[Pd(IPr*^OMe)(acac)Cl]: Tuning the N-Heterocyclic Carbene in Catalytic C–N Bond Formation

Abstract

A new N-heterocyclic ligand IPr*OMe [N,N′-bis­(2,6-bis­(diphenylmethyl)-4-methoxyphenyl)­imidazol-2-ylidene] has been synthesized and compared with its earlier methyl-substituted congener IPr* [N,N′-bis­(2,6-bis­(diphenylmethyl)-4-methylphenyl)­imidazol-2-ylidene]. IPr*OMe was successfully bound to palladium, and the catalytic activity of the resulting complex [Pd­(IPr*OMe)­(acac)­Cl] was investigated in Buchwald–Hartwig arylamination. Catalytic activity was compared to that of [Pd­(IPr*)­(acac)­Cl].

The use of the sterically demanding IPr* and related ligands in catalysis

“…40 As palladium has proven to be an efficient metal in numerous coupling reactions, Nolan and co-workers reported the synthesis of several IPr* Pd-complexes, bearing IPr* and variations of this core motif: [Pd(IPr*)(cin)Cl] (Scheme 6, 4a cin = cinnamyl) with a %V bur of 44.6, 41 35 Interestingly 4a is the bulkiest palladium complex reported to date, while the presence of OMe substituents, as in 4d, improved in certain case the catalytic activity of the complexes. 33 These complexes have been used successfully in several reactions. In Buchwald-Hartwig amination, complexes 4a and 4e exhibited improved efficiency at low catalyst loading when compared with the less bulky [Pd(IPr)(acac)Cl] complexes (Table 1).…”

“…47 Aryl amination has also been achieved with [Pd(IPr* OMe )-(cin)Cl] (4b) 48 and [Pd(IPr* OMe )(acac)Cl] (4d). 33 Complex 4d was compared to [Pd(IPr*)(acac)Cl] (4c), and showed higher activity, which might be explained by a slightly stronger s-donor character compared to IPr*. Complex 4d allowed the coupling of secondary biaryl anilines with highly deactivated aryl chlorides (Scheme 9).…”

“…Complex 4d allowed the coupling of secondary biaryl anilines with highly deactivated aryl chlorides (Scheme 9). 33 Moreover, Navarro and co-workers were able to show that such coupling reactions (10 examples, 90 to 99%) can be performed under air using their complex [Pd(IPr*)(TEA)Cl] (TEA: triethylamine) at 1 mol%. 49 While the efficiency of numerous NHC systems has been shown in routine C-C cross-coupling reactions, only a few of these studies involve the IPr*-ligand.…”

Synthesis and characterisation of an N-heterocyclic carbene with spatially-defined steric impact

“…Free carbenes tend to be at least somewhat airand moisture-sensitive, but there exist a variety of methods by which transition metal complexes can be prepared without the need to liberate and isolate these reactive species. 9 This theory has been put forward to explain the successful use in catalysis of various ligands such as the IBiox series of NHCs, 7,11 IPr* and derivatives, 9,[12][13][14] and IPent 8 and its analogues 15 (Figure 1). [7][8][9][10] Essentially, the theory is that an 'ideal' ligand should be bulky enough to protect and stabilise coordinatively unsaturated intermediate species -such as putative 'NHC-Pd 0 ' or 'NHC-Ni 0 ' complexes for cross-coupling -yet flexible enough to allow the coordination and reaction of substrates at the metal centre without impediment.…”

Cross coupling reactions catalyzed by (NHC)Pd(II) complexes

“…Complex C96 was proven to be superior to its [Pd(IPr*)(acac)Cl] congener (Table 14, entries [18][19][20][21]. 135 In this context, (IPent Cl )PdCl 2 (o-Picoline) (C97)…”