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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Palladium-Catalyzed Amide Formation via Masked Isocyanates

Brzezinski, David 22 December 2020 (has links)
Amides are one of the most common functional groups in biological systems and in bioactive molecules. Arguably the most direct way to form amides is via the condensation of an amine onto a carboxylic acid. This reaction is notoriously difficult and has stimulated much development, including the developments of new reagents and catalysts to perform this transformation under milder conditions. More broadly, amide formation continues to be of high importance and the incorporation of emerging transformations utilizing new disconnections are complimentary to existing routes. Isocyanates are the simplest electrophiles containing the desired NCO motif and have a large presence in the polymer (e.g. polyurethane) and paint industries. In addition, isocyanates have been utilized for amide formation with various nucleophiles in a stoichiometric and catalytic fashion, but the inherent functional group intolerance associated with the high reactivity of isocyanate largely remains. Efforts have been made to address such limitations of isocyanates, including the use of a blocking group which allow for in situ release of the isocyanate while using a bench stable masked (blocked) isocyanate precursor. Changes to the blocking group structure have direct correlations to the stability and reactivity of the precursor, which helps in suppressing common side reactions observed with free isocyanates such as polymerization or oligomerization. Incorporation of a blocking group strategy in catalytic amide forming reactions has the power to unlock the potential of isocyanates with reactivity that would not be attainable with free isocyanates. Reports imparting this strategy exemplify the power of a blocking group with increased applicability and functional group tolerance compared to reactions with the free isocyanate counterpart. The implementation of this strategy for catalytic amide formation is sparse including only two reports with a rhodium catalyst. Utilization of different metals could broaden the scope of reactivity allowing for extensions that the rhodium (I) catalyst cannot do. The development of a palladium-catalyzed amide synthesis via masked isocyanates was targeted (Chapter 2). Indeed, implementation of a blocking group strategy with alkyl and aryl isocyanates allowed for efficient synthesis of amides with electron rich and mildly deficient aryl boroxine nucleophiles. Catalysis was achieved with 1 mol% of Pd(OAc)2 and 2 mol% of SPhos at 50 ℃ with Et3N to aid in the deblocking of the isocyanate. Several control experiments were iii conducted to obtain mechanistic insight including what mechanism may be operative as well as the necessity of this blocking group strategy. Kinetic studies were performed using the variable time normalization analysis method and have yielded the following information: 1) the presence of catalyst decomposition, 2) that the rate determining step involved the catalyst, boroxine, and masked isocyanate, and 3) that the rate determining step is likely the insertion into the isocyanate. In summary, palladium catalysts can achieve catalysis with masked isocyanates to facilitate amide formation under appropriate conditions. With limited reports of masked isocyanates in catalysis, this reactivity could act as a steppingstone for developments of reactivity that are held back with the use of free isocyanates.

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