Cyclobutane ring current

Studies of the stereochemical dependence of 1H chemical shifts in cyclobutanes, benzocyclobute-nes and /J-lactams have been collected463. In a number of cases the protons or proton groups are shielded if they are cis to vicinal halogen, hydroxy or phenyl substituents, due to a ring-current effect. Carbonyl groups, however, can give rise to deshielding effects. 

Some of the more remarkable effects of strain are found in NMR chemical shifts. Cyclopropane derivatives usually have upheld proton chemical shifts with regard to the corresponding cyclohexane derivatives, whereas cyclobutanes commonly have downfield shifts.The upfield shift for cyclopropane protons have sometimes been attributed to a ring current in the three-membered ring, but there is little evidence for such a phenomenon. The unusual shift for these protons has proven valuable in demonstrating the presence of a three membered ring. 

The ring current is therefore in the opposite direction, adding to the applied field at the centre of the ring, and the protons experience therefore an enhanced field 1.37. The effect may be rather less in cyclobutanes than in cyclopropanes, because the cyclobutane ring is flexible, allowing the ring to buckle from the planar structure, and the C—H bonds thereby avoid the full eclipsing interactions inevitable in cyclopropanes, and compensated there by the aromaticity they create. 

Cyclohexenones with substituents at the S- and 6-positions tend to give the product which would be predicted by steric approach control to an excited state with a trigonal P-carbon (equations 34-36). As a final cautionary note, while these hypotheses are often useful for prediction of stereochemical results, the true source of the selectivity may involve the rates of cleavage of the intermediate diradicals to the ground state versus rates of cyclobutane ring closure. Insufficient data are currently available to explain these results completely. 

Currently, industry uses many polymers that crosslink mostly by 2n + 2n type dimerizations with formations of cyclobutane rings. This is like the crosslinking reaction of the cinnamate group. Such materials are used in photoresists, photolithography, and in many ultraviolet light... 

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