Martix Isolation IR Spectroscopic And Theoretical Study On The Tunneling Reactions Of Heteroarylnitrenes

Arylnitrenes(Ar-N)are neutral reactive nitrogen intermediates prossesing a sextet of electrons in their outer shell,which are facilely obtained by the decomposition of the corresponding azide precursors through thermally or photolytically initiated N2-elimination.However,rapid inter-and/or intramolecular reactions occurring under the formation conditions render the studies on the fundamental physical and chemical properties of this class of electron-deficient species challenging.Due to the wide application of heteroarylnitrenes in the photo affinity labeling of biomolecules,modification of polymer materials and design and synthesis of organic molecules,a large number of researchers have been attracted to reveal the structure-activity relationship of nitrenes by using low-temperature matrix isolation spectroscopy,time-resolution spectroscopy and quantum chemical theoretical calculation.In this paper,3-azide-2-formyl thiophene and 3-azide-2-formylfuran with simple structure have been synthesized by means of low temperature vacuum hydrazine device and common sample separation methods.The thermochemical and photochemical of 3-azide-2-formylthiophene and 3-azide-2-formylfuran have been studied by using low temperature matrix isolation spectroscopy,and the reaction mechanism involved in the experimental phenomena have been explained by using quantum-mechanical computational approaches.Moreover,according to JWKB approximate solution method and tunneling limit empirical formula,the quantum tunneling effect of 3-nitrene-2-formylfuran and 3-nitrene-2-formylthiophene and their derivatives in the process of intramolecular 1,4-H migration was theoretically analyzed.This paper is divided into two parts as follows1.Two kinds of compounds,3-azide-2-formylthiophene and 3-azide-2-formylfuran,were synthesized according to the related literature.The process of photodegradation and thermal decomposition were studied by means of·low-temperature matrix isolation infrared spectroscopy.The experimental results show that under 266 nm light,one molecule N2 of the above two azides will be removed to form the corresponding nitrenes,and each nitrenes will have two conformations due to the different orientation of the carbonyl group.Then,with 532 nm green light,the two nitrenes will react in a ring-closure manner to form 3,2-thienoisoxazole and 3,2-furoisoxazole,respectively.In addition,trace amounts of 1,4-H shift product 3-imino-4,5-dihydrothiophene-2-ketene of 3-nitrene-2-formylthiophene were also observed during photodegradation.In contrast to the above photolysis phenomenon,3-azide-2-formylthiophene will decompose into two conformational 3-imino-4,5-dihydrothiophene-2-ketene at 1000 K,and the two conformations will undergo reversible transformation under photoinduced conditions.At the same temperature,3-azide-2-formylfuran will completely decompose to produce a series of small fragments.In view of the above experimental phenomena,high-precision quantum chemical calculation method is used to identify and analyze the products and decomposition mechanism.2.Based on the experimental phenomena of 2-formyl phenylnitrene,3-nitrene-2-formylfuran and 3-nitrene-2-formylthiophene,the quantum tunneling effect in the process of intramolecular 1,4-H migration of these three kinds of nitrenes are theoretically analyzed by using JWKB approximate solution and tunneling limit empirical formula.At the same time,the O atom in the formyl group is replaced by S atom,NH group and CH2 group respectively with these three nitrenes as the parent molecules,thus a series of nitrenes with similar structure are derived,and the possibility and speed of quantum tunneling effect are theoretically predicted.The results show that the P(E)value of 2-formyl phenylnitrene is 7.14 × 10-18,the TL value is 2.4,and the corresponding half-life is 2.38 h,which is consistent with the time required for spontaneous 1,4-H migration observed in the experiment.While the P(E)values of 3-nitrene-2-formylfuran and 3-nitrene-2-formylthiophene are at least six orders of magnitude smaller and the TL values are also relatively high relative to 2-formyl phenylnitrene,implying that both nitrene are not only less likely to have a tunneling effect but also at a slower rate,which is consistent with the experimental phenomenon that spontaneous 1,4-H migration of the above two nitrenes are not observed in a certain time range.In addition,when the O atom in the formyl group is replaced by the S atom of the same main group,the P(E)value and TL value of the derived nitrenes are close to that of the parent nitrenes,i.e.,2-thioformyl phenylnitrene is likely to observe spontaneous intramolecular 1,4-H shift.However,the half-life of 2-thioformylthiophene/furan nitrenes are too large to be observed in a certain time range.When the NH group replaces the O atom in the formyl group,the P(E)values of the anti conformations in the derived three kinds of nitrenes are mostly orders of magnitude higher than the P(E)values of the parent nitrenes,but because their TL values are all less than 2,it means that even though the three kinds of nitrenes are likely to have tunneling effect in the H migration,but may be difficult to observe in the matrix isolation experiment due to the excessive rate.However,when the O atom is replaced by the CH2 group,the possibility of tunneling effect in the syn or anti conformation of the derived nitrenes are very small.