Urea and Thiourea Catalysed Reactions

It was reported by Curran ef fll. [19] in 1994 that a symmetrical A, A -diarylurea effectively catalyses the allylation of cyclic sulfmyl radicals with allyltributylstannane (Table 9.5). High trans selectivity in the product was observed when a stoichiometric amount of urea catalyst was used (run 4), suggesting activation in the transition state by double hydrogen bonding between the urea hydrogen atoms and the sulfmyl oxygen atom. 

Catalytic activity of the same urea and its variations was examined in the Claisen rearrangement of 3-methoxyvinyl vinyl ether [20] (Table 9.6). Great rate acceleration was observed in the use of thiourea, even though completion of reaction could not be estimated due to slow decomposition of the catalyst under the reaction conditions used (run 6). 

Schreiner et al. developed thiourea catalyst as a promising hydrogen donor, which has more benefit in solubility, synthesis and catalytic mrn over number compared with urea catalyst, in the Diels-Alder reaction of A-crotonyloxazolidinone and cyclopentadiene [22,23] (Table 9.7). A,A -Di[3,5-bis(trifluoromethyl)phenyl]thiourea accelerates the reaction and improves stereoselectivity (run 4) similar to a metal catalyst such as aluminium chloride (AICI3) (mn 2) or titanium chloride (TiCls) (run 3). 

The thiourea catalysed Diels-Alder reaction of MVK and cyclopentadiene gives useful information for further mning the stmcture of the thiourea catalyst flexible side chains on nitrogen atoms of thiourea are ineffective due to the large entropy in complexation with an hydrogen acceptor [22] (Table 9.8). 

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