Borylene Transfer from Transition Metal Borylene Complexes

Transition metal borylene complexes (LnM=BR) were predicted to be possible sources of the borylene fragment (:BR) in 1973, decades before the first complex had even been isolated. Over the past 15 years, routes to a small range of borylene complexes have been developed by a handful of research groups, including a family of active borylene donor complexes. Through these and other borylene donors, the borylene transfer to transition metal fragments has recently emerged as the most reliable and mild route to new transition metal borylene complexes, with the synthesis of a number of novel "second-generation" borylene complexes. Borylene transfer to alkynes was found to be a dependable route to the unusual "borirene" architecture, and more recently, the insertion of a borylene fragment into an unactivated alkenyl C-H bond has hinted at further borylene insertions into element-element bonds. This review covers borylene transfer reactions to both transition metal and main group substrates, from the first adventitious reports in the late 1990s up to the current state of intentional employment of the borylene transfer reaction in synthesis.