Rearrangement of N-Heterocylic Carbenes Involving Heterocycle Cleavage

Abstract

Materials and Methods.All reactions involving metal complexes were conducted in oven-dried glassware under a nitrogen atmosphere using standard glovebox techniques. Solvents were prepared by passage through alumina. All commercially obtained reagents were used as received. Organic reagents were purchased from Sigma-Aldrich and metal salts obtained from Strem. NiClPh(PPh 3 ) 2 1 and NiBrMes(PPh 3 ) 2 2 were prepared according to literature procedures. The synthesis of compound 1 is described elsewhere. 3 1 H, 13 C and 31 P NMR spectra were recorded on a Varian Mercury 300 spectrometer (at 300 MHz, 75 MHz and 121 MHz respectively) or a Varian Inova 500 spectrometer (at 500 MHz, 125 MHz and 203 MHz respectively) and are reported relative to Me 4 Si (δ 0.0) for 1 H and 13 C, and H 3 PO 4 (δ 0.0) for 31 P. Data for 1 H NMR spectra are reported as follows: chemical shift (δ ppm), multiplicity, coupling constant (Hz) and integration. Data for 13 C and 31 P NMR spectra are reported in terms of chemical shift. N-(2,6-Diisopropylphenyl)-N-benzylidene-N´-(2-hydroxy-3-(adamant-1-yl)-5-methylphenyl)-ethylene diimine triphenylphosphine nickel(II) (2).1-(2,6-Diisopropylphenyl)-3-(2-hydroxy-3-(adamant-1-yl)-5-methylphenyl)-4,5-dihydroimidazolium chloride (1) (162 mg, 0.320 mmol, 1.00 equiv) and potassium hexamethyldisilazide (134 mg, 0.670 mmol, 2.10 equiv) were weighed together in a vial in the glovebox. THF (~10 mL) was added to the mixture of solids, providing a light yellow solution with a light precipitate. This was added to a round-bottomed flask and allowed to stir for ten minutes. At this point, a solution of NiClPh(PPh 3 ) 2 (223 mg, 0.320 mmol, 1.00 equiv) in THF (5 mL) was added, giving a dark green solution with precipitate. This solution was allowed to stir at room temperature for 1 hr and then was filtered through Celite. The solvent was removed under reduced pressure until ca. 2 mL remained. Pentane (~15 mL) was added and the solution was allowed to sit at -40 °C overnight, yielding a dark green solid (163 mg, 0.190 mmol, 59% yield). Crystals suitable for X-ray crystallography were grown by layering pentane over a concentrated solution of 2 in THF and storing this layered solution at -40 °C for two days. The 1 H NMR spectrum of 2 showed some broad peaks attributed to a fluxional process on the NMR timescale. This is most likely due to restricted rotation of the diisopropylphenyl and phenyl moieties, which are adjacent in the X-ray crystallographic structure. The 13 C NMR spectrum of 2 features many fewer resonances than expected. This may be due to the fact that several resonances may overlap, e.g. from the triphenylphosphine and adamantyl moieties. In addition, the fluxional processes described above may weaken some signals. 1

Robust and Electron‐Rich cis‐Palladium(II) Complexes with Phosphine and Carbene Ligands as Catalytic Precursors in Suzuki Coupling Reactions

“…The successful [15] Similar to the situation of the ligand precursors described above, the more downfield carbene carbon atom in saturated carbene complexes indicates that the carbene carbon atom is much more electrophilic, such that intramolecular nucleophilic attack might occur readily. [41] Noticeably, these carbene signals are all singlets; the zero 2 J coupling constants strongly indicate that they are cis complexes, which was confirmed subsequently by X-ray structural studies.…”

“…[40] Ring-opening reactions of NHCs has also been observed by Grubbs et al on a saturated carbene nickel complex. [41] The cleavage is, however, carried out by means of an intramolecular attack by a phenyl group on the coordinated carbene.…”

Nickel N-heterocyclic carbene complexes and their utility in homogeneous catalysis

“…In particular, the (533−534), [163] (535), [164] (536), [165] (543−544), [166] (545−546), [167] (548 [168]−557), [169] (558), [51] (559), [71] (560), [161] (561), [170] (562−563), [141] (565), [171] (566−567), [172] (568), [173] (569), [126d] (570−573), [174] (574), [175] (575−576), [176] (577), [177] (578), [178] (579), [106] (580−581), [179] (582−583), [151] (584−585), [180] (586−588), [181] (595−596), [182] (597−599), [183] (600−603), [184] (604), [185] (605−608), [186] (609), [185] (610−613), [187] (614−618), [188] (619−621), [189] (622−623), [185] (624−630), [190] (631), [191] (633−635), [192] (636), [193] (637), [194] (647) [195] and (648) [196] complexes were obtained by this pathway from the treatment of the free or the in situ generated NHC The third route is the oxidation method (Scheme 15) using which the (589−590), [197] (638) …”

DFT study of the reactivity of methane and dioxygen with d10-L2M complexes

“…Among this family of organometallic complexes, N-heterocyclic carbene (NHC) complexes have been particularly well studied since the seminal work of Öfele and Wanzlick [4]. NHC complexes have seen a wide range of applications from Arduengo's pioneering studies [5] to Grubbs' olefin metathesis catalysts [6]. In some sense, NHC's lie between Schrock-and Fischer-type carbenes in terms of their electronic structure.…”