Peri hydrogen interactions

Hepworth and coworkers95 have studied the effects of iV-alkyl and heterocycloalkyl substituents on the light absorption properties of, Victoria Blue 35 and 36 (Figure 12), the 1-naphthyl analogue of Crystal Violet. The steric interaction found between the N-terminal group and the peri-hydrogen in the naphthyl residue is correlated with H NMR data. 

There are three isomeric binaphthyls 1, T, the 1,2, and 2,2. There is enthalpy of formation data only for the first one as the solid,ii° 170 kj/mol. No doubt it is nonplanar in the gas phase, thus minimizing any steric interaction between the 8,8 peri hydrogens. The enthalpy of reaction 32 where Ar, Ar = 1-naph-thyl is +14 kJ/mol in the solid phase. Evidently, the more aromatic benzene undergoes a more endothermic reaction to form biphenyl. 

Figure 1.18. Steric interactions between the cyano groups and peri hydrogens in 6,13-TCPQ.

The increased stabilization upon thienoannulation as compared to benzo-annulation may be caused by the more effective stabilization of the positive charge, by diminished angular ring strain, and by limited peri interactions between hydrogen atoms (due to the heteroatoms) and hence by nearly perfect planarity. 

Since in naphtalene substrates a steric interaction exists between the substituent at the stereogenic center and the peri H, the preferred conformations A and B bear the small hydrogen substituent proximate to this atom so that the attack of oxygen can be controlled by steric and... 

Apart from the construction of phenanthrenes, carbene complexes have also been used for the synthesis of more extended polycyclic arenes. An unusual dimerization of chromium coordinated ortbo-ethynyl aryl carbenes results in the formation of chrysenes (Scheme 37) [81]. This unusual reaction course is presumably due to the rigid C2 bridge that links the carbene and alkyne moieties, and thus prevents a subsequent intramolecular alkyne insertion into the metal-carbene bond. Instead, a double intermolecular alkyne insertion favored by the weak chromium-alkyne bond is believed to occur forming a central ten-membered ring that may then rearrange to the fused arene system. For example, under typical benzannulation conditions, carbene complex 97 affords an equimolar mixture of chrysene 98a and its monochromium complex 98b. The peri-interactions between the former alkyne substituent (in the 5- and 11-positions) and the aryl hydrogen induce helicity in the chrysene skeleton. 

It has been suggested that substituent preference for the axial position in 9,10-dihydroheteroanthracene derivatives is related to repulsions with hydrogens peri to the substituent. If heteroatoms with longer C—M bonds are present the substituents will extend further away from the m -hydrogens and repulsive interactions should decrease. Thus organometallic heterocycles may not exhibit a substituent preference. 

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