2-Naphthaldehyde: Photochemical Reactivity & Industrial Intermediate

2-Naphthaldehyde is an organic compound that serves as an important intermediate in various industrial applications. It is characterized by its aromatic aldehyde structure and is widely used in the production of pharmaceuticals, fragrances, and agrochemicals. Its appearance is typically a pale yellow to light brown crystalline powder with a distinctive aromatic odour. It has a melting point of 58–61°C, a boiling point of 160°C (19 mmHg), and a density of approximately 1.2 g/cm³. 2-Naphthaldehyde can be used as a reactant: In asymmetric three-component Mannich reaction; For the synthesis of Hantzsch 1,4-dihydropyridines and For the synthesis of pyrazolo[1,2−b]phthalazinediones.

The ortho photocycloaddition of 2-naphthaldehyde

Upon direct excitation, 1-naphthaldehyde and 2-naphthaldehyde undergo photochemical transformations at the aldehyde carbonyl group. Like for many other aromatic aldehydes, the typical reaction products are oxetanes (Paternò–Büchi reaction), alcohols (photoreduction) or carbonyl addition products (photoaddition). The reaction of 1-naphthaldehyde in 2,3-dihydrofuran for example has been reported to deliver oxetane 3 in a yield of 55% . The reactivity pattern is determined by the nπ* character of the reactive excited states (singlet or triplet) or it is due to electron transfer pathways which generate a radical anion by reduction of the carbonyl group. In the absence of a Lewis acid the aldehydes are not sufficiently reactive to undergo a PET with silanes such as benzyltrimethylsilane (TMSBn). The addition of magnesium perchlorate (4 eq.) allows for the desired transformation and product 4 was obtained in the reaction of 2-naphthaldehyde and TMSBn (2 eq.). In the present study, we have compared the photochemical reactivity of 1- and 2-naphthaldehyde towards a typical olefin component (2,3-dimethyl-2-butene) in the absence and in the presence of catalytic amounts of strong Lewis acids. The Lewis acids induced a complete reversal of the type selectivity from carbonyl reactions in the absence of a Lewis acid to reactions at the C1/C2 double bond of the aromatic core. To the best of our knowledge, a complete type selectivity reversal in the photochemistry of aldehydes has not yet been reported. Previous studies in photochemistry have mainly focussed on the choice of irradiation wavelength or the addition of a triplet sensitizer.[1]

Although 1- and 2-naphthaldehyde had been previously shown to undergo Paternò–Büchi and photocycloaddition reactions with olefins we closely examined the yet unreported reaction with 2,3-dimethylbutene in order to rule out any involvement of the aromatic π system. Upon irradiation at λ = 366 nm product formation was observed and three major products could be isolated and identified for both aldehydes. Isosbestic points at 255 nm, 288 nm, and 340 nm indicate that there are only two UV active species present in solution. The absorption of the Lewis acid complex stretches into the visible region and the solution of the complex is yellow-coloured. Likewise 2-naphthaldehyde showed a less pronounced but clearly identifiable absorption maximum at λ = 398 nm. If the reaction mixture was warmed to 0 °C after complete photochemical conversion, a further transformation to this product continued which was complete after another 12 hours. The product was identified as alcohol which could be isolated in 74% yield. The reaction cascade thus offers access to a formal [3 + 2] cycloaddition product of 2-naphthaldehyde with C–C bond formation at the aldehyde carbon atom and at carbon atom C1. In summary, we have found that naphthaldehydes change their photochemical reactivity pattern dramatically in the presence of EtAlCl2 or AlBr3 as Lewis acids. The site of reactivity is shifted from the carbonyl group to the C1/C2 arene double bond where an initial ortho photocycloaddition with olefins occurs. Consecutive Lewis acid promoted reactions lead to an opening of the cyclobutane ring and to the formation of a C2-alkylated naphthalene in the case of 1-naphthaldehyde and to a formal [3 + 2] cycloaddition product for 2-naphthaldehyde.

References

[1]Stegbauer S, Jeremias N, Jandl C, Bach T. Reversal of reaction type selectivity by Lewis acid coordination: the ortho photocycloaddition of 1- and 2-naphthaldehyde. Chem Sci. 2019 Jul 29;10(37):8566-8570. doi: 10.1039/c9sc03315g. PMID: 31803430; PMCID: PMC6839505.

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