cyclonickellated complexes with hydroxylamines: formation of κO-hydroxylamine and κN-imine adducts and a κO, κN-aminoxide derivative

This report discusses the reactivities of hydroxylamines with a family of nickellacyclic complexes prepared by C-H nickellation of aryl phosphinites. Treating the dimeric complexes κ^C,κ^P-{2-OPR₂,4-R'-C₆H₄}₂Ni₂(μ-Br)₂ (R = i-Pr; R' = H, Cl, OMe, NMe₂) or their monomeric acetonitrile adducts κ^C,κ^P-{2-OPR₂,4-R'-C₆H₄}Ni(Br)(NCMe) with hydroxylamines showed three types of reactivities depending on the Ni complex, the reaction solvent, and the substrate used: (1) the benzyl-protected substrate PhCH₂ONH₂ gave simple N-bound adducts with all Ni complexes; (2) the parent Ni dimer (R' = H) reacted with Et₂NOH and (PhCH₂)₂NOH in CH₂Cl₂ to give, respectively, the zwitterionic amine oxide κ^C,κ^P-{2-OPR₂-C₆H₅}Ni(κ^O-ONHEt₂)Br and the bidentate aminoxide (i-R₂POPh)Ni{κ^O,κ^N-ON(CH₂Ph)₂}Br; (3) the analogous reaction of substituted Ni complexes (R' = Cl, OMe, NMe₂) with hydroxylamines in acetonitrile gave adducts of imines derived from dehydration of Et₂NOH and (PhCH₂)₂NOH. The latter reactivity proceeds optimally in acetonitrile, but it also occurs to a lesser extent in C₆D₆ if the reaction is allowed to go for more than 24 h. Different mechanistic scenarios have been considered to rationalize the observed hydroxylamine → imine transformation.

Structure-Reactivity Relationships of Buchwald-Type Phosphines in Nickel-Catalyzed Cross-Couplings

The dialkyl-ortho-biaryl class of phosphines, commonly known as Buchwald-type ligands, are among the most important phosphines in Pd-catalyzed cross-coupling. These ligands have also been successfully applied to several synthetically valuable Ni-catalyzed cross-coupling methodologies and, as demonstrated in this work, are top performing ligands in Ni-catalyzed Suzuki Miyaura Coupling (SMC) and C-N coupling reactions, even outperforming commonly employed bisphosphines like dppf in many circumstances. However, little is known about their structure-reactivity relationships (SRRs) with Ni, and limited examples of well-defined, catalytically relevant Ni complexes with Buchwald-type ligands exist. In this work, we report the analysis of Buchwald-type phosphine SRRs in four representative Ni-catalyzed cross-coupling reactions. Our study was guided by data-driven classification analysis, which together with mechanistic organometallic studies of structurally characterized Ni(0), Ni(I), and Ni(II) complexes allowed us to rationalize reactivity patterns in catalysis. Overall, we expect that this study will serve as a platform for further exploration of this ligand class in organonickel chemistry as well as in the development of new Ni-catalyzed cross-coupling methodologies.

The Deuterated "Magic Methyl" Group: A Guide to Site-Selective Trideuteromethyl Incorporation and Labeling by Using CD3 Reagents

In the field of medicinal chemistry, the precise installation of a trideuteromethyl group is gaining ever-increasing attention. Site-selective incorporation of the deuterated "magic methyl" group can provide profound pharmacological benefits and can be considered an important tool for drug optimization and development. This review provides a structured overview, according to trideuteromethylation reagent, of currently established methods for site-selective trideuteromethylation of carbon atoms. In addition to CD3 , the selective introduction of CD2 H and CDH2 groups is also considered. For all methods, the corresponding mechanism and scope are discussed whenever reported. As such, this review can be a starting point for synthetic chemists to further advance trideuteromethylation methodologies. At the same time, this review aims to be a guide for medicinal chemists, offering them the available C-CD3 formation strategies for the preparation of new or modified drugs.