Photochemistry of 2-diphenylmethoxyacetophenone. Direct detection of a long-lived enol from a Norrish Type II photoreaction.

Photolysis of 2-diphenylmethoxyacetophenone (1) in deaerated acetonitrile at room temperature is expected to proceed via Norrish Type II reaction to generate benzophenone (2) and acetophenone (4) as the main isolated photoproducts. Although the acetophenone enol intermediate (3) is generally considered a putative intermediate that is a precursor of acetophenone, there are few reports of the direct spectroscopic detection of 3 when a Type II reaction is conducted in solution at room temperature. Presumably, rapid enol-ketone isomerisation under the reaction conditions causes 3 to have a very short lifetime that lowers its concentration below limits detectable by standard organic spectroscopic methods such as (1)H NMR. We report that 3 is readily observed at room temperature by (1)H NMR spectroscopy and showed a remarkably long lifetime of almost 1 h under our reaction conditions. It was found that the acetophenone enols from the classic Norrish Type II reactions of valerophenone and butyrophenone could also be readily detected by (1)H NMR in acetonitrile at room temperature, but that their lifetimes were similar (tens of minutes) from both precursors, but were considerably shorter than that of 1. The reason for the differences in the lifetime of the acetophenone enol is probably due to small amounts of adventitious catalysts that arise during the photolysis.

Adiabatic ring opening in tethered naphthalene and anthracene cycloadducts.

The cycloadducts of tethered naphthalene and anthracene derivatives undergo photochemical ring opening to an electronically excited product with adiabatic yields up to 90%.

Guest rotations within a capsuleplex probed by NMR and EPR techniques.

With the help of (1)H NMR and EPR techniques, we have probed the dynamics of guest molecules included within a water-soluble deep cavity cavitand known by the trivial name octa acid. All guest molecules investigated here form 2:1 (host/guest) complexes in water, and two host molecules encapsulate the guest molecule by forming a closed capsule. We have probed the dynamics of the guest molecule within this closed container through (1)H NMR and EPR techniques. The timescales offered by these two techniques are quite different, millisecond and nanosecond, respectively. For EPR studies, paramagnetic nitroxide guest molecules and for (1)H NMR studies, a wide variety of structurally diverse neutral organic guest molecules were employed. The guest molecules freely rotate along their x axis (long molecular axis and magnetic axis) on the NMR timescale; however, their rotation is slowed with respect to that in water on the EPR timescale. Rotation along the x axis is dependent on the length of the alkyl chain attached to the nitroxide probe. Overall rotation along the y or z axis was very much dependent on the structure of the guest molecule. The guests investigated could be classified into three groups: (a) those that do not rotate along the y or z axis both at room and elevated (55 degrees C) temperatures, (b) those that rotate freely at room temperature, and (c) those that do not rotate at room temperature but do so at higher temperatures. One should note that rotation here refers to the NMR timescale and it is quite possible that all molecules may rotate at much longer timescales than the one probed here. A slight variation in structure alters the rotational mobility of the guest molecules.

Chiral photochemistry within a confined space: diastereoselective photorearrangements of a tropolone and a cyclohexadienone included in a synthetic cavitand.

The value of a supramolecular assembly to enforce a closer interaction between a chiral auxiliary and a reaction center has been established using photoreactions of tropolone and cyclohexadienone derivatives. Two probe molecules utilized to establish the concept undergo 4 e- electrocyclization and oxa-di-pi-methane rearrangement from excited singlet and triplet state, respectively. The chiral auxiliaries investigated here has no/little effect in acetonitrile solution during phototransformations of the probe molecules to yield products with new chiral centers. On the other hand the same ones are able to enforce diastereoselectivities to the extent of approximately 30% when the reactions occur within the restricted space of a capsule made up of a synthetic cavitand commonly known as octa acid. Extensive NMR studies have been utilized to characterize the guest-host supramolecular structures. The results presented here should be of value in the overall understanding of chiral induction in photochemical reactions.

Chiral Photochemistry in a Confined Space: Torquoselective Photoelectrocyclization of Pyridones within an Achiral Hydrophobic Capsule.

Chiral induction during the photoelectrocyclization of pyridones included within octa acid (OA) capsule has been established. Chiral induction is brought about by a chiral auxiliary appended to the reactive pyridone moiety. Importantly, the same chiral auxiliary while ineffective in acetonitrile solution is found to be effective within the confined space of OA capsule. The diastereomeric excess of 92% obtained here is comparable only to that in solid state. OA capsule, we believe, provides restriction to the rotational motions of the reactant pyridone and chiral auxiliary and thus places the chiral auxiliary in a selective conformation with respect to the reactive pyridone part. A correlation between the position of the methyl group on the pyridone ring and diastereoselectivity was noted. Structures of the host-guest complexes were examined by (1)H NMR and the data was used to obtain preliminary information concerning the mechanism of chiral induction within the confined spaces of OA capsule.

Templation of the excited-state chemistry of alpha-(n-alkyl) dibenzyl ketones: how guest packing within a nanoscale supramolecular capsule influences photochemistry.

Excited-state behavior of eight alpha-alkyl dibenzyl ketones (alkyl = CH3 through n-C8H17) that are capable of undergoing type II and/or type I photoreactions has been explored in isotropic solution and within a water-soluble capsule. The study consisted of two parts: photochemistry that explored the excited-state chemistry and an NMR analysis that revealed the packing of each guest within the capsule. The NMR data (COSY, NOESY, and TOCSY experiments) revealed that ternary complexes between alpha-alkyl dibenzyl ketones and the capsule formed by two cavitands are kinetically stable, and the guests fall into three packing motifs modulated by the length of the alpha-alkyl chain. In essence, the host is acting as an external template to promote the formation of distinct guest conformers. The major products from all eight guests upon irradiation either in hexane or in buffer solution resulted from the well-known Norrish type I reaction. However, within the capsule the excited-state chemistry of the eight ketones was dependent on the alkyl chain length. The first group consisted of alpha-hexyl, alpha-heptyl, and alpha-octyl dibenzyl ketones that yielded large amounts of Norrish type II products within the host, while in solution the major products were from Norrish type I reaction. The second group consists of alpha-butyl and alpha-pentyl dibenzyl ketones that yield equimolar amounts of two rearranged starting ketones within the capsule (combined yield of ca 60%), while in solution no such products were formed. The third group consisted of alpha-methyl, alpha-ethyl, and alpha-propyl dibenzyl ketones that within the capsule yielded only one (not two) rearranged starting ketone in larger amounts (21-35%) while in solution no rearrangement product was obtained. Variation in the photochemistry of the guest within the capsule, with respect to the alpha-alkyl chain length of the guest, highlights the importance of how a small variation in supramolecular structure can influence the selectivity within a confined nanoscale reactor.

Making a difference on excited-state chemistry by controlling free space within a nanocapsule: photochemistry of 1-(4-alkylphenyl)-3-phenylpropan-2-ones.

The free space within a reaction cavity plays a determining role during the excited-state reaction of 1-(4-alkylphenyl)-3-phenylpropan-2-ones included within a capsule formed by two molecules of a deep cavity cavitand. By controlling the free space within the reaction cavity through remote alkyl substitution on the reactant ketone it is possible to control the yield of the rearrangement product shown above.