*2.4. Kumada–Tamao–Corriu Reaction*

The use of organometallic reagents to form carbon–carbon bonds is a standard procedure in organic synthesis but there are still many instances where the simple stoichiometric reaction is unsuccessful for one or more reasons. A number of transition metal catalysts have been developed for specific cases such as the Negishi coupling of organozinc reagents with aryl or alkenyl halides [127], or the related Kumada–Tamao–Corriu reaction involving the analogous coupling with Grignard reagents (Scheme 31) [8]. There are a number examples of the latter involving thiosemicarbazone complexes although the majority of the reports describe only one instance of a coupling of an aryl bromide and aryl magnesium bromide and thus do not permit a good assessment of wider applicability (Figure 6). Thus, there are accounts of ruthenium complexes derived from thiosemicarbazones. The mixed ligand complexes of Ru(II) **31**, [RuCO(EPh3)2L] and [RuCO(PPh3)(py)L] (where E = P or As and L is a dibasic tridentate ligand derived from the condensation of ethylacetoacetate or methylacetoacetate and thiosemicarbazide) catalysed the coupling of PhMgBr and PhBr as reported by Thilagavathi et al. [64]. Using a 200:1 substrate to catalyst ratio, rather low conversions were reported. Analogous complexes derived from chalcone thiosemicarbazone gave similar results [128]. Somewhat better yields were reported by Raja et al. for the complexes **32** [RuCO(EPh3)L] and [RuCO(py)L], where L is a tetracoordinated dianionic ligand derived from the reaction of 2-hydroxyaryl aldehyde, thiosemicarbazide and furfuraldehyde [62]. The coupling of PhMgBr with *p*-bromoanisole catalysed by a Ru(III) complexes **33** containing a monoanionic 2-acetylpyridine thiosemicarbazone ligand has also been reported by Manikandan et al. [81]. A 300:1 substrate to catalyst ratio was used and conversions of 28–48% were obtained.

Priyarega et al. reported nickel thiosemicarbazone complexes **34** with Ph3P supporting ligand that catalyse the formation of biphenyl in good yield [129]. From the experimental data, it is stated that a large amount (0.05 mol) of complex is used for 0.01 mol of PhBr but presumably this is a typographical error with the correct amount of complex to be probably 0.05 mmol. Güveli et al. have prepared a series of thiosemicarbazone complexes **35** derived from *o*-hydroxyacetophenone in which either *O*,*N*,*S*or *O*,*N*,*N*-tridentate coordination is observed [130]. In addition to structural and computational studies, the authors also examined the coupling of PhMgBr with PhBr in the presence of these compounds. The *ONN*-complexes gave higher yields compared to the *ONS*-complexes and this was ascribed to the larger size of the S atom and also to the higher charge on the metal. A range of aryl halides were employed by Anitha et al. in their study of Ni(II) complexes containing *O*,*N*,*S*-tricoordinating thiosemicarbazone ligands, which have also been described above as catalysts for the Suzuki–Miyaura and Sonogashira reactions (see complexes **26** in Figure 4) [119]. Moderate to excellent yields of biaryls were obtained under mild conditions (Et2O, 4 h) and with catalyst loadings of 0.2 mol% (Scheme 32). TONs of up to 93 and TOFs of up to 2 h–1 were recorded. Reactions for aryl halides with electron withdrawing substituents were found to give slightly higher yields than those with electron-donating

substituents, while ortho-substituted aryls gave lower yields. Overall, their catalytic efficiency was found to compare favorably with previously reported catalysts. monoanionic 2-acetylpyridine thiosemicarbazone ligand has also been reported by Manikandan et al*.* [81]. A 300:1 substrate to catalyst ratio was used and conversions of 28–48% were obtained. monoanionic 2-acetylpyridine thiosemicarbazone ligand has also been reported by Manikandan et al*.* [81]. A 300:1 substrate to catalyst ratio was used and conversions of 28–48% were obtained.

