Twisting and planarization in push–pull ethylenesElectronic supplementary information (ESI) available: tables of X-ray crystallographic bond lengths and angles of compounds 8–19 (including selected torsional angles and hydrogen bonds), X-ray crystal structures of compounds 10, 11, 13, 16, and 18 (Figs. S1–S5) and packing diagrams for 17·H2O and 19·H2O (Figs. S6–S7). See http://www.rsc.org/suppdata/p2/b1/b109624a/

Twisted Push - Pull Ethylenes

“…1) as expected from previous structural determinations of pushpull substituted Meldrum's acid derivatives. [3] The most relevant dimensions for 5 are the formal double bond C5-C7 1.395(4) Å, and the torsional angles about C5-C7 of 18-32 • . The range reflects the fact that the C5-C7 torsional angle may be defined by various combinations of atoms attached to C5 and C7 (see details in the Accessory Publication).…”

“…The presence of water molecules can help to 'tie back' RNH groups and thus prevent intramolecular hydrogen bonding, thereby changing a structure from nearly planar to nearly perpendicular. [3] This is the case in compound 2, which contains crystal water. A theoretical analysis of the twisting and rotational barriers in push-pull-substituted model ethylenes of the general formula (X,Y)C=C(CHO) 2 using density functional theory (DFT) has been published, [13] and the push-pull effect has been quantified in terms of occupation numbers of the bonding π and antibonding π * orbitals of the central C=C bond.…”

“…[4] Very large dihedral angles, 51-89 • , have also been determined in (N,N-disubstituted) 1,1-diamino-2,2-dinitroethylenes. [5] Rotational barriers [6] and X-ray crystal structures [3,[7][8][9][10][11] of several other push-pull ethylenes are described in the literature. The twisting has been discussed in terms of contributions from the π-electron energy (favouring planarity) and the steric strain energy (opposing planarity).…”

“…5-Methylene derivatives of Meldrum's acid 1 with one or two donor substituents X andY in position 7 can have dihedral angles as high as 83 • (compound 2) as determined by X-ray crystallography (Scheme 1). [3] A similar angle of 86 • has been determined for compound 3. [4] Very large dihedral angles, 51-89 • , have also been determined in (N,N-disubstituted) 1,1-diamino-2,2-dinitroethylenes.…”

“…[1,4] Hydrogen bonding between NH and CO groups in 1 (X and/or Y = NHR) stabilize the nearplanar forms (e.g. 4), [3,12] and large substituents on nitrogen cause increasingly large dihedral angles due to steric hindrance. The presence of water molecules can help to 'tie back' RNH groups and thus prevent intramolecular hydrogen bonding, thereby changing a structure from nearly planar to nearly perpendicular.…”