NHC‐Stabilized Triorganozincates: Syntheses, Structures, and Transformation to Abnormal Carbene–Zinc Complexes

Abstract

The chemistry of organozinc compounds has been inextricably entwined with the development of organic synthetic methods since Franklands seminal discovery of zinc alkyls in 1849. [1,2] Compared to diorganozinc reagents, the corresponding anionic zincate derivatives are considerably better nucleophiles. [3][4][5][6][7][8] Consequently, triorganozincates [R 3 Zn]M (M = alkali metals) and tetraorganozincates [R 4 Zn]M n (M = alkali metals, n = 2; M = alkali earth metals, n = 1) have been extensively utilized in organic transformations such as halogen-metal exchanges, [9][10][11][12] nucleophilic additions, [1,2] deprotonative metalations, [6][7][8] and epoxide ring-openings. [9,10] Recently, Mulvey et al. employed TMP-zincates (TMP = 2,2,6,6-tetramethylpiperidide) in developing a zincationanion trapping strategy [13] and in realizing the unusual transformation of a diamine into an unsaturated diazaethene through a lithium/zinc bimetallic system. [14,15] The advantages of organozincates may be attributed to their intrinsic "bimetallic" character. Indeed, due to their synergic reactivity, bimetallic systems often outperform their monometallic components and have shown greater potential in organic transformations. [6] Lithium zincate investigations reveal that not only the Li/Zn ratio [3,4,9] but also the substituents [5] may have remarkable effects on their reactivity. Thus, the TMP ligand has contributed to the renaissance of metallation chemistry involving organozincate compounds. [6][7][8] We show here that extensions of the ligand systems can result in organozincates with novel reactivity.While N-heterocyclic carbenes (NHCs) have become ubiquitous ligands for organic [16] and transition-metal [16] catalysis, they are also capable of stabilizing highly reactive main group molecules. [17][18][19][20][21][22] NHC-zinc complexes may act as polymerization catalysts. [23][24][25][26] However, all the reported NHC-based zinc compounds are neutral and C2-bound. Considering the unique application of C4-bound NHC [i.e., Scheme 1. Synthesis of NHC-stabilized triorganozincates 2 and 3.

Zincocene and Dizincocene N‐Heterocyclic Carbene Complexes and Catalytic Hydrogenation of Imines and Ketones

“…The bridging ligands exhibited slightly elongated ZnO bond lengths of 2.016(1) Å, whereas the nonbridging alkoxy ligands displayed ZnO distances of 1.927(1) Å. The terminal aI t Bu ligands showed longer bond lengths of ZnC 2.027(2) Å, which is slightly shorter compared to previously reported NHC adducts of zinc 23a…”

“…However, when a freshly prepared solution of 11 in CD 2 Cl 2 was kept in liquid N 2 and warmed prior to acquisition of NMR data, 1 H resonances at d = 7.56 and 6.86 ppm and a 13 C resonance at 155.3 ppm were consistent with the presence of an abnormal carbene. [23] Two sets of resonances inferred two alkoxide (OCHPh 2 ) À fragments leading to speculation of a dimeric structure in solution. The corresponding reaction of 1 with benzophenone under D 2 effected the analogous conversion, prompting the formulation of the product as [Zn-(OCDPh 2 ) 2 (aItBu)] ([D 1 ]11).…”

Experimental and Theoretical Insights into the Electronic Properties of Anionic N-Heterocyclic Dicarbenes through the Rational Synthesis of Their Transition Metal Complexes

“…Though the chemistry of NHDC species C and their adducts with main group elements have been well developed, 6,9,10 the corresponding binuclear transition metal complexes remain relatively scarce. Bimetallic Zn(II) 11 and Au(I) 12 NHDC derivatives based on IPr skeleton (IPr = 1,3-bis(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene) were originally obtained by transmetallation of main group metal cations in appropriate precursors with ZnEt2 and (PPh3)AuCl, respectively. Another approach exploiting C−H 13 or C−I 14 NHC backbone bond activation in bidentate and pincer complexes resulted in the formation of binuclear Pd(II) or Co(I) NHDC complexes.…”

“…Scheme 1. Reactivity of the Abnormal Carbene Moiety in Complex Cp(CO)2Mn(μ-dIMes)Li (2) Inspired by the pioneering works of Robinson 11 and Hevia 12 we wondered whether transmetallation at the abnormal carbene position in complex 2 (Scheme 1, path iv) could afford a viable route to heterobimetallic transition metal NHDC complexes based on the ubiquitous IMes architecture. We report herein the synthesis of carbonyl-containing Mn(I) and Fe(II) NHDC complexes, the comprehensive study of their electronic structure and the quantification of the donation properties of dIMes ligand.…”