Sterically overcrowded molecules

The 71 subunits, ethene and benzene, differing in both size as well as topology, but containing the same tetra-substitution pattern, seem to approach identity15,16. ESR spin delocalization data of their organosilicon radical cations as well as additional information from own structures of sterically overcrowded molecules and many more registered in the Cambridge Structural Database stimulate speculation that both tetra-substituted molecules... 

Keywords Molecular States / Photoelectron Spectroscopy / EPR Spectroscopy / Radical Anions and Cations / Sterically Overcrowded Molecules / DFT Calculations / Long-Range Interactions... 

Hexaaryl- and hexapyridyl[3]radialenes are sterically overcrowded molecules. According to NMR spectra, the (hetero)aryl rings are tilted against the plane of the radialene core and adopt a propeller conformation in solution several crystal structure analyses [24, 26, 38,40] reveal the double three-bladed propeller structure with a nearly planar radialene core. 

Part 27 of Structures of Sterically Overcrowded and Charge Perturbed Molecules, for part 26 see H. Bock, C. Nather, Z. Havlas, A. John and C. Arad, Angew. Chem. in print. 

Results from X-ray studies of three annulenes are presented In Table 8. According to Hiickel s rule [14]annulene (14-ANN) and [18]annulene (18-ANN) should be aromatic and most probably planar molecules, while [16]annulene (16-ANN), as a [4n]annulene, should be antiaromatic. The [14]annulene molecule is nonplanar, with a structure that approaches C2h symmetry. The cause of the nonplanarity is the steric overcrowding in the center of the molecule. While the spread of the individual bond lengths implies possible significant differences, there is no significant pattern to the values obtained. 

The electronic spectra of the [2.2]paracyclophanes provide valuable information regarding the extent and mechanisms of transannular electronic interactions in strained 7t-electron systems. A rigid system like the [2.2]paracyclophane molecule is of great value as a model for checking theoretical data with a view to interpretating U V spectra of sterically hindered molecules ( overcrowded compounds ). 

Much is known empirically on B-NMR data (39). Restricting boron compounds to noncyclic molecules with no or one nitrogen atom and two, three, or four carbon atoms bonded to boron and omitting sterically overcrowded alkyl groups R and R, we find four classes with the following typical chemical shift ranges (Et20 BF3 as the external standard) ... 

Overcrowded Molecules.—Many condensed aromatic molecules that are forced into nonplaruir configurations by steric repulsion have been studied during recent years. An example is 3,4-5,6-dibenzophenan-threne,... 

Another aspect of steric overcrowding which has not attracted much attention, although obviously of fairly general potential occurrence, is illustrated by the dipyridothiophene (14). In a hypothetical planar molecule the distance between the two peri-chlorine atoms would be 2.05 A. In practice this is reduced to a more acceptable 3.06 A, principally as a result of displacement of the peri -chlorines by about 0.6 A from the least-squares plane (74JCS(P2)1925). 

The sterically overcrowded alkenes shown in Scheme 6 have been exploited in our group since, from the perspective of molecular switches design, they combine a number of attractive structural features. Steric interactions between the groups attached to the central olefmic bond force these molecules to adapt a non-planar helical shape. The chirality of these so-called inherently dissymmetric alkenes 3, is therefore the result of distortion of the entire molecular structure. Beside the heli-cene-like geometry, both a cis- and a trans-stilbene chromophore are present in the same molecule. 

When, as in spiroarsoranes of type 138, an additional benzo-anneland across the 3,4-positions of one 2,2 -biphenylylene ligand renders the potential tetragonal-pyramidal transition state 137 highly improbable because of prohibitive steric overcrowding, inner flexibility of the molecule can express itself now only by... 

FIGURE 14.5. Biphenyl, a chiral molecule, (a) Biphenyl, showing steric overcrowding with respect to hydrogen atoms, and (b) the two enantiomers of its derivative 2,2 -disulfonic acid. 

The concept of cis-trans (Z-E) isomerism, originally used for the description of the relative geometry of olefins, has been extended to many other functions which feature a double bond character (pseudo double bonds), such as amides, as well as single bonds with a partial or complete limited rotation due to steric or stereoelec-tronic effects. Cis-trans isomerization (CTI) therefore exists in non-re-bonded or overcrowded molecules that switch from a given stable conformational state to another. This is the case of biaryl compounds which have been utilized in organic chemistry as the basis of molecular switches and rotors [1,2]. Nature has also exploited CTI of single bonds to increasing molecular diversity, in particular with the bulky thyroxin, a thyroid hormone, and the well-known disulfide bond which plays a critical role in the structure of peptides and in the conformation of proteins. 

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