Synthesis and Structure of N-Heterocyclic Carbene Complexes with Tethered Olefinic Groups:  Application of the Ruthenium Catalyst in Olefin Metathesis

Abstract

Silver(I) and rhodium(I) complexes bearing the bisallyl-substituted N-heterocyclic carbene ligand (4R,5S)-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (allyl 2 SIMes) have been prepared in a straightforward synthesis. The reaction of (4R,5S)-4,5-diallyl-1,3bis(2,4,6-trimethylphenyl)-4,5-dihydro-3H-imidazol-1-ium tetrafluoroborate (1a) with Ag 2 O affords the ionic biscarbene complex [(allyl 2 SIMes) 2 Ag] + BF 4 -(2), while the reaction of (4R,5S)-4,5-diallyl-1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-3H-imidazol-1-ium chloride (1b) with Ag 2 O leads to the monocarbene complex (allyl 2 SIMes)AgCl (3). Sequential treatment of 1a with KOtBu and dimeric [RhCl(cod)] 2 (cod ) cyclooctadiene) yields the rhodium carbene complex (allyl 2 SIMes)RhCl(cod) (4). However, the reaction of 1a with the first-generation Grubbs catalyst (PCy 3 ) 2 Cl 2 RudC(H)Ph (Cy ) cyclohexyl) leads to ring-closing metathesis of the two allylic groups, yielding 1,3-bis(2,4,6-trimethylphenyl)-3a,4,7,7a-tetrahydro-3Hbenzimidazol-1-ium tetrafluoroborate (5). Subsequent reaction of this new imidazolium salt with KOtBu and 1 equiv of (PCy 3 )Cl 2 RudC(H)(C 6 H 4 OiPr-2) forms [1,3-bis(2,4,6-trimethylphenyl)-3a,4,7,7a-tetrahydro-3H-benzimidazolin-2-ylidene]dichloro(2-isopropanolatobenzylidene)ruthenium(II) (8). All new complexes have been thoroughly characterized, including X-ray crystallographic analyses of 2, 3, and 8. The most intriguing feature of 8 is the presence of an innocent CdC bond that is part of a highly active olefin metathesis catalyst, which offers many options for further functionalization of the ligand backbone. The catalytic activity of complex 8 has been evaluated for the ring-closing metathesis of N,N-diallyl-4-toluenesulfonamide.

The Pivotal Role of Symmetry in the Ruthenium‐Catalyzed Ring‐Closing Metathesis of Olefins

“…The C6/C11 and C13/ C18 phenyl rings are twisted with respect to the NHC ring by 78.24(1) and 69.1(2)8, respectively. These conformations are associated with the C À H···p [25] interaction between the C20 À H20 group of the benzylidene moiety and the centroid C of the C6/C11 phenyl ring (H20···CA C H T U N G T R E N N U N G (C6/C11) = 2.70 , C20-H20···C = 1418), and the CÀH···Cl interaction [26] between the C18ÀH18 moiety of the C13/C18 phenyl group and the Cl1 atom (H18···Cl1 = 2.78 , C18-H18···Cl1 = 1378). In 9 b, the Ru atom is positioned 0.38 above the basal plane toward the C24 carbene atom.…”

Pyrazole‐Bridged NHC Ligands and Their Dimetallic (Allyl)palladium Complexes

“…[1][2][3] The original and most commonly used NHCs usually bear two identical bulky hydrocarbon groups at the nitrogen atoms of the imidazole ring, and the required symmetric imidazolium salts that serve as precursors for such NHCs are conveniently accessible by standard preparation procedures. [4] Over the last years manifold variations have been introduced into this chemistry, like substitutions at the backbone of the imidazole or NHCs having different groups in the periphery, [5][6][7] including linkers to connect several NHCs within one ligand molecule and/or side arms containing additional heteroatoms as potential donor atoms in pincer complexes. [8,9] Carbenes bearing appended N-heterocycles such as pyridine (e.g., type I in Scheme 1) [10,11] have attracted particular attention and have shown promising potential in catalytic applications.…”

NHC Backbone Configuration in Ruthenium-Catalyzed Olefin Metathesis

“…The RCM activity of complexes 2-antiGII and GII was explored and an increased reactivity especially at elevated temperatures was actually found. The first example of a ruthenium complex bearing an NHC ligand with a syn configuration of the backbone was reported by Köhler et al, in 2005 (Figure 4) [36]. With the aim of synthesizing complex 4-synGII with syn allyl substituents on the backbone as functional groups for immobilization of the catalyst to a solid support, they instead formed a new NHC ligand featuring an olefinic group in the ligand backbone, then employed it to prepare the complex 5-synHGII.…”

“…In the ring-closing metathesis of N,N-diallyl-4-methylbenzenesulfonamide, complex 5-synHGII was slightly less active than the benchmark catalysts HGIItol, especially at low catalyst loadings. [36]. With the aim of synthesizing complex 4-synGII with syn allyl substituents on the backbone as functional groups for immobilization of the catalyst to a solid support, they instead formed a new NHC ligand featuring an olefinic group in the ligand backbone, then employed it to prepare the complex 5-synHGII.…”