Catalytic decomposition of diazomalonates and other diazoesters using Rh(II)- and Cu(II)-complexes in the presence of α,β-unsaturated δ-(N-aryl)amino esters gives rise to the formation of multi-functionalized pyrrolidines with yields of up to 82%. The reaction apparently occurs as a domino process involving the initial N-ylide formation followed by intramolecular Michael addition to the conjugated system of amino esters to afford the pyrrolidine heterocycle. The whole process can also be classified as a [4 + 1]-annulation of the δ-amino α,β-unsaturated ester with the carbenoid intermediate.
The formation of C-H insertion products in the course of direct photolysis of α,α-diphenylsubstituted diazo ketones of the tetrahydrofuran series was rationalized by secondary photochemical processes which give rise to benzophenone acting as a sensitizer. Triplet excited states of diazo ketones generated by the action of benzophenone are capable of undergoing bimolecular transformations.Singlet excited states generated by direct irradiation of diazo carbonyl compounds are capable of undergoing only very fast unimolecular transformations. Among the latter, the most typical is Wolff rearrangement which involves elimination of nitrogen molecule and leads to the formation of ketenes and carboxylic acid derivatives [1]. This reaction is especially effective with cyclic and heterocyclic diazo ketones, and the yields of 1,2-nucleophilic rearrangement (ring contraction) products reach 80-90% and more [1].On the other hand, bimolecular processes were also observed with some diazo ketones of the tetrahydrofuran series. For example, we previously [2] described bimolecular photochemical transformation of 4-diazo-2,2,5,5-tetraphenyltetrahydrofuran-3-one which reacted with the solvent (tetrahydrofuran) to produce the corresponding substituted hydrazone, i.e., the process was accompanied by elimination of nitrogen. It should be specially emphasized that the described reaction was the first example of photochemical transformation of diazo ketones without elimination of nitrogen.Insofar as we failed to effect similar transformation of tetraalkyl-substituted analogs [3], we presumed that the presence of phenyl substituents in the α-positions with respect to the diazo and/or carbonyl group in the initial diazo ketone is crucial. Presumably, α-phenyl groups are responsible for the formation of long-lived triplet states from diazo ketones.Thus the goal of the present work was to study the above previously unknown photochemical reaction [4] using as substrates regioisomeric α,α-diphenyl-substituted diazo ketones Ia and Ib belonging to the tetrahydrofuran series [5]. Initially, analysis of the reaction mixtures showed that the main path of photochemical transformations of both diazo ketones Ia and Ib is the Wolff rearrangement (to 73%) leading to the formation
Benzophenone-sensitizedr eactions of 4-diazotetrahydrofuran-3(2H)-onesw ith H-donors such as tetrahydrofuran, 1,4-dioxane,c yclohexane and diethyle ther,o ccur without elimination of nitrogen and give rise to the corresponding N-substituted hydrazones or bis-hydrazonoethanes due to af ormal insertion of the terminal Natom of diazo group into a-CÀHb onds of ethers and aliphatic hydrocar-bons, with yields of up to 78 %. Long-wavelength UV irradiation (l > 310 nm) is most suitable fort his process, whereas oxygen molecules adequately quench the triplet excited state of the diazoketone and reduce the preparative yields of CÀH-insertion products. Hence this photochemicalr eaction of diazoketones can be used for CÀHf unctionalization of different aliphatic compounds.Scheme1.Photochemical insertion of the terminalNatom of a4 -diazodihydrofuran-3(2H)-onei nto an a-CÀHb ondofT HF.
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