Stille vs. Suzuki - cross-coupling for the functionalization of diazocines.

Diazocines are azobenzene derived macrocyclic photoswitches with well resolved photostationary states for the (E)- and (Z)-isomers, which improves their addressability by light. In this work, effective procedures for the stannylation and borylation of diazocines in different positions are reported. Their use in Stille cross-coupling and Suzuki cross-coupling reactions with organic bromides is demonstrated in yields of 47-94% (Stille cross-coupling) and 0-95% (Suzuki cross-coupling), respectively.

ortho-Functionalization of azobenzenes via hypervalent iodine reagents.

ortho-Functionalized azobenzenes are much sought after molecular switches, as they may be tuned to absorb in the visible range of light and the (Z)-isomers can have high thermal half-lives. To enable straightforward access to these targets, we have developed a synthetic route via novel ortho-substituted azobenzene-functionalized diaryliodonium salts. Selective transfer of the azobenzene moiety to O-, N-, C- and S-nucleophiles under mild, transition metal-free conditions gives access to an unprecedented range of ortho-substituted azobenzenes. The photoswitching properties of the reagents were investigated and the structure was determined by X-ray crystallography.

Active Ester Functionalized Azobenzenes as Versatile Building Blocks.

Azobenzenes are important molecular switches that can still be difficult to functionalize selectively. A high yielding Pd-catalyzed cross-coupling method under mild conditions for the introduction of NHS esters to azobenzenes and diazocines has been established. Yields were consistently high with very few exceptions. The NHS functionalized azobenzenes react with primary amines quantitatively. These amines are ubiquitous in biological systems and in material science.

Synthesis of Phenols and Aryl Silyl Ethers via Arylation of Complementary Hydroxide Surrogates.

Two transition-metal-free methods to access substituted phenols via the arylation of silanols or hydrogen peroxide with diaryliodonium salts are presented. The complementary reactivity of the two nucleophiles allows synthesis of a broad range of phenols without competing aryne formation, as illustrated by the synthesis of the anesthetic Propofol. Furthermore, silyl-protected phenols can easily be obtained, which are suitable for further transformations.

Contrasting Frustrated Lewis Pair Reactivity with Selenium- and Boron-Based Lewis Acids.

The activation of π-bonds in diynyl esters has been investigated by using soft and hard Lewis acids. In the case of the soft selenium Lewis acid PhSeCl, sequential activation of the alkyne bonds leads initially to an isocoumarin (1 equiv PhSeCl) and then to a tetracyclic conjugated structure with the isocoumarin subunit fused to a benzoselenopyran (3 equiv PhSeCl). Conversely, the reaction with the hard Lewis acidic borane B(C6 F5 )3 initiates a cascade reaction to yield a complex π-conjugated system containing phthalide and indene subunits.