Photosensitized [2+2]‐Cycloadditions of Alkenylboronates and Alkenes

Abstract

A new strategy for the synthesis of highly versatile cyclobutylboronates via the photosensitized [2 +2]-cycloaddition of alkenylboronates and alkenes is presented. The process is mechanistically different from other processes in that energy transfer occurs with the alkenylboronate as opposed to the other alkene. This strategy allows for the synthesis of an array of diverse cyclobutylboronates. The conversion of these adducts to other compounds as well as their utility in the synthesis of melicodenine C is demonstrated.

Boronic Ester Enabled [2 + 2]-Cycloadditions by Temporary Coordination: Synthesis of Artochamin J and Piperarborenine B

“…While several strategies are known for cyclobutylboronate synthesis, − [2 + 2]-cycloadditions retain their own importance because (1) the reactions are convergent, (2) the alkene starting materials are generally widely available, and (3) products can be prepared that would be inaccessible by other approaches. , With respect to photochemical [2 + 2]-cycloaddition, , prior work has demonstrated that alkene triplet excited states can be captured with alkenylboronates. More recently, our lab has demonstrated that alkenylboronates can be photosensitized and undergo reaction with a variety of activated alkenes (e.g., styrenes, dienes) . In the latter case, the Bpin unit acted as an activating group to allow for the cycloaddition to proceed in good yield.…”

“…The initial reaction optimization was conducted with allyl alcohol ( 1 ) and E -styrenylBpin ( 2 ). The choice to use E -styrenylBpin ( 2 ) stems from the fact that the sytrenyl component could be sensitized with common visible-light-activated sensitizers, , thus avoiding the use of high-energy UV light at this stage of development. Treatment of allyl alcohol ( 1 ) and E -styrenylBpin ( 2 ) in the presence of fac- Ir­(ppy) 3 did not lead to product formation.…”

“…Radical addition allows for formation of VII , which upon intersystem crossing (ISC) and radical recombination, leads to formation of the product. On the other hand, if the conditions are modified such that the Bpin acts as an activating group and not a coordinating group (as previously described), product 48 is generated (Scheme B). Here, triplet energy transfer occurs with 2 to form VIII .…”

Manganese catalyzed acceptorless dehydrogenative borylation of styrenes

“…The [2+2]-cycloaddition of alkyl boronate ester 2 with 4-methoxy styrene under photochemical irradiation of 456 nm blue LED with fac-Ir(ppy)3 as a photosensitizer and subsequent oxidation with NaBO3‧4H2O lead to cyclobutanol derivative 34 in 67% yields and 4:1 diastereomeric ratio. 64 To further demonstrate the synthetic utility, the developed methodology was employed to devise the synthesis of several bioactive compounds. We have also utilized vinyl boronate ester 18 to synthesize stilbene derivatives 35-37.…”

The Impact of Boron Hybridisation on Photocatalytic Processes

“…It is pertinent to note this allowed a series of boron functionalities of different hybridisation states to be compatible with model reaction conditions. Recently Brown and co‐workers provided a striking example demonstrating EnT‐catalysed intermolecular [2+2] cycloaddition of alkenyl boronic esters (Scheme 18, bottom) [103] . Mechanistic studies identified a triplet energy transfer mechanism highlighting that energy is transferred to the alkenyl (styrene or diene) organoboron species.…”