Abstract
The intramolecular C-H borylation of (hetero)arenes and alkenes using electrophilic boranes is a powerful transition metal free methodology for forming C-B bonds. These C-H borylation reactions are preceded by intermolecular bond (both dative and covalent) formation, with examples proceeding via initial C-B and N-B bond formation dominating this field thus both are discussed in depth herein. Less prevalent intramolecular electrophilic C-H borylation reactions that proceed by intermolecular O-B, S-B and P-B bond formation are also summarised. Mechanistic studies are presented that reveal two mechanisms for C-H borylation, (i) electrophilic aromatic substitution (prevalent with B-X electrophiles); (ii) σ-bond metathesis mediated (prevalent with B-H and B-R electrophiles). To date, intramolecular electrophilic C-H borylation is utilised mainly for accessing boron containing conjugated organic materials, however recent developments, summarized herein alongside early studies, have highlighted the applicability of this methodology for forming synthetically versatile organo-boronate esters and boron containing bioactives. The multitude of synthetic procedures reported for intramolecular electrophilic C-H borylation contain many common features and this enables key requirements for successful C-H borylation and the factors effecting regioselectivity and substrate scope to be identified, discussed and summarized.
Highlights
The formation ofC–B bonds is of significant import across many fields
The past twenty years in particular have seen the utility of organoboranes expand with the incorporation ofC–B units established as a useful function imparting tool in sensors[2] and organic materials (Fig. 1).[3]
While its use is well established in the organic materials field, intramolecular electrophilic C–H borylation is much more rarely utilised to access organoboranes for use in functional group transformations or for accessing the boracycles found in sensors and bioactive molecules
Summary
The formation of (sp2)C–B bonds is of significant import across many fields. Historically, this is due to the power of organoboranes in functional group transformations coupled with their low toxicity and ease of handling (relative to other organometallic reagents).[1]. He completed his MChem under the supervision of Dr Duncan Browne and Dr Louis Morrill on the study of organocatalysis under mechanochemical conditions He joined the Ingleson group in 2018 where he is currently working on directed electrophilic C–H borylation using trihaloboranes. Since its discovery in the 1950s this transformation has been most widely used to make boron containing organic materials containing three and four coordinate boron centres.[8] While its use is well established in the organic materials field, intramolecular electrophilic C–H borylation is much more rarely utilised to access organoboranes for use in functional group transformations or for accessing the boracycles found in sensors and bioactive molecules This is in part due to the misconception that C–H borylation always requires forcing conditions and reactive catalysts (e.g. AlCl3). Due to a number of commonalities in mechanism and C–H borylation conditions, this review covers the formation of borylated products that are 3 and 4 coordinate at boron
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