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-diborylnorbornenes. The utility of the gem-diborylnorbornene 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. Additionally, these polymers have been successfully submitted to postpolymerization modification reactions. 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.
Highlights
Organoboron reagents have had an enormous impact on the development of new chemical reactions[1] and have extended the scope of accessible complex molecular scaffolds.[2−4] Organoboronate compounds are attractive owing to their wide availability and air stability,[5] making them versatile reagents in organic synthesis.[6,7]
Key challenges include (1) the steric repulsion introduced by the two groups of the bulky Bpin units in the TS of the cycloaddition reaction;[37] (2) whether the regio- and stereoselectivity of the cycloaddition can be controlled when two unsymmetrical boron groups are placed on the geminated carbon of dienophile 2′;13 and (3) whether the reaction can proceed in a regioselective manner when borylated dienes react with 2 (Figure 2B).[34]
We have developed a method that addresses the long-standing challenge of regio- and stereoselective Diels− Alder cycloadditions with poly-alkenylboranes. This was achieved by introducing a new method that enables the use of gem-diborylalkenes as a reasonable reactive dienophile for the DA reaction
Summary
Organoboron reagents have had an enormous impact on the development of new chemical reactions[1] and have extended the scope of accessible complex molecular scaffolds.[2−4] Organoboronate compounds are attractive owing to their wide availability and air stability,[5] making them versatile reagents in organic synthesis.[6,7]. Many synthetic methods utilize transformations of C−B bonds,[2] the development of polyborylated reagents would enable greater structural diversity, an important objective.[8] over the past decade, much effort has been expended to synthesize new functionalized classes of polyboronates, which have been shown to be excellent building blocks for the modular construction of new compounds.[9−12]. Among polyboron-containing structural motifs, (unsymmetrical)[13] gem-diboron derivatives (2, 3′) are a well-known emerging class with good potential for novel synthetic applications (Figure 1A).[14−17] The special properties and structures of bisnucleophile gem-diboryl compounds 2 and 3′ (termed geminated organodimetallics)[18,19] have attracted increasing attention from synthetic chemists, in constructing C−C/C−heteroatom bonds. As a part of a general program to investigate the reactivity and selectivity of gem-diboryl compounds in new synthetic applications, we sought to prepare variants bearing the gemdiboryl-norbornene group (4) because these strained compounds (∼27 kcal/mol of inherent strain) might offer new opportunities toward the ring-opening metathesis polymerization (ROMP) reaction[31] and lead to unprecedented gemdiboryl-based polymer II′ (Figure 1D)
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