**4.4 NHC-transition-metal-mediated catalysis—Flow chemistry**

Continuous flow chemistry is a revolutionary technology that has developed rapidly during the past few years. The small size of channel reactors enables

#### **Figure 19.** *Representative examples of NHC-metal catalyzed addition reactions [85–88].*

### *Imidazolium-Based N-Heterocyclic Carbenes (NHCs) and Metal-Mediated Catalysis DOI: http://dx.doi.org/10.5772/intechopen.102561*

increased mass transfer, efficient heat transfer, and enhanced reaction efficiency. Moreover, this method enables the handling of dangerously toxic reagents remotely and thus reduces the potential health risks [89–92]. Cole-Hamilton et al. [93] reported the first continuous flow-based olefin metathesis catalytic system with a homogeneous catalyst. In this method, the Ru-NHC catalyst was immobilized into silica pores, and CO2 was passed as the carrier gas (**Figure 20**). It gave the metathesis products with an overall turnover number > 10,000.

Jenson et al. [94] reported a continuous nanofiltration method in which the metathesis catalyst (Ru-NHC) was allowed to react homogeneously; upon the reaction completion, the catalyst is trapped into a nano-filter setup, and it is flushed back to continue the reaction cycle. Unlike other conventional methods, there are no modifications done to the catalyst to recover and reuse it. Fabrication of NHC-metal complexes onto solid supports such as silica is another strategy used in NHC-centered flow chemistry [95–98]. Functionalized catalyst is packed into a reactor, and the reagents are forced to pass through it to yield final products (**Figure 21**).

**Figure 20.**

*Continuous flow reaction setup for olefin metathesis [93].*

**Figure 21.**

*Solid-supported NHC-metal catalyst and continuous flow reaction [93, 94, 99].*
