Bond cooperativity effects, which are typical of 'resonant' chains or rings of pi-conjugated hydrocarbons, can also occur in hydrogen-bonded systems in the form of sigma-bond and pi-bond cooperativity or anticooperativity. sigma-Bond cooperativity is associated with the long chains of O-H...O bonds in water and alcohols while sigma-bond anticooperativity occurs when the cooperative chain is interrupted by a local defect reversing the bond polarity. sigma-Bond cooperativity is the driving force controlling resonance-assisted hydrogen bonds ( RAHBs), while sigma-bond anticooperativity has never been considered so far and is investigated here by studying couples of hydrogen-bonded beta-enolone and/or beta-enaminone six-membered rings fused through a common C=O or C-C bond. The effect is studied by X-ray crystal structure determination of five compounds [( 2Z)-1-( 2-hydroxyphenyl)3-phenyl-1,3-propanedione enol ( 1), ( 2Z)-1-( 2-hydroxy-5-chlorophenyl)- 3-phenyl-1,3-propanedione enol ( 2), ( 2Z)- 1( 2-hydroxy-5-methylphenyl)-3-phenyl-1,3-propanedione enol ( 3), ( 2Z)-1-( 2-hydroxy-4-methyl-5-chlorophenyl)-3-phenyl-1,3-propanedione enol ( 4) and dimethyl( 2E)-3-hydroxy-2{[( 4-chlorophenyl) amino] carbonyl} pent-2-enedioate ( 5)] and by extensive analysis of related fragments found in the CSD ( Cambridge Structural Database). It is shown that fusion through the C=O bond is always anticooperative and such to weaken the symmetric O-H...O...H-O and N-H...O...H-N bonds formed, but not the asymmetric O-H...O...H-N bond. Fusion through the C - C bond may produce either cooperative or anticooperative hydrogen bonds, the former being more stable than the latter and giving rise to a unique resonance-assisted ten-membered ring running all around the two fused six-membered rings, which can be considered a type of tautomerism never described before.
pi-bond cooperativity and anticooperativity effects in resonance-assisted hydrogen bonds (RAHBs)
BERTOLASI, Valerio;PRETTO, Loretta;GILLI, Gastone;GILLI, Paola
2006
Abstract
Bond cooperativity effects, which are typical of 'resonant' chains or rings of pi-conjugated hydrocarbons, can also occur in hydrogen-bonded systems in the form of sigma-bond and pi-bond cooperativity or anticooperativity. sigma-Bond cooperativity is associated with the long chains of O-H...O bonds in water and alcohols while sigma-bond anticooperativity occurs when the cooperative chain is interrupted by a local defect reversing the bond polarity. sigma-Bond cooperativity is the driving force controlling resonance-assisted hydrogen bonds ( RAHBs), while sigma-bond anticooperativity has never been considered so far and is investigated here by studying couples of hydrogen-bonded beta-enolone and/or beta-enaminone six-membered rings fused through a common C=O or C-C bond. The effect is studied by X-ray crystal structure determination of five compounds [( 2Z)-1-( 2-hydroxyphenyl)3-phenyl-1,3-propanedione enol ( 1), ( 2Z)-1-( 2-hydroxy-5-chlorophenyl)- 3-phenyl-1,3-propanedione enol ( 2), ( 2Z)- 1( 2-hydroxy-5-methylphenyl)-3-phenyl-1,3-propanedione enol ( 3), ( 2Z)-1-( 2-hydroxy-4-methyl-5-chlorophenyl)-3-phenyl-1,3-propanedione enol ( 4) and dimethyl( 2E)-3-hydroxy-2{[( 4-chlorophenyl) amino] carbonyl} pent-2-enedioate ( 5)] and by extensive analysis of related fragments found in the CSD ( Cambridge Structural Database). It is shown that fusion through the C=O bond is always anticooperative and such to weaken the symmetric O-H...O...H-O and N-H...O...H-N bonds formed, but not the asymmetric O-H...O...H-N bond. Fusion through the C - C bond may produce either cooperative or anticooperative hydrogen bonds, the former being more stable than the latter and giving rise to a unique resonance-assisted ten-membered ring running all around the two fused six-membered rings, which can be considered a type of tautomerism never described before.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
