Stereoselective Diels-Alder Reactions of gem-Diborylalkenes: Toward the Synthesis of gem-Diboron-Based Polymers via ROMP

Abstract

Although gem-diborylalkenes are known to be among the most valuable reagents in modern organic synthesis, providing a rapid access to a wide array of transformations, including the construction of C−C and C‐heteroatom bonds, their use as dienophile-reactive groups has been rare. Herein we report the Diels-Alder (DA) reaction of (unsymmetrical) gem-diborylalkenes. These reactions provide a general and efficient method for the stereoselective conversion of gem-diborylalkenes to rapidly access 1,1-bisborylcyclohexenes. Using the same DA reaction manifold with borylated-dienes and gem-diborylalkenes, we also developed a concise, highly regioselective synthesis of 1,1,2-tris- and 1,1,3,4-tetrakis(boronates)cyclohexenes, a family of compounds that currently lack efficient synthetic access. Furthermore, DFT calculations provided insight into the underlying factors that control the chemo-, regio, and stereoselectivity of these DA reactions. This method also provides stereodivergent syntheses of gem-diboryl-norbornenes. The utility of the gem-diboryl-norbornene building blocks was demonstrated by ring-opening metathesis polymerization (ROMP), providing a highly modular approach to the first synthesis of the gem-diboron-based polymers. Given its simplicity and versatility, we believe that this novel DA and ROMP approach holds great promise for organoboron synthesis as well as organoboron-based polymers and that it will result in more novel transformations in both academic and industrial research.<br>

A Stereodivergent Approach to the Synthesis of gem-Diborylcyclopropanes

“…This is in good agreement with the fact that the aromatic-planar Bdan group is less bulky than the Bpin group. 14,16 Although it would appear that the diastereoselectivity here is driven primarily by sterics, we estimated that a second consideration of the possible interactions in TS-I (follow the purple color) through the hydrogen bonds between the NH of the Bdan moiety and the oxygen atoms might play a role in this selectivity (Figure 4). These actions are supported by the low diastereoselectivity of 4i, which resulted from the cyclopropanation reaction of aryl diazotrifluoromethyl compound 2-CF3, where both carbene substitutes, Ph vs. CF3, are only slightly sterically differentiated, along with the absence of hydrogen bond interaction (Figure 3).…”

“…Thus, two possible diastereomers are defined by the respective positions of the substituents on the diazo and boryl groups, i.e., Bpin and Bdan, respectively. However, based on our reported stereoselective Diels-Alder reaction, 16 it was anticipated that the [2+1] cycloaddition might proceed with good stereoselectivity (Figure 3).…”

“…15 We sought to obtain product 5 using our recently reported conditions for the late-stage trifluoroboration of gem-diborylalkanes by employing KF (Path b, Figure 4). 15,16 In this way, this would be the first time that this protocol is applied to gem-diborylcyclopropanes 3 bearing an all-carbon quaternary center adjacent to the diboryl unit. 14,15 We were pleased to observe the desired cyclopropane-mono-BF3K product, 5a, in good yield (Figure 4).…”

“…Based on our last reports on the reactivity and selectivity of gem-diborylalkenes (1) in new addition reactions, i.e., the stereoselective Diels-Alder reaction 16 and the radical mediated transformation 17,18 (Figure 1B), we posited that these reactive electron-deficient gem-diborylalkenes 19 (1) have the potential to serve as "carbenophiles" for the cyclopropanation reactions. [16][17][18] We hypothesized that this could be achieved by the transition metal-catalyzed cyclopropanation reaction of diazo-compounds (e.g., 2) with gem-diborylalkenes 1 (Figure 1D). In this regard, and during the preparation of this manuscript, a recent sophisticated report by Fernández and co-workers described a stereoselective cyclopropanation reaction of gem-diborylalkenes 19 1 via Pd-catalyzed (trimethylsilyl)diazomethane (Figure 1C).…”

Vinyl-Addition Polymerizations of Borylated Allenes

“…22−38 While these precedents demonstrate the versatility of unifying boron chemistry with polymer science, there are lingering obstacles to the preparation of borylated materials. For example, the synthetic scope is largely limited to polymers derived from aromatic boronates (although materials bearing aliphatic boronates were recently prepared using radical 24,25 or metal-catalyzed polymerizations 26 ). 15−21 Jakle reported early examples of preparing well-defined borylated polystyrenes either through a substituent exchange reaction 15 or through ATRP.…”