Phenyl group, easily oxidized

The synthesis of molecular tweezers 10 and 11 are outlined in Scheme 1. Their spacers differ in two ways from the hypothetical system of Fig. 2 They are not fully oxidized and they carry a phenyl substituent. The phenyl group is an artifact of the synthesis, but does provide the advantage of being easily functionalized... 

The tertiary hydrogen of the triamino compounds (leuco base) is easily oxidized to the carbinol (color base) which under the influence of hydrochloric acid dehydrates to form a highly colored hydrochloride. In the dehydration one of the phenyl groups rearranges to a quinonoic structure which has chromophoric properties ... 

Rubrene. This organic semiconductor consists of a tetracene core with four additional phenyl groups connected by single bonds. In contrast to the desired crystalline phase the amorphous one is not stable against oxidation [10], The two states can be distinguished easily, since the amorphous phase lacks the typical red color found for crystalline films. 

Little meaningful physical data on these complexes has appeared yet. The study of their reactivity is complicated by the effects of the substituents on the cyclobutadiene ring and the other ligands present. Until more information is available on the properties of the recently prepared unsubstituted cyclobutadieneiron tricarbonyl (XVIII) 38) and similar molecules, it is hard to be certain which properties are due to the presence of a cyclobutadiene group. Thus while cyclobutadieneiron tricarbonyl (XVIII) is easily oxidized by ferric chloride in ethanol (as are other diene-iron tri-carbonyl complexes, albeit to different types of product), tetraphenyl-cyclobutadieneiron tricarbonyl (XIII) is very resistant to this reagent, and indeed to most others, presumably mainly due to the steric hindrance of the phenyls. 

Sodeoka and co-workers subsequently reported a Pd"-catalyzed asymmetric oxidative Mannich-type reaction of malonate 5 with THIQ 4 using DDQ as the stoichiometric oxidant. The reactive iminium intermediate from the N-Boc-protected THIQ 4 could be generated in situ by the slow addition of DDQ. The desired product 6 was obtained in 82% yield with 86% ee (Scheme 2.3). It is notable that an easily removable protecting group (Boc) can be used in this reaction, whereas Af-phenyl-protected substrates have been used in the previously reported CDC reactions with alkynes. More detailed mechanistic studies of this oxidative Mannich-type reaction were presented in a subsequent report from the Sodeoka group. Under the optimized reaction conditions, they examined the generality of the reaction. This oxidative Mannich-type protocol was also suitable for... 

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