**4.1 NHC-transition metal catalyzed cross-coupling reactions**

The coupling reactions catalyzed by transition metal-NHC complexes involve both C-C bond formation reactions such as Mizoroki-Heck [61, 62], Sonogashira [63–65], Negishi [66–68], Hiyama [69–71], Suzuki-Miyaura [72–74], Kumada coupling [75, 76], C-X (X = B, N, O) bond formation reactions such as Buchwald-Hartwig amination [77–79] and Ullmann coupling [80–82] etc. Recently, the coupling reactions catalyzed by the NHC-transition metal complexes have found to be efficient with commercially viable metal salts such as iron, cobalt, and nickel. For example, the cross-coupling of methyl sulfonates and amines to afford sulfonamides uses Ni/NHC system to form the S-N bond [83]. Few well-known NHC-metal catalyzed reactions are shown in (**Figure 16**).

Recently, the Pd-NHC catalyzed metathesis of carbon-sulfur bonds seems to be one of the significant reactions, which is capable of altering the trends in the retrosynthetic approach for the preparation of organosulfur compounds [60] (**Figure 17**).

### **4.2 NHC-transition-metal-mediated metathesis reactions**

One of the best examples to demonstrate the significance of organometallic complexes is their role as catalysts in olefin metathesis, which is having numerous synthetic applications and commercial significance. There are different types of

**Figure 16.**

*Few well-known NHC-transition metal-based catalytic applications [13, 61–74].*

**Figure 17.** *Carbon-sulfur bond metathesis using Pd-NHC complex [60].*

**Figure 18.** *NHC-Ru catalysts for Z-selective olefin cross-metathesis reaction [84].*

olefin metathesis, namely ring-opening metathesis polymerization (ROMP), ringclosure metathesis (RCM), olefin cross metathesis, and enyne cross-metathesis. The ruthenium complex with a nonlabile NHC group and a labile phosphine group known to exhibit higher ring-closure metathesis (RCM) activity with better thermal stability and has tolerance toward many functional groups [14]. In general, the metathesis catalyst prefers the formation of thermodynamically favored E-isomer. However, subsequent modification on the NHC group yielded Z-isomer in significant yields. Chelating NHC-Ru complexes gave upto 95% Z selectivity. The unique steric and electronic parameters of chelating-Ru-based NHCs promote the olefin to attack cis to NHC and trans to the chelating group. This side-bound reaction mechanism results in Z-isomer as predominant product. One example of such Z-selective olefin metathesis using NHC chelated Ru-catalyst is shown (**Figure 18**) [84].
