Synthesis of 2-Nickela(II)oxetanes from Nickel(0) and Epoxides: Structure, Reactivity, and a New Mechanism of Formation

Abstract

2-Nickelaoxetanes have been frequently invoked as reactive intermediates in catalytic reactions of epoxides using nickel, but have never been isolated or experimentally observed in these transformations. Herein, we report the preparation of a series of well-defined nickelaoxetanes formed via the oxidative addition of nickel(0) with epoxides featuring ketones. The stereochemistry of the products is retained, which has not yet been reported for nickelaoxetanes. Theoretical calculations support a bimetallic ring-opening/closing pathway over a concerted oxidative addition. Initial reactivity studies of a nickelaoxetane demonstrated protonolysis, oxidatively induced reductive elimination, deoxygenation, and elimination reactions when treated with the appropriate reagents.

The Importance of Ligand‐Induced Backdonation in the Stabilization of Square Planar d10 Nickel π‐Complexes

“…DFT calculations were performed on each of the species in Table using simplified diphosphine ligands (see Figure SI 07 for details, Supporting Information). Molecular structures derived from B3LYP/def2‐TZVP calculations of the dtbpe complexes, as well as those using a simplified diphosphine ligand (dmpe=1,2‐bis(dimethylphosphino) ethane), yield good agreement with solid‐state molecular structures of 1 – 12 . The effect of decreasing the steric bulk and electron donation in the supporting diphosphine ligand does not affect the general structural trends and conclusions, which are consistent with those observed in the experimental data.…”

“…In previous work, we noted that the 31 P{ 1 H} NMR spectroscopic data of a number of (dtbpe)Ni [dtbpe=1,2‐bis(di‐ tert ‐butyl)phosphinoethane] π‐complexes were consistent with typical d 10 Ni 0 complexes (Table ) . In contrast, we also noted that the distorted square planar geometry with significant elongation of the π‐bond was most consistent with a d 8 Ni II formulation, in keeping with the metallaepoxide extreme of the Dewar–Chatt–Duncanson (DCD) model of bonding (Scheme ).…”

“…Ni(COD) 2 ( 13 ) was purchased from Strem and used as received. Compounds 1 – 2 , 3 , 4 , 5 – 6 , 7 , 8 – 10 , 11 , 12 , 14 , and 15 were prepared according to literature procedures. All other chemicals were purchased from commercial suppliers and used as received.…”

“…We have recently become interested in exploring the fundamental organometallic chemistry of earth-abundant, first-row transition metals. For example, we are exploring the organometallicc hemistry of nickel, [10][11][12][13][14] which has undergone ar enaissance in recent years. [15][16][17][18][19][20][21] Our focus has been the structure and reactivityo fn ickel p-complexes, which have been reported in aw ide range of catalytic processes, including the coupling of CO 2 and ethylene, [5,[22][23][24][25][26][27][28][29][30][31][32] intermolecular Tischenkoc oupling, [33][34][35] benzoxasilole synthesis, [36,37] the aldol reaction, [38] allylic alkylation, [39] allylic amination, [40] allylic amidation, [41] epoxide functionalization, [42] and Suzuki-Miyaura coupling.…”

Catalytic Functionalization of Styrenyl Epoxides via 2‐Nickela(II)oxetanes

“…[11] In contrast, recent work with both Al/Co [14] and Rh [15] has demonstrated epoxide isomerization that proceeds through nucleophilic attack typically form ketones,a lthough an otable exception is the Pd system reported by Kulsagaram and Kulawiec [16] To exploret he ring-opening step, we prepared epoxide cis-12 and reacted it with 1.M onitoring the reaction at 60 8Cu sing 31 P{ 1 H} NMR spectroscopy reveals the formationo fa na symmetrici ntermediate ( 2 J P, P = 29 Hz) as well as af inal nickel(II) product, 13 ( 2 J P, P = 13 Hz). Although our previously isolated nickelaoxetanes were also formed with retention of configuration, [8] earlier work by Jamsion invoked the formation of nickelaoxetane intermediates via an S N 2-type mechanism (i.e.,w ith inversion of configuration). Surprisingly,t he 1 HNMR spectrum of 13 showed an ABX spin system inconsistent with the expected nickelaoxetane product.…”