Reactions of p-tert-butylcalix[8]arene (1) with various alkylating agents, including para-substituted benzyl bromides, tert-butyl bromoacetate, and 2-chloro-N,N-dimethylacetamide, were carried out in THF-DMF in the presence of a base. With strong bases (Nail or BaO/Ba(OH)(2)) octasubstituted derivatives were obtained, usually in good yield, while in the presence of weak bases (K2CO3 or CsF) more or less complicated reaction mixtures were obtained, wherefrom 1,3,5,7-tetraethers with C-4 symmetry were isolated in yields up to 49%. The eight equivalent ArCH2Ar groups of these compounds give rise to a singlet in the H-1-NMR spectrum, indicating a conformational freedom of the macrocycle, as confirmed by VT-NMR studies. The origin of the 1,3,5,7-regioselectivity can be explained, assuming that alkylation proceeds via a sequence of alternating monodeprotonation and alkylation steps. In each individual monodeprotonation step, those phenoxide monoanions are formed preferentially which are stabilized by two flanking hydrogen bonds. However, this cannot be the whole explanation. In fact, when methyl iodide was used as the electrophile in the reaction with 1, the main products were the 1,2,4-trimethoxy and 1,2,3,4-tetramethoxy derivatives, whereas the 1,3,5,7-tetramethyl ether was not detected in the reaction mixture. This finding leads us to believe that in the reaction with Mel some factor other than stability of oxyanions prevails, possibly the molecular dimension of the electrophile. This seems to be confirmed by the observation that n-butyl iodide, of intermediate dimension between MeI and arylmethyl bromides, gives alkylation products typical of both reaction courses.
Study On the Alkylation of P-tert-butylcalix[8]arene - Partially O-alkylated Calix[8]arenes
NERI, Placido;
1994-01-01
Abstract
Reactions of p-tert-butylcalix[8]arene (1) with various alkylating agents, including para-substituted benzyl bromides, tert-butyl bromoacetate, and 2-chloro-N,N-dimethylacetamide, were carried out in THF-DMF in the presence of a base. With strong bases (Nail or BaO/Ba(OH)(2)) octasubstituted derivatives were obtained, usually in good yield, while in the presence of weak bases (K2CO3 or CsF) more or less complicated reaction mixtures were obtained, wherefrom 1,3,5,7-tetraethers with C-4 symmetry were isolated in yields up to 49%. The eight equivalent ArCH2Ar groups of these compounds give rise to a singlet in the H-1-NMR spectrum, indicating a conformational freedom of the macrocycle, as confirmed by VT-NMR studies. The origin of the 1,3,5,7-regioselectivity can be explained, assuming that alkylation proceeds via a sequence of alternating monodeprotonation and alkylation steps. In each individual monodeprotonation step, those phenoxide monoanions are formed preferentially which are stabilized by two flanking hydrogen bonds. However, this cannot be the whole explanation. In fact, when methyl iodide was used as the electrophile in the reaction with 1, the main products were the 1,2,4-trimethoxy and 1,2,3,4-tetramethoxy derivatives, whereas the 1,3,5,7-tetramethyl ether was not detected in the reaction mixture. This finding leads us to believe that in the reaction with Mel some factor other than stability of oxyanions prevails, possibly the molecular dimension of the electrophile. This seems to be confirmed by the observation that n-butyl iodide, of intermediate dimension between MeI and arylmethyl bromides, gives alkylation products typical of both reaction courses.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
