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Synthesis of Some Novel Thioxanthenone‐Fused Azacrown Ethers, and Their Use as New Catalysts in the Efficient, Mild, and Regioselective Conversion of Epoxides to β ‐Hydroxy Thiocyanates with Ammonium Thiocyanate
Author(s) -
Sharghi Hashem,
Beni Alireza Salimi,
Khalifeh Reza
Publication year - 2007
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.200790139
Subject(s) - chemistry , regioselectivity , catalysis , ammonium thiocyanate , epoxide , reagent , steric effects , thiocyanate , ammonium , yield (engineering) , organic chemistry , medicinal chemistry , combinatorial chemistry , polymer chemistry , materials science , metallurgy
The regioselective ring‐opening reactions of some epoxides with ammonium thiocyanate in the presence of a series of new 9 H ‐thioxanthen‐9‐one‐fused azacrown ethers, i.e. , 7 – 11 ( Scheme 1 ), and also of dibenzo[18]crown‐6 ( 12 ), Kryptofix ® 22 ( 13 ), and benzo[15]crown‐5 ( 14 ) were studied ( Tables 1 and 2 ). The epoxides were subjected to cleavage by NH 4 SCN in the presence of these catalysts under mild conditions in various aprotic solvents. Reagents and conditions were identified for the synthesis of individual β ‐hydroxy thiocyanates in high yield and with more than 90% regioselectivity. The results can be discussed in terms of a four‐step mechanism ( Scheme 2 ): 1 ) formation of a complex between catalyst and NH 4 SCN, 2 ) release of SCN − from the complex, 3 ) reaction of the released SCN − at the sterically less hindered site of the epoxide, and 4 ) regeneration of the catalyst. The major advantages of this method are the high regioselectivity, the simple regeneration of the catalyst, the reuse of it through several cycles without a decrease of activity, and the ease of workup of the reaction mixtures.