Resolution and thermal racemisation of nickel 1,19-dimethyltetradehydrocorrin salts

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Abstract
Nickel 8,12-diethyl-1,2,3,7,13,17,18,19-octamethyltetradehydrocorrin has been partially resolved as its (+)-D-camphorsulphonate salt; this provides conclusive evidence for the trans arrangement of the angular 1- and 19-methyl groups. The formation of nickel trans-1,19-dimethyltetradehydrocorrin salts by cyclisation of the corresponding 1,19-dideoxy-1,19-dimethylbiladiene-ac salts is discussed in terms of an orbital symmetry-allowed conrotatory electrocyclic process. This mechanism is supported by the finding that trityl perchlorate can convert the nickel biladiene complex (IV) into the corresponding nickel 1,19-dimethyltetradehydrocorrin (II). (+)-Nickel 1,19-dimethyltetradehydrocorrin salts are shown to undergo thermal racemisation, the rate of racemisation being influenced by the nature of the anion. This observation is related to the mechanism of the thermal rearrangement of nickel 1,19-dialkyltetradehydrocorrin salts to nickel pophins.

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