Seven-membered NHC

In 2009, Buchmeiser and co-workers reported the synthesis of a novel ruthenium complex 54 based on a seven-membered NHC ligand [68] (Fig. 3.22). To examine the catalytic activity of complex 54 in the RCM reaction, the authors subjected the complex to a series of typical RCM reactions by using substrates 1, 3, and 5. Pre-catalyst 54 showed only moderate reactivity with 1 and 3 and no reaction occurred with 5. 

Keywords Acyclic NHC Anionic N-heterocyclic carbenes Mesoionic compounds Mesomeric betaines NHC catalysis NHC complexes NHC organocatalysis Seven-membered NHC... 

The synthesis and characterization of a series of complexes 35 bearing six-and seven-membered NHC and general formula (NHC)Pd(dvtms) (dvtms = divin)dtetramethyldisiloxane) was recently reported [44]. In order to investigate the effect of increasing ring size on the catalytic activity, the complexes were... 

Another complex (120) featuring a seven-membered NHC was reported by Buchmeiser and coworkers in 2009 (Figure 11.26) [104]. 120 displayed excellent reactivity in CM of 27 and 28 and ROMP of 25. In contrast, only moderate activity was found for the RCM of 17 and tetrasubstituted olefins could not be ring-closed by this catalyst. 

Stahl and co-workers then sought to develop chiral NHC ligands in order to achieve asymmetric catalysis. Toward this end, axially chiral seven-membered NHCs were developed and tested in their racemic forms initially, giving comparable results to IMes (Equation (12.9)). ... 

Clark s group also reported on ring-closing enyne metathesis for the preparation of six- and seven-membered cyclic enol ethers 428 n= 1,2) as potential building blocks for the synthesis of marine polyether natural compounds such as brevetoxins and ciguatoxins. Metathesis products 428 were obtained from ene-ynes 427 in 72-98% yield when the NHC-bearing catalyst C was used (Scheme 84) [179]. 

Fig. 3.21 Catalysts 52 and 53 with six-membered NHCs 3.1.2.4 Seven-Membered Central Ai-Heterocycle...

In the presence of an imidazolium salt and a base, oxidative cyclization of a Ni(0) species upon the diene and an aldehyde takes place first and forms an oxanickellacycle 25, which equilibrates with a seven-membered oxanickella-cycle 26, naturally possessing a cis double bond. cr-Bond metathesis through 26 with hydrosilane affords (Z)-allylsilane (Z)-23. The role of NHC ligand (AT-heterocyclic carbene, generated by H+ elimination from imidazolium C2H by a base) is not clear at present a Ni(0)-NHC complex is believed to effectively produce 26. 

No enetetramine formation has been observed with NHCs derived from seven-membered heterocycles. The C2 resonance in these derivatives is shifted further downfield (5 250-260 ppm) in the NMR spectra. The most striking difference in comparison to all NHCs discussed previously is the twisted, nonplanar conformation of the seven-membered heterocycle both in the azolium precursor and in the carbene ligand. The twisted conformation appears to be essential to accommodate the strain within the seven-membered heterocycle. 

We also note that another possibility of introducing chirality within the heterocycle is the use of a seven-membered N-heterocyclic carbene. Most of the structures with four, five or six-membered rings possess a nearly planar heterocycle with the exception of seven-membered ring NHCs [63]. 

The Chi group recently achieved the first NHC-eatalyzed diastereo- and enantioselective [3 + 4] cycloaddition of azomethine imines and enals via 1,4-dipolarophile intermediates generated by oxidative catalytic remote y-car-bon aetivation of enals. Dinitrogen fused seven-membered heterocyclic products were produced with high enantiomerie purity (up to 81% yield and 99% ee). Racemic azomethine imines ean be used as 1,3-dipolar substrates to afford dinitrogen-fused seven-membered heterocyclic compounds with high enantiomeric purity (s-factor up to 339). The key vinyl enolate intermediate is generated by oxidative y-carbon activation of enal via NHC catalysis as the reactive 1,4-dipolarophile (Scheme 7.116). 

Very recently, the Chi group disclosed the NHC-catalysed [4 + 3] (ycload-dition of enals and azomethine imines to provide seven membered heterocyclic adducts 132 with excellent diastereoselectivity and enantioselectivity (Scheme 20.56). ... 

In 2009, Stahl and coworkers described the synthesis of the enantiomeri-cally resolved seven-membered ring NHC-Pd dimer 87 [75]. This complex was examined as a chiral catalyst under aerobic conditions in the intramolecular oxidative amination reaction of alkene 88 (Scheme 3.49). In the best case, an enantioselectivity of 63% ee was obtained with low yield (35%), while the other substrates underwent cyclization to afford essentially racemic products. 

Concerning ring size expansion, since the first anionic sbc-membered example derived from malonic acid described by Cesar et al. [8], other sophisticated five-[5a,9], sbc-[10], and seven-membered [11] NHCs have been described and coordinated to rhodium and iridium, including sbc-membered amino/amido carbenes [10b] and caffeine-based diamino carbenes [12]. Also related to this type of ligands, Conejero and coworkers described a general method for the preparation of 2-pyridylidene-based rhodium complexes by decarboxylation of pyridinium car-boxylates [13], a method previously described by Crabtree and coworkers [14]. 

Figure 1.12 Stable NHCs with six- and seven-membered heterocycles.

NHCs featuring an aliphatic seven-membered ring scaffold of type 132b68c,69,i98 obtained by deprotonation of amidinium salts with... 

The N-C-N angles in NHCs derived from six- and seven-membered heterocycles were larger that in their five-membered analogues ( 115° in 132a and 133a, 116.6° in 132b, 113.4° in 133b). 

Zuo and Louie also found that NHC-Ni complexes catalyzed the rearrangement of cyclopropylen-yne derivatives. However, the selectivity of the reaction was strongly dependent on the NHC-Ni catalyst as well as on the substrate substitution. With a SIPr/Ni 1 1 system, a hindered vinylcyclopro-pylene-yne gave the isomerized seven-membered ring as the sole product while the corresponding methyl derivative afforded a disubstituted tetrahydrofuran derivative. The authors then described a successful and general access to the later by using the ItBu/Ni 1 1 system (Scheme 10.6). 

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