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Bond Angle Versus Torsional Deformation in an Overcrowded Alkene: 9‐(2,2,2‐Triphenylethylidene)fluorene
Author(s) -
Bell Thomas W.,
Catalano Vincent J.,
Drew Michael G. B.,
Phillips Daniel J.
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20021104)8:21<5001::aid-chem5001>3.0.co;2-a
Subject(s) - steric effects , chemistry , molecular geometry , bond length , crystallography , fluorene , double bond , single bond , ring strain , stereochemistry , crystal structure , ring (chemistry) , polymer chemistry , molecule , group (periodic table) , organic chemistry , polymer
Competition between bond angle and torsional strain in sterically crowded alkenes generally causes twisting in tetrasubstituted alkenes, while most structurally characterized trisubstituted alkenes are planar. To investigate structural effects of steric repulsion between a planar aromatic ring and a vicinal triphenylmethyl (trityl) group, 9‐(2,2,2‐triphenylethylidene)fluorene ( 1 a ) was synthesized by reaction of 9‐bromomethylenefluorene with triphenylmethyllithium. For comparison with a less strained analogue, 9‐ethylidenefluorene ( 1 b ) was prepared by reaction of fluorenone with ethylmagnesium bromide. The X‐ray crystal structures show that the difference between bond angles at the 9‐fluorenyl carbon atom is much larger for 1 a (12.9°) than 1 b (2.6°). Bond angle and torsional deformations were compared theoretically (HF/6‐31+G*) with the tert ‐butyl analogue ( 1 c ), 1,2,2‐tri‐ tert ‐butylethene ( 7 ), and 2,4,4‐trimethyl‐2‐pentene ( 8 ) and crystallographically with six known 1,1‐diaryl‐2‐ tert ‐alkylethenes ( 2 ). The trisubstituted alkenes formed three groups with 1) large angle distortion with moderate twisting ( 1 a, 1 b , and 7 ), 2) moderate bending with a large range of torsional angles ( 2 ), and 3) little bending or twisting ( 1 b and 8 ). For the entire series, there appears to be a delicate balance between angle and torsional deformation, but twisting appears to produce smaller relief from steric strain than angle bending. In the crystallographically characterized trisubstituted alkenes, the choice between the two is mainly determined by more subtle packing forces.