The coupling of PhMgBr with *p*-bromoanisole catalysed by a Ru(III) complexes **33** containing a

The coupling of PhMgBr with *p*-bromoanisole catalysed by a Ru(III) complexes **33** containing a

*Catalysts* **2020**, *10*, x FOR PEER REVIEW 23 of 42

*Catalysts* **2020**, *10*, x FOR PEER REVIEW 23 of 42

of wider applicability (Figure 6). Thus, there are accounts of ruthenium complexes derived from thiosemicarbazones. The mixed ligand complexes of Ru(II) **31**, [RuCO(EPh3)2L] and [RuCO(PPh3)(py)L] (where E = P or As and L is a dibasic tridentate ligand derived from the condensation of ethylacetoacetate or methylacetoacetate and thiosemicarbazide) catalysed the coupling of PhMgBr and PhBr as reported by Thilagavathi et al*.* [64]. Using a 200:1 substrate to catalyst ratio, rather low conversions were reported. Analogous complexes derived from chalcone thiosemicarbazone gave similar results [128]. Somewhat better yields were reported by Raja et al*.* for the complexes **32** [RuCO(EPh3)L] and [RuCO(py)L], where L is a tetracoordinated dianionic ligand

of wider applicability (Figure 6). Thus, there are accounts of ruthenium complexes derived from thiosemicarbazones. The mixed ligand complexes of Ru(II) **31**, [RuCO(EPh3)2L] and [RuCO(PPh3)(py)L] (where E = P or As and L is a dibasic tridentate ligand derived from the condensation of ethylacetoacetate or methylacetoacetate and thiosemicarbazide) catalysed the coupling of PhMgBr and PhBr as reported by Thilagavathi et al*.* [64]. Using a 200:1 substrate to catalyst ratio, rather low conversions were reported. Analogous complexes derived from chalcone thiosemicarbazone gave similar results [128]. Somewhat better yields were reported by Raja et al*.* for the complexes **32** [RuCO(EPh3)L] and [RuCO(py)L], where L is a tetracoordinated dianionic ligand

**Scheme 31.** Kumada–Tamao–Corriu reaction. **Scheme 31.** Kumada–Tamao–Corriu reaction. **Scheme 31.** Kumada–Tamao–Corriu reaction.

**Figure 6.** Representative metal complexes of thiosemicarbazones as catalysts for the Kumada–Tamao– Corriu reaction. **Figure 6.** Representative metal complexes of thiosemicarbazones as catalysts for the Kumada–Tamao– Corriu reaction. **Figure 6.** Representative metal complexes of thiosemicarbazones as catalysts for the Kumada–Tamao– Corriu reaction. catalytic efficiency was found to compare favorably with previously reported catalysts.

**Scheme 32.** Kumada–Tamao–Corriu reaction of aryl halides with phenylmagnesium chloride catalysed by Ni complexes **26**. **Scheme 32.** Kumada–Tamao–Corriu reaction of aryl halides with phenylmagnesium chloride catalysed by Ni complexes **26**.

studied. These predominantly concern the formation of carbon–nitrogen bonds for systems where non-catalysed coupling is not possible or very difficult. Reactions in this category include the Pdcatalysed arylation of amines (the Buchwald–Hartwig coupling shown in Scheme 33) [131,132], the Pd-catalysed formation of C–N, C–O or C–S bonds using aryl boronic acids and suitable heteroatom derivatives (the Chan-Lam coupling shown in Scheme 34) [133,134], and also the metal-catalysed Ullmann reaction (Scheme 35) [135,136]. Representative metal complexes catalyzed carbon–

**Scheme 33.** Buchwald–Hartwig reaction.

**Scheme 34.** Chan-Lam coupling.

heteroatom coupling reactions are shown in Figure 7; see also in Figures 2 and 4.

**3. Carbon–Heteroatom Coupling Reactions** 
