The transesterification of boronate esters with diols is tunable over 14 orders of magnitude. Rate acceleration is achieved by internal base catalysis, which lowers the barrier for proton transfer. Here we report a photoswitchable internal catalyst that tunes the rate of boronic ester/diol exchange over 4 orders of magnitude. We employed an acylhydrazone molecular photoswitch, which forms a thermally stable but photoreversible intramolecular H-bond, to gate the activity of the internal base catalyst in 8-quinoline boronic ester. The photoswitch is bidirectional and can be cycled repeatedly. The intramolecular H-bond is found to be essential to the design of this photoswitchable internal catalyst, as protonating the quinoline with external sources of acid has little effect on the exchange rate.

5-Iodo-1,2,3-triazole (iodotriazole) can be prepared from a copper(I)-catalyzed reaction between azide and terminal alkyne in the presence of an iodinating agent, with 5-protio-1,2,3-triazole (protiotriazole) as the side product. The increasing utilities of iodotriazoles in synthetic and supramolecular chemistry drive the efforts in improving their selective syntheses based on a sound mechanistic understanding. A routinely proposed mechanism takes the cue from the copper(I)-catalyzed azide-alkyne cycloaddition, which includes copper(I) acetylide and triazolide as the early and the late intermediates, respectively. Instead of being protonated to afford protiotriazole, an iodinating agent presumably intercepts the copper(I) triazolide to give iodotriazole. The current work shows that copper(I) triazolide can be iodinated to afford iodotriazoles. However, when the reaction starts from a terminal alkyne as under the practical circumstances, 1-iodoalkyne (iodoalkyne) is an intermediate while copper(I) triazolide is bypassed on the reaction coordinate. The production of protiotriazole commences after almost all of the iodoalkyne is consumed. Using (1)H NMR to follow a homogeneous iodotriazