Triptycene scaffolds have largely been explored in supramolecular and polymer chemistry but not much known about their use as a reagent and catalyst. As triptycene–based hypervalent iodine reagents are currently unknown, we have synthesized iodotriptycene derivatives including chiral iodotriptycenes and have investigated their catalytic use in organic transformations for α–oxytosylation of propiophenone.
Our main target was to synthesize iodotriptycene compounds including chiral reagents and to explore their potential reactivity as a catalyst in selective organic transformations. Initially, we prepared triptycene core structure bearing iodine at position–1 or –2 in one of the aromatic rings (Schemes 1 and 2) and were fully characterised including X-ray structures. Both were investigated in a catalytic manner (10 mol%) for the α-oxytosylation of propiophenone (Scheme 6) and are efficient (Table 1, entries 1 and 2). A hypervalent iodotriptycene compound, 2-(diacetoxyiodo)-triptycene was also prepared (scheme 3).
Two different chiral iodotriptcenes were prepared (Schemes 4 and 5). Diastereomers syn-8 and anti-8 were separated using preparative TLC and their structures were confirmed by single crystal X-ray. Enantiomers of syn-8 separated by chiral HPLC methods were investigated in the α-oxytosylation of propiophenone (Scheme 6) but the enantioselectivities in the products were low (Table 1, entries 3 and 4).
As the position of iodine is not very close to the chiral center in syn-8, 5–iodo–1,4–dimethoxytriptycene was prepared using 1,4–dimethoxyanthracene that avoids the formation of diastereomers (Scheme 5). Initially, we planned to isolate the enantiomers of this compound 12 using chiral HPLC method but, could not performed it due to poor solubility in the solvent that was required for the separation. So, enantiomers of 12 were separated using chiral resolving agent (1S)-(–)-camphanic chloride. The absolute configuration of isolated enantiomers was determined by their single crystal X-ray structures as (9S,10S) – (–) –12 and (9R,10R)–(+)-12 and the optical rotations of these enantiomers were also determined. The HPLC purity was very good and the enantiomeric excess was >99%. These were investigated in the α–oxytosylation of propiophenone (Scheme 6) but the enantioselectivities in the products were low (Table 1, entries 5 and 6).
Conclusion: Established a reliable approach for the synthesis of iodotriptycenes ● Iodotriptycene works as a catalyst in organic transformations (eg. α-oxytosylation) ● Triptycene based hypervalent iodine compounds (eg. 2-(diacetoxyiodo)-triptycene) and chiral iodotriptycenes have been synthesized and investigated in an enantioselective reaction of α-oxytosylation of propiophenone.
We have synthesized different 1- and 2-iodotriptycenes including chiral iodotriptycenes. They are investigated in organic transformations of α-oxytosylation of propiophenone and are found efficient catalyst but generally with low enantioselectivity. Hypervalent iodotriptycene such as 2-(diacetoxyiodo)-triptycene has also been prepared. To the best of our knowledge this work shows a first oxidative iodotriptycene catalyst in organic synthesis. In addition, we have published a minireview on chiral triptycenes.
