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Generation and Application of Nucleophilic Allylmetal Species by Catalytic 1,4-Metal Migration

Periodic Reporting for period 1 - Nu14Mig (Generation and Application of Nucleophilic Allylmetal Species by Catalytic 1,4-Metal Migration)

Reporting period: 2015-06-11 to 2017-06-10

"""Allylmetal species"" are versatile synthetic intermediates in chemistry, which, in their simplest form, are composed of a metal atom bonded to a three-carbon fragment (which also contains a carbon-carbon double bond). Allylmetal species are used in a wide range of reactions to produce numerous important chemical building blocks. In the Host Laboratory, a new method to access allylmetal species was discovered, which involves an addition to a carbon-carbon triple bond to give one organometallic intermediate. The metal in this intermediate then undergoes migration to another position four carbon atoms away. Overall, this migration is called ""1,4-migration"". In the original discovery, rhodium was used as the metal, and the resulting allylrhodium species was used to react with another group in the same molecule to form a cyclic compound. The advantage of this method for producing allylmetal species is that the chemical precursors used are relatively simple, which avoids the need to prepare more complex, more highly functionalized precursors that might otherwise be required using more conventional methods.

The original objectives of this project were to:
(a) Define the scope and limitations of this method in producing a wide range of cyclic compounds.
(b) Develop variants of these reactions that are able to give products selectively as a single enantiomer (which is defined as a non-superimposable mirror-image form). The ability to produce compounds as single enantiomers is very important because the exact function of compounds within biological systems (e.g. if the compounds are to be used as medicines) often depends upon which mirror image they exist in.

Ultimately, work in this project was not successful in the original area of study, but in a related area involving the generation of sulfur-based reactive species by visible light in conjunction with an iridium catalyst (Scheme 1). This new method was successful in producing a range of cyclic products in good yields under mild reaction conditions, and at the time of writing, this work is being finished up for publication.




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"At the beginning of this project, the Fellow attempted to expand upon the preliminary results already obtained in the Host Laboratory by increasing the scope of the reactions. However, this work proved to be of limited success. Therefore, the Fellow decided to focus upon another, related line of enquiry; in particular, it was of interest whether it would be possible replace the expensive rhodium catalyst with a cheaper, more readily available cobalt-based catalyst system. The replacement of precious metals with base metals is an important grand challenge in catalysis, with broad implications for sustainability and society. We selected cobalt on the basis that it might exhibit similar properties with rhodium, as they are both group 9 metals. Yoshikai and co-workers have also recently demonstrated the ability of low-valent cobalt to undergo 1,4-migration reactions (e.g. see Angew. Chem., Int. Ed. 2012, 51, 9610; Angew. Chem., Int. Ed. 2013, 52, 10496). Interesting, cobalt catalysis provided completely different products from rhodium catalysis, as a result of an anti¬-carbometallative cyclization and not from 1,4-cobalt migration. These results were mechanistically interesting, and significant effort was made to improve upon the low yields obtained initially. Although a comprehensive evaluation of reaction parameters was conducted, this failed to provide satisfactory and consistently reproducible results.

Fortunately, however, a new discovery was made in a related area that also produced highly functionalized cyclic compounds (Scheme 1). Once again, this reaction is initiated by an addition to a carbon-carbon triple bond, followed by reaction of the resulting species with another group in the same molecule. However, the initial reactive agent is a sulfur species containing a single unpaired electron (this species is called a ""sulfonyl radical""). This reactive species is generated by the action of visible light in combination with an iridium-based catalyst, on a class of chemical reagent called ""sulfonyl azides"" and this new synthetic turned out to be versatile and relatively broad in scope.

Public Engagement and Outreach Activities:
The School of Chemistry, University of Nottingham conducted a Science fair to attract and create enthusiasm among schoolchildren towards science. The event took place on the mornings of Tuesday 22nd, Wednesday 23rd and Thursday 24th March, 2016, 10am-12pm. The Fellow actively participated in the fair, and demonstrated Alchemy experiments such as “turning copper 2p coins either to silver or to gold”. A total of 80 pupils actively participated in the sessions on each day, and the Fellow improved their general communication skills greatly by talking science with the general public/children.

Dissemination Activities:
At the time of writing, the results of this work are being prepared for publication as a full article. There are some final experiments which need to be completed, and this is being done by a colleague of the Fellow. We anticipate this will be completed in the next month or two, with manuscript submission to follow shortly thereafter. This work has also been presented in research seminars by the Host Scientist in industry and at a conference within the UK.

Training and Career Progression of the Fellow:
When the Fellowship began, the Fellow had expertise in carbohydrate chemistry, supramolecular chemistry, and crystal engineering rather than in the development of new organic reactions using transition metal catalysis, which is the main area of this project. Therefore, the Fellow had to learn new skills and gain new knowledge, but did this successfully. At the end of this project, the Fellow is now highly trained in this area. Currently, the Fellow is searching for a new postdoctoral/research scientist position to consolidate his skills in preparation for an independent research career."
The main finding of this fellowship is a new way to generate sulfonyl radicals under mild reaction conditions from sulfonyl azides using visible light and an iridium photocatalyst. The key ingredient to get this sulfonyl radical generation to work is one particular solvent - tetrahydrofuran (THF). Notably, sulfonyl azides usually react by losing dinitrogen with nitrogen-nitrogen bond cleavage, which is thermodynamically very favorable. However, in this case, the particular combination of reagent, catalyst, visible light, and solvent ultimately results in sulfur-nitrogen bond cleavage to give a sulfur radical. We expect that the principles behind this mild method of radical initiation could be used to design related reactions in future, which could open up new methods for complex molecule synthesis.
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