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Title: Copper mediated transformations of alkyl boronic esters
Author: Grayson, James
ISNI:       0000 0001 2432 2563
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2020
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Alkyl boron reagents have a lot of potential as synthetic building blocks. However, in comparison to their aryl boron counterparts, catalytic transformations of alkyl boronic esters are highly underdeveloped. For example, the Chan-Evans-Lam coupling of aryl boronic acids to form aryl amines is a well explored method, but examples of similar couplings with alkyl boronic esters are scarce. The successful development of such a transformations would give a powerful synthetic method to prepare valuable alkyl amines. Conditions have been developed for the copper-mediated coupling of a range of 2° alkyl pinacol boronic esters and anilines. This functional group tolerant method uses stoichiometric Cu(OAc)2 and inert argon atmosphere to overcome the formation of oxidation side-products. Complementary to current reports, coupling is shown to be selective for amine over amide functionalities. Low conversion of 2° amines and non-benzylic boronic esters show a limitation of these conditions. Preliminary mechanistic data suggests the formation of a radical intermediate. The conditions have been applied to analogous O- and S- coupling partners showing varied success. This method has been extended to the coupling of 3° boronic ester and anilines. To our knowledge, this is the first example of the use of 3° boronic esters in a Chan-Evans-Lam reaction. This is an attractive method for the formation of synthetically challenging 3° amines, which cannot be prepared through reductive amination. Cu-catalysed conditions for the direct conversion of benzylic boronic esters to the corresponding carbonyl have been developed. This mild and functional group tolerant method uses an ambient atmosphere of air as the terminal oxidant. The oxidation occurs selectively even in the presence of an unprotected alcohol. Preliminary investigations suggest the reaction proceeds through an alkyl boron to Cu transmetallation, peroxide formation, and rearrangement to give the carbonyl. To our knowledge this is the first example of a coppercatalysed C-B bond oxidation to the carbonyl oxidation state.
Supervisor: Partridge, Benjamin Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available