Jan Geevers

Reactions of thiophenes with acetylenes in polar solvents; a novel annulation reaction

Although for a long time thiophenes have been considered to be inert in cycloaddition reactions, recently some have been reported to react with acetylenes via either a (2+2)or a (4+2)-cycloaddition reaction l-4 . Particularly some of the (2+2)-cycloadditions of thiopheneslr2, and of benzo[b]thiophenes5-7, proved to be versatile reactions in the synthesis of l-heterocycloheptatrienes. For example, reaction of 3-pyrrolidinothiophenes (1) with dimethyl acetylenedicarboxylate in apolar solvents at low temperature yielded the corresponding thiepins (A), a class of anti-aromatic compounds that could not be obtained by other routes. We now report that in polar solvents the reactions of A with dimethyl acetylenedicarboxylate take an entirely different course.

Preparation and X-ray structures of complexes of 18-membered crown ethers with polyfunctional guests: Urea and (O-alkyliso)uronium salts

The preparation and X-ray structure determinations of six complexes of urea and (O-n-butyliso)uronium salts with crown ethers are presented. Urea forms isostructural 5:1 adducts with 18-crown-6 (1) and aza-18-crown-6 (2), in which two urea molecules are each hydrogen bonded to two neighbouring hetero atoms of the macroring. The remaining urea molecules form two-dimensional layers alternating with crown ether layers. In both complexes the macroring has theg+g+a ag−a ag−a g−g−a ag+a ag+a conformation withCi symmetry. In the solid 1:1 complex of O-n-butylisouronium picrate with 18-crown-6 (3) two types of conformations of the macroring were observed: theg+g+a ag−a ag+a ag−g−ag−a ag+a conformation with approximateCm symmetry and to a lesser extent theg+g+a ag−a ag+a g+g+a ag−a ag+a conformation with approximateC2 symmetry. Both conformations allow the guest to form three hydrogen bonds to the macrocyclic host. Three complexes of 18-crown-6 and uronium salts have been prepared and characterized by X-ray crystallography. The 1:1 complexes with uronium nitrate (4) and uronium picrate (5) both exhibit the sameC2 conformation and the same hydrogen bonding scheme as in the least occupied form of the previous complex. A 1:2 complex with uroniump-toluenesulphonate (6) has a different hydrogen bonding scheme (two hydrogen bonds per cation to neighbouring oxygen atoms of the macroring) and a different conformation of the host molecule (theag+a ag−a ag+a ag−a ag+a ag−a conformation with almostD3d symmetry). An attempt to prepare a solid uronium nitrate complex with diaza-18-crown-6 in the same way as the 18-crown-6·uronium nitrate (1:1) complex did not yield the expected result. Instead X-ray analysis revealed that the uronium ion is dissociated, resulting in the nitrate salt of the diprotonated diaza crown ether (7).

Liquid-liquid phase transfer of uronium salts via complexation by (di)benzo crown ethers. X-Ray structure of the benzo-27-crown-9 uronium perchlorate (1:1) complex

The complexation of uronium perchlorate with crown ethers of different ringsizes (18–33 ring atoms) has been studied by using two-phase extraction experiments. Crown ethers with 27 or more ring atoms are the best hosts to transfer uronium salts from an aqueous phase to an organic phase. The amount of uronium perchlorate transferred was measured by coulometric titration of the stoichiometric amount of ammonia produced by enzymatic degradation of urea. The crystal and molecular structure of the 1:1 complex of uronium perchlorate with benzo-27-crown-9 has been determined by X-ray crystallography. The uronium cation is encapsulated in the crown ether cavity with all its hydrogen atoms bonded to the macrocyclic host. A 1:1 complex of uronium picrate with benzo-21-crown-7 was isolated and is assumed to be a perching complex.

Crystal and molecular structure of benzo-27-crown-9 guanidinium perchlorate (1 : 1): an encapsulated complex

In the crystal structure of the complex of benzo-27-crown-9 and guanidinium perchlorate (1 : 1) a complementary binding relationship between host and guest is found; all guanidinium hydrogen atoms are involved in hydrogen bonding to oxygen atoms of the macrocycle.