Alkenes diamination, cyclic ureas

Three different approaches to the synthesis of five-membered cyclic ureas have recently been described that involve Pd-catalyzed alkene diamination reactions. In a series of very interesting papers, Muniz has described the conversion of alkenes bearing pendant ureas to imidizolidin-2-one derivatives using catalytic amounts of Pd(OAc)2 in the presence of an oxidant such as PhI(OAc)2 or CuBr2 [61, 62]. For example, these conditions were used to effect the cyclization of 79 to 80 in 78% yield (Eq. (1.34)) [62aj. These reactions proceed via a mechanism similar to that shown above in Eq. (1.33), except that the heteropalladation may occur in a syn- rather than anti- fashion, and the reductive elimination occurs with intramolecular formation of a C—N bond rather than intermolecular formation of a C—O bond. The alkene diamination reactions have also been employed for the synthesis of bisindolines (Eq. (1.35)) [63] and bicyclic guanidines (Eq. (1-36)) [64]. 

Scheme 4.10 Cyclic ureas from intramolecular diamination of alkenes, n = 1, 2.

The quest for a metal-catalyzed diamination of alkenes was reahzed by Muniz, who introduced N-sulfonylated ureas 132 as nitrogen sources for this type of processes (Scheme 16.35). The reaction proceeds through two consecutive C-N bond-forming events to the cyclic ureas 138, which were investigated in detail in a mechanistic study [96]. 

The reaction has recently been developed further by use of the milder reagents PhIO and PhI(OAc)2, respectively, within the iodine(III)-mediated intramolecular pyrrolidine formation from alkenes. For example, the combined use of iodoso-benzene and trimethylsilyl triflate generates reaction conditions that enable an efficient aminooxygenation of urea precursors 61. Because of the acidic reaction conditions, the final carbon-heteroatom bond formation cannot proceed toward diamination, and aminooxygenation products 62 are obtained selectively. The free aminoalcohols are conveniently obtained by acidic cleavage of the cyclic carbamate [51]. 

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See also in sourсe #XX -- [ ]

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