Allenylboronic Acid Pinacol Ester: A Selective Partner for [4 + 2] Cycloadditions

Abstract

We have studied the reaction of allenylboronic acid pinacol ester with cyclopentadiene with experimental and computational methods. The reaction occurred efficiently with complete Diels–Alder periselectivity and regioselectivity at the proximal double bond. The concerted mechanism for the observed transformation was computed to be favored over competitive addition to the distal double bond, [3,3]-sigmatropic rearrangements, and stepwise radical mechanism. This unprecedented Diels–Alder reaction enables the construction of synthetically versatile boron-substituted cycloadducts.

Diels–Alder Reactivity of Allenylboronic Acid Pinacol Ester and Related Dienophiles: Mechanistic Studies and Distortion/Interaction-Activation Strain Model Analysis

“…The choice of these levels of theory was based on the results described in literature and in our previous study for the reaction of 4a. 13 The nature of the stationary points was verified by normal-mode analysis, and Intrinsic Reaction Coordinate (IRC) 51 calculations were performed to link the TSs to the corresponding minima. To examine the more important interactions in the TSs, natural bond orbital calculations were carried out.…”

“…9 Recently, we reported the novel use of a boron-substituted allene as a DA dienophile. 13 Allenes can participate in [4 + 2], [3 + 2], and [2 + 2] cycloadditions under thermal and photochemical conditions. 14−16 The reaction between allenylboronic acid pinacol ester (4a) and cyclopentadiene (2) proceeds in a completely peri-and regioselective fashion, yielding product 5a in a 50:50 endo/exo ratio (Scheme 1, eq b).…”

“…The Diels–Alder (DA) cycloaddition reaction is one of the most useful transformations in organic synthesis due to its ability to yield six membered rings in a single step and generate up to four stereocenters in a stereospecific and stereo- and regioselective manner. − In our group, the DA reactions of unsaturated organoboron compounds have been studied extensively, both experimentally and computationally. − We investigated a variety of DA reactions using alkenylboronic esters as dienophiles, such as the reaction between vinylboronic acid pinacol ester ( 1a ) with cyclopentadiene ( 2 ) which gave rise to the DA cycloadducts ( 3a ) with a 96% yield when performed in toluene at 170 °C (Scheme , eq a) . Recently, we reported the novel use of a boron-substituted allene as a DA dienophile . Allenes can participate in [4 + 2], [3 + 2], and [2 + 2] cycloadditions under thermal and photochemical conditions. − The reaction between allenylboronic acid pinacol ester ( 4a ) and cyclopentadiene ( 2 ) proceeds in a completely peri- and regioselective fashion, yielding product 5a in a 50:50 endo / exo ratio (Scheme , eq b).…”

Regioselectivity Switch Towards the Development of Innovative Diels‐Alder Cycloaddition and Productive Applications in Organic Synthesis

“…Allenes systems have witnessed to huge expansion in recent decades because of their distinctive reactivity and architecture, [49] allowing them to perform [4+2] cycloaddition and excel in organic synthesis [76] . In the year 2021, Pellegrinet and colleagues reported an excellent example of regioselective Diels‐Alder cycloaddition with boron masked dienophile [77] . They tried to explore the reactivity pattern of allenylboron compounds in continuation of their previous findings on Diels‐Alder cycloaddition of alkenylboronic esters under various conditions such as Lewis acid aided, microwave, or heating [78–80] .…”

“…[76] In the year 2021, Pellegrinet and colleagues reported an excellent example of regioselective Diels-Alder cycloaddition with boron masked dienophile. [77] They tried to explore the reactivity pattern of allenylboron compounds in continuation of their previous findings on Diels-Alder cycloaddition of alkenylboronic esters under various conditions such as Lewis acid aided, microwave, or heating. [78][79][80] As allenylboron compounds have been employed in a range of chemical transformations, such as propargylations, allenylations, Suzuki cross -couplings, and related coupling reaction protocols, their potential in Diels-Alder cycloaddition has been revealed for the first time as allenyl derivative.…”