ACS catalyst: Professor Ye Longwu's research group of Xiamen University and Professor Lu Xin's research group made a real cash catalytic oxidation cyclization of terminal diyne to produce inner vinyl carbene complex

Researchers can construct complex ring molecules through the metal carbene / alkyne series reaction Therefore, the metal carbene / alkyne series reaction has attracted extensive attention of scientists However, the vinyl metal carbenes produced by this kind of reactions are only confined to the outer carbenes, while the formation of the inner vinyl carbenes remains to be explored Recently, Professor Ye Longwu's research group and Professor Lu Xin's research group of Xiamen University reported on the oxidation and cyclization of terminal diacetylene catalyzed by gold Under very mild conditions, the inner vinyl carbene complex can be obtained By using this reaction, the readily available diynes are converted into various valuable naphthoquinones and carbazoquinones The reaction has a wide range of substrates and functional group tolerance The related achievements were published in ACS catalyst (DOI: 10.1021 / acscalal 8b04455) under the title of "generation of endocyclic vinyl carbene complexes via gold catalyzed aerobic cycling of terminal diynes: toward naphtoquinones and carbazolequinones" At first, the author optimized the reaction conditions by using the diyne (1a) as the model substrate (Table 1) By screening a series of gold catalysts, oxidants and additives, the author found that when brettphosauntf 2 (10 mol%) was used as catalyst, 2-bromopyridine-n-oxide (3a) as oxidant and trifluoroacetic acid (2.0 equivalent) as additive, the target product 2A (entry 6) could be obtained in 64% yield When agntf 2, PTCL 2 and Zn (OTF) 2 were used as catalysts, the target product (entries 8-10) could not be obtained The results of oxidant screening showed that when 8-ethyl quinoline-n-oxide was used as oxidant, 70% yield could be obtained (entry 13) In addition, when the coordination anion of the gold catalyst is changed from NTF 2 - to sb F 6 -, the reaction efficiency is improved obviously, and 2A (entry 15) can be obtained in 81% yield (source: ACS catalyst.) then, under the optimal reaction conditions, the author explored the reaction substrate (Table 2) The results show that all kinds of diyne substrates 1 can react smoothly, and the corresponding 1,4-naphthoquinone compounds 2 can be obtained in medium to good yields The target product 2A - 2K (entries 1 − 11) was obtained in 59-83% yield It is worth noting that a series of functional groups such as the protected hydroxyl group and amino group are well tolerated in this transformation (entries 6-11) The electron deficient aryl substrate is also suitable for the oxidation and cyclization reaction, and can be converted to the corresponding naphthoquinone compound 2m - 2O (entries 13 − 15) In addition, the authors found that naphthodiethyn1p can also react smoothly (entry 16) (source: ACS catalyst.) next, the author extended the reaction substrate to other heterocyclic diynes (Table 3) The results showed that all the diynes with different N protecting groups, even the easily decomposed N-acetyl diynes, could react smoothly, and the corresponding product 6a-6e (entries 1 − 5) was obtained in 59-65% yield The substrate containing electron donor or electron acceptor on indole ring is also suitable for the reaction, but the reaction rate of the latter is greatly reduced (entries 8, 9) (source: ACS catalyst.) finally, based on the above experimental results, the author proposed the reaction mechanism (scheme 3) At first, N-oxide carried out nucleophilic attack on Au (I) - linked diyne a to form ethylene fund intermediate B The n − O bond in B breaks, and the cyclopropene intermediate C1 is obtained, but the expected α - carbonyl fund carbene intermediate C is not obtained C1 is isomerized into another cyclopropene intermediate D the Au (I) l in D is coordinated with C = C, which is further converted into the inner ring ethylene fund carbene intermediate E Another molecule, N-oxide, has nucleophilic attack on carbene carbon atom of e to form intermediate F When Au (I) l migrates to 1 l of diacetylene, G releases 2 l of the final product The whole reaction process is highly exothermic, and the free energy released is 152.1 kcal / mol (source: ACS Catal.) in summary, the author developed a new type of gold catalyzed oxidation cyclization of terminal two alkynes Under very mild conditions, the inner vinyl carbene complex can be obtained By using this reaction, the readily available diynes are converted into various valuable naphthoquinones and carbazoquinones The reaction has the advantages of wide range of substrates and good tolerance of functional groups.