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. 2014 Feb 7;79(3):1386-98.
doi: 10.1021/jo4027929. Epub 2014 Jan 28.

Titania-promoted carboxylic acid alkylations of alkenes and cascade addition-cyclizations

David W Manley  1 , Roy T McBurney, Phillip Miller, John C Walton, Andrew Mills, Christopher O'Rourke
Affiliations

Titania-promoted carboxylic acid alkylations of alkenes and cascade addition-cyclizations

David W Manley et al. J Org Chem. .

Abstract

Photochemical reactions employing TiO2 and carboxylic acids under dry anaerobic conditions led to several types of C-C bond-forming processes with electron-deficient alkenes. The efficiency of alkylation varied appreciably with substituents in the carboxylic acids. The reactions of aryloxyacetic acids with maleimides resulted in a cascade process in which a pyrrolochromene derivative accompanied the alkylated succinimide. The selectivity for one or other of these products could be tuned to some extent by employing the photoredox catalyst under different conditions. Aryloxyacetic acids adapted for intramolecular ring closures by inclusion of 2-alkenyl, 2-aryl, or 2-oximinyl functionality reacted rather poorly. Profiles of reactant consumption and product formation for these systems were obtained by an in situ NMR monitoring technique. An array of different catalyst forms were tested for efficiency and ease of use. The proposed mechanism, involving hole capture at the TiO2 surface by the carboxylates followed by CO2 loss, was supported by EPR spectroscopic evidence of the intermediates. Deuterium labeling indicated that the titania likely donates protons from surface hydroxyl groups as well as supplying electrons and holes, thus acting as both a catalyst and a reaction partner.

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Figures

Scheme 1
Scheme 1. P25-Promoted Alkylations of N-Phenylmaleimide with Simple Carboxylic Acids
Scheme 2
Scheme 2. P25-Promoted Alkylations of N-Phenylmaleimide with Diverse Carboxylic Acids
Accompanied by dimers (see the text). Obtained as a 1:1 mixture of two diastereoisomers.
Scheme 3
Scheme 3. Titania-Promoted Reactions of Aryloxyacetic Acids with N-Substituted Maleimides
Scheme 4
Scheme 4. Substrates and Products for Intramolecular Reactions
Yields in brackets are maximum yields from NMR monitoring of sol–gel tube reactions. Yield estimated by 1H NMR analysis (nd = not determined).
Figure 1
Figure 1
Reaction profile for acid 8a obtained by NMR monitoring.
Scheme 5
Scheme 5. Proposed Mechanism for TiO2-Promoted Reactions of Carboxylic Acids and Maleimides
Scheme 6
Scheme 6. SET Photoxidation Events Taking Place between Carboxylic Acids RXCH2CO2H and the TiO2 Surface
Scheme 7
Scheme 7. Deuterium Labeling Test Reactions
See this image and copyright information in PMC

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