Suzuki iodobenzene

Similar Suzuki couplings have been performed by Hu and coworkers utilizing a poly(dicyclohexylcarbodiimide)/palladium nanoparticle composite [152]. This PDHC-Pd catalyst showed remarkable activity and stability under microwave irradiation. Near quantitative conversion (95% isolated yield) was obtained after 40 min of microwave heating of a mixture of iodobenzene with phenylboronic acid in dioxane. Re-using the immobilized catalyzed showed no significant loss of efficiency, as the fifth cycle still furnished a 90% isolated yield of the desired biphenyl. 

Keywords iodobenzene, alkylboronic acid, Suzuki coupling, palladium-doped... 

Keywords iodobenzene, p-methylphenylboronic acid, Suzuki coupling, KF/ AI2O3, palladium powder, microwave irradiation, 4-methylbiphenyl... 

The cA-bis(boryl)alkenes 482 are obtained by bis-boration of terminal alkynes catalysed by a Pt complex [185]. Pd and Rh complexes are inactive. Then the (Z)-1,2-diphenylalkene 483 is prepared by Pd-catalysed Suzuki-Miyaura coupling of 482 with iodobenzene. 

To test these ideas the reaction of p-tolylboronic acid and iodobenzene (eq. 1) was studied.17 19 As seen in Table 1, the reaction does not occur with Pd(0) and basic alumina, but does to varying degrees using the transition metal and bases adsorbed on alumina. Because KF/A1203 was the most effective base in inducing the Suzuki reaction, it was used in all subsequent reactions. It is worth noting that the Pd(0) - KF/A1203 system could be reused for further Suzuki reactions by addition of more KF to the alumina. 

A less extensive study of the reaction of boronic acids with different structural characteristics - aryl, vinyl, allyl, and alkyl- with iodobenzene on Pd-KF/Al203 was also undertaken (eq. 3) (Table 4), Only the allylic boronic acid, 3-propenylboronic acid, did not undergo the Suzuki coupling although it was largely consumed. 

Table 4. Suzuki Coupling of Boronic Acids with Iodobenzene°...

The triethoxysilyl endgroup is a popular functional group to bind the catalyst to a polymeric support [238]. Polymeric supports include silica gel, MCM-41 (mesoporous silica gel) and ITQ-2 (delaminated zeolite) [247]. Corma et al. used this approach to synthesise gold(I) and palladium(II) NHC complexes for Suzuki cross-coupling reactions between iodobenzene and various arylboronic acids (see Figure 4.78) [247]. The results were very modest at 35-80% dependent upon the substitution pattern of the arylboronic acid. Yields with gold(I) catalysts were marginally better than those for palladium(II) complexes. 

Not until 2005 were the first organoboron derivatives of pyridazines reported in the literature [57]. Harrity reported the preparation of pyridazin-4-ylboronic esters (99) via cycloaddition of tetrazines (98) with alkynyl boronic esters. Some of these boronic sters have been used in Suzuki reactions with iodobenzene. Although the organoboron compounds 99 are electron deficient (and some are also sterically hindered) protodeboronation usually proved no major problem. 

The amino-modified support was reacted with adipic acid dichloride and, after hydrolysis, was coupled with 4-iodophenylhydrazine 128 to yield the immobihzed iodobenzene 129. Intermediate 129 was then employed in Pd-mediated C-C couplings (Stille, Heck, Suzuki, Sonogashira) to yield, for example, compounds 130 and 131. The final products 132 and 133 were obtained in good to excellent yields by oxidation of 130 and 131, respectively. 

Palladium-catalyzed reactions such as Suzuki, Heck, Trost-Tsuji and Stille have been modified in the presence of KF/A1203 without solvent under microwave irradiation conditions by Villemin et al. [10], These reactions rapidly afforded the product in moderate to excellent yields. For instance, a reaction of iodobenzene with arylboronic acids was completed in 2-15 min. Recently, improved methods under microwave irradiation have been reported [11-16] (Scheme 5.5). 

Much research has been carried out on the Ugand-free palladium-catalysed Suzuki reaction. Already in 1989, Beletskaya reported a Ugand-free Suzuki reaction in water, between iodobenzoates and phenylboronic acid using Pd(OAc)2 as catalyst [81]. Later, Novak took up the quest to develop a highly active catalyst for the Suzuki reaction and since he noted that this reaction suffers from phosphine inhibition he decided to test three Ugand-free palladium catalyst precursors Pd(OAc)2, [(Ti -CsHsjPd PdCl]2, and Pd2(dba)3.QH6 [82]. All three catalysts performed well in the Suzuki reaction between 4-nitro-iodobenzene and phenylboronic acid. In the reaction with 4-nitro-bromobenzene the first and last catalyst were clearly superior with yields of 96-98% (Scheme 10.7). Novak suggests that... 

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