Unstable N-alkoxycarbonyl quaternary salts of axaaromatics were trapped in sifu by bis(tributylstannyl)acetylene followed by the treatment with trifluoroacetic acid to give 2-ethynyl addocts in good yields. The same compounds were obtained only ia low yields when ethynyluibutyltin was used as a nucleophile. The reaction was revealed te be available for various aromatics including pyrldine. pyridazine, imidazole, thiazele. oxazole, and benzediazines.
Recently we have been investigating the application of N-alkoxycarbonyl quatemary salts of azaaromatics for the introduction of nucleophiles .2 Although they are unstable and unisolable except the salts of pyridines, the in situ trapping of them was successful when the reagent used was unreactive to alkoxycarbonyl groups.
Using the methods, the introduction of ally1 group with allyltributyltinf was performed for various kind of azaaromatics.4 The results prompted us to search for the other nucleophiles which are suitable for this system.
Ethynylation of azaaromatics has been one of the most important methods for substituents intrcduction.~ It was reported that the reaction of pyridines with alkynyl Grignard reagents in the presence of an alkyl chloroformate affoided alkynyl adducts in good yields, including ethynyl derivative.6 In this reaction, rapid formation of N-alkoxycarbonyl pyridinium salts was crucial for the product formation in order to prevent the side reaction, in which the Grignard reagents underwent the nucleophilic attack to carbonyl group of alkyl chloroformate. Therefore, this reaction system cannot be applied to other azaaromatics because of the instability or the low concentration of their alkoxycarbonyl quatematy salts in solution.
At first, we tried the ethynylation using ethynyltributyltin, expecting that the reaction would progress in the same manner as in the cases of allyltributyltin 2~~ and alkynyhin.7 However, the yields were very low because of the insufficient nucleophilicity of the reagent. Hence the other reagents were investigated and bis(tributylstarmyl)acetylene was revealed to afford good results, which are described in this paper.8
When pyridine la was allowed to react with ethynyltributyltin (reagent B) in the presence of 1-chlomethyl chloroformate, 2-ethynyl adduct was obtained in only 11% yield (Table . entry 3). Replacement of B with bis(tributylstannyl)acetylene (reagent A) and succeeding treatment with trifluoroacetic acid (TFA) afforded ethynyl adduct 2 in 62% yield. although it was necessary to use 1-chloroethyl chlorofomtate (Table , entries 1 and 2). 13089 13090 T. ITOH et al. 1) ethynylation reagent CICO&, CH$l2 2) CF&QH (in the case of A as the reagent) 1 2 Scheme 1
Table 1. The Reaction of Pyidines and Pyridazine with Ethynyltin A or B in the Rresence of Allcyl Chloroformate Entry X R' R2 R3 Reagent a) Conditions Products Yield (%) 1 CH H H CHClMe A rt. 30h 2a 62 2 CH H H Et A rt. 24h 2b 0 3 CH H H CHClMe B rt.