No abstract

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

Non-fluorinated vinyldiazo compounds with trans-configuration irrespective of the nature of 3-R(1)-substituent (R(1) = H, Me, TBSO) even under ambient conditions easily cyclize to produce pyrazoles, while cis-stereoisomers undergo similar ring closure only at elevated temperatures or decompose to produce vinyloxocarbene reaction products. The 3-CF3-substituted analogues with cis- or trans-configuration do not produce pyrazoles at all, but on heating furnish only vinylcarbene derived products. DFT calculations of theoretical energy barriers adequately explain the different experimental reactivity found for stereoisomeric vinyldiazocarbonyl compounds, and a new model for their interconversion through the corresponding pyrazoles has been suggested.

Pyridazines attract much attention due to their biological activity [1], some of them are used in the treatment of Parkinson, Alzheimer, and other neurodegenerative diseases [2,3]. Therefore the development of new methods of preparation of this class compounds is an urgent problem of the synthetic organic chemistry.In keeping with published reports 3,4,6-trisubstituted pyridazines can be synthesized in good yields from 2-vinyl derivatives of 2-diazoketones by the intramolecular diaza-Wittig reaction [4][5][6][7][8][9]. The only example of the synthesis of the tetrasubstituted pyridazine with the use of analogous procedure was described in [10], where by the diaza-Wittig reaction of the posphazine of 4-diazopyrrolidinetrione with ethyl acetoacetate bicyclic pyrrolo[3,4-c]pyridazine was obtained.The aim of our study was the elucidation of the possibility to obtain 3,4,5,6-tetrasubstituted pyridazines by the intermolecular diaza-Wittig reaction of phosphazines of acyclic diazoketoesters with 1,3-diketones and ketoesters .We used as diazo substrates methyl and ethyl 2-diazo-3-oxo-4,4,4-trifl uorobutanoates (Iа, Ib), аnd as 1,3-dicarbonyl compounds ketoesters IIa, IIb and diketones IIc, IId. In the preparation of the phosphazines from diazoketoesters I we applied the triphenylphosphine (IIIа) commonly used in the Staudinger reaction [11][12][13] and more nucleophilic [14] tris(dimethylamino)phosphine (IIIb).The reaction of diazoketoesters Iа, Ib with ketoester IIа was carried out similarly to procedure [10] using PPh 3 . It presumably proceeded with the intermediate formation of triphenylphosphazines IVа, IVb and was of low effi ciency. The monitoring of the reaction progress by 1 Н NMR spectroscopy showed that under these conditions a fairly complex mixture of products formed containing alongside the target compounds also a diazoacetic ester phosphazine and the other reaction products, and according to the data of 1 Н NMR spectrum of the reaction mixture the yield of pyridazines V did not exceed 30-32%.

Phosphazenes of vinyldiazocarbonyl compounds having cis stereochemistry of the functional groups on the vinyl bond readily produce pyridazines by a diaza-Wittig process, whereas their counterparts with trans configuration remain intact under similar reaction conditions. Upon UV irradiation trans-phosphazenes furnish pyridazines through a tandem trans-to-cis isomerization followed by intramolecular cyclization. At elevated temperatures trans-(triphenyl)phosphazenes dissociate to give the initial vinyldiazo compounds, which produce pyrazoles in high yields. The first theoretical study on the mechanism of the diaza-Wittig process by DFT calculations at the M06-2X/6-31G(d) level of theory suggest that for the cis-phosphazenes a rapid tandem [2+2] cycloaddition/cycloelimination process with low energy barriers is preferred over trans isomers.