Arylation of Carbonyl Compounds Catalyzed by Rhodium and Iridium 1,3‐R₂‐Tetrahydropyrimidin‐2‐ylidenes: Structure‐Reactivity Correlations

Abstract

Six different well-defined rhodium and iridium N-heterocyclic carbene complexes, i.e., RhCl-(1,3-dimesityltetrahydropyrimidin-2-ylidene)(COD) þ Rh 2 (CF 3 COO) 3 (COD) À (8). Compounds 4 and 8 were characterized by X-ray analysis. The activity of the rhodium complexes increased in the order 5 > 3 > 1 > 2, indicating the necessity of strongly electron-withdrawing groups at the metal centers, thus increasing their nucleophilicity. In due consequence, the "softer" iridium complexes 4 and 6 exhibited significantly reduced catalytic activity albeit with enhanced selectivity. The syntheses of the metal complexes as well as a detailed study on their reactivity in the arylation of carbonyl compounds using equimolar amounts of arylboronic acid and carbonyl compound in the presence of 0.08 -1 mol % catalyst are presented.

CO2 and SnII Adducts of N‐Heterocyclic Carbenes as Delayed‐Action Catalysts for Polyurethane Synthesis

“…Though many of these compounds were stable up to 100 8C in the absence of a polyol based on a hydroxy-functionalized polyA C H T U N G T R E N N U N G (acrylate) and a commercial triisocyanate, that is, cyclotrimeric hexamethylene diisocyanate (HDI-cyclotrimer, Figure 2), and some of these compounds were in fact active catalysts for PUR synthesis, no latent behavior was observed at all when these compounds were mixed with both the polyol and the HDI-cyclotrimer. We therefore prepared a series of CO 2 -protected NHCs [10] based on tetrahydropyrimidin-2-ylidenes, [11,12] of which catalysts 1 and 2 ( Figure 2) in fact turned out to satisfy our demands. The high latency of these pre-catalysts can be rationalized by the fact that tetrahydropyrimidin-2-ylidenes are characterized by a small angle defined by the two substituents at N-1 and N-3, which provides sufficient steric protection of the C-2 carbon as well as by the efficient charge delocalization (vide infra).…”

Rh(1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD) tetrafluoroborate, an unsymmetrical Rh-homoazallylcarbene: synthesis, X-ray structure and reactivity in carbonyl arylation and hydrosilylation reactions

“…As already briefly mentioned particularly the use of 2 gave raise to arylation reactions with TONs up to 1200 [20]. With one exception (Table 3, entry 5), the use of 1 under conditions identical to the ones reported previously gave raise to enhanced turn-over numbers, exceeding the previously reported ones up to a factor of 3.…”

“…In terms of carbonyl arylation reactions (Scheme 2), originally reported by Miyaura et al [25,26], Rh (III) (NHC) complexes have been used by Fü rstner [27], the use of Rh (I) (NHC) complexes has been reported by our group [20].…”

“…Recently, our group [15][16][17][18][19][20][21] reported on novel NHC complexes of Ag (I), Pd (II), Rh (I), Ir (I), and Ru (II) based on 1,3-R 2 -3,4,5,6-tetrahydropyrimidin-2-ylidenes (R = mesityl, 2-propyl). For carbonyl arylation and hydrosilylation reactions the novel NHC-complexes MX(1,3-R 2 -3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD) (M = Rh, Ir; X = Cl, Br; R = 2-Pr, mesityl; COD = g 4 -1,5-cyclooctadiene) have been used and were found to possess excellent activity [20,21]. However, inspired by fruitful discussions in course of the International Symposium on Homogeneous Catalysis (ISHC) in Munich, 2004, we were interested whether the corresponding cationic complexes would allow for even enhanced reactivity and/or selectivity in these reactions.…”

Synthesis of New C2-Symmetric Six-Membered NHCs and Their Application for the Asymmetric Diethylzinc Addition of Arylaldehydes

“…To date, most of the research has employed five-membered NHCs based on imidazole or imidazoline. The so-called "expanded-ring" NHCs with six- [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] and seven- [38][39][40][41][42][43][44][45][46][47][48][49] heterocyclic rings have recently attracted attention, as these "non-standard" NHCs show quite different properties, such as stronger basicity (nucleophilicity) and greater steric demand [50]. Structurally, the larger ring sizes of these unusual NHCs will lead to a comparatively large N-CNHC-N angle and consequential smaller C NHC -N-C R angle, which in turn results in better protection of metal centers and subsequently better performance in catalysis.…”