C–H nickellation of phenol-derived phosphinites: regioselectivity and structures of cyclonickellated complexes

Abstract

This report describes the results of a study on the ortho-C-H nickellation of the aryl phosphinites i-PrP(OAr) derived from the following four groups of substituted phenols: 3-R-CHOH (R = F (b), Me (c), MeO (d), Cl (e)); 3,5-R-CHOH (R = F (f), Me (g), Cl (h), OMe (i)); 2-R-CHOH (R = Me (j), Ph(k)); and 2,6-R-CHOH (R = Me (l), Ph (m)). No nickellation was observed with the phosphinites derived from the 3,5-disubstituted phenols g and h, and the 2,6-disubstituted phenols l and m; in all other cases nickellation occurred at an ortho-C-H to generate either the Br-bridged dimers [{κ,κ-(i-Pr)POAr}Ni(μ-Br)] (1b-1f, 1j, and 1k) or the monomeric acetonitrile adduct {κ,κ-ArOP(i-Pr)}Ni(Br)(NCMe) (1i-NCMe). Analysis of C-H nickellation regioselectivity with 3-R-CHOH pointed to the importance of substituent sterics, not electronics: nickellation occurred at the least hindered position either exclusively (for R = Me (c), MeO(d), and Cl (e)) or predominantly (for R = F (b); 6 : 1). This conclusion is also consistent with the observation that C-H nickellation is possible with the 3,5-disubstituted aryl phosphinites bearing F and OMe, but not with the more bulky substituents Me or Cl. For the 2-substituted aryl phosphinites, C-H nickellation occurs at the unsubstituted ortho-C-H and not on the R substituent, regardless of whether the alternative C-H moiety of the substituent is sp (R = Me (j)) or sp (R = Ph (k)). The system thus reveals a strong preference for formation of 5-membered metallacycles. Consistent with this reactivity, no nickellation occurs with (2,6-R-CHO)P(i-Pr). Tests with the parent dimer derived from i-PrP(OPh) showed that conversion to the monomeric acetonitrile adduct is highly favored, going to completion with only a small excess of MeCN. All new cyclonickellated complexes reported in this study were fully characterized, including by single crystal X-ray diffraction studies. The solid state structures of the dimers 1b and 1d showed an unexpected feature: two halves of the dimers displayed non-coplanar conformations that place the two Ni(ii) centers at shortened distances from each other (2.94-3.16 Å). Geometry optimization studies using DFT have shown that such non-coplanar conformations stabilize the complex, implying that the "bending" observed in these complexes is not caused by packing forces. Indeed, it appears that the occurrence of coplanar conformations in the solid state structures of these dimers is a simple consequence of packing forces rather than an intrinsic property of the compound.

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

“…While this type of catalyst deactivation is well known with Pd for ligands without 2,6substituted B-rings, for the C-H activation to occur with Ni rapidly, under such mild conditions, and with a non-chelating ligand is remarkable. [49][50][51][52][53][54][55][56][57] It is also notable that this cyclometalation occurs significantly faster than in the analagous CyJohnPhos/Pd system reported by Fink. 15 Computationally, we were unable to locate a reasonable transition state from ii, the L2Ni(II) structure, or the µ-Cl dimer that would lead to 5.…”

A Wacker-Type Strategy for the Synthesis of Unsymmetrical POCsp3E-Nickel Pincer Complexes

“…We conclude, therefore, that the zwitterionic species α and β give more accurate descriptions of this complex. The 1 H and 13 C NMR spectra of 3 (Figures S29−S41) indicate that the α form shown in Scheme 5 and featuring a nickellacyclopropane moiety is the prevalent resonance structure of compound 3 in solution. Thus, the two C−H moieties bound to the metal center gave signals in the aliphatic region: δ 1 H at 2.04 and 2.95 ppm; δ 13 C at 18.63 and 37.17 ppm.…”

“…This study began by preparation of the target substrate 1 and examination of its reactivity under the conditions used for our previous nickellation studies. , Thus, 1 was treated in acetonitrile with {( i -PrCN)­NiBr 2 } n and Et 3 N (molar ratio of 1.0:1.2:1.2), and the reaction progress was monitored by NMR spectroscopy. 31 P NMR spectral analysis of the greenish mixture obtained after 2 h of stirring at room temperature showed that the initial signal for 1 had been replaced almost completely by two new 31 P resonances at 103 and 202 ppm.…”

“…12 In other words, ring substituents were found to be largely inert to nickellation. 13 The only exception to this regioselectivity was observed with 2-allyl-1-naphthyl-OP(i-Pr) 2 (Scheme 2, reaction B). 14 This observation was quite unexpected as the normally less facile nickellation of a substituent C sp 3 −H moiety proceeded in preference over reactivity at the ring C−H.…”

“…Earlier studies had shown that in most cases nickellation of aryl phosphinites occurs regioselectivity at the unsubstituted C–H moiety ortho to the OPR 2 position, as illustrated in Scheme (reaction A) . In other words, ring substituents were found to be largely inert to nickellation . The only exception to this regioselectivity was observed with 2-allyl-1-naphthyl-OP­( i -Pr) 2 (Scheme , reaction B) .…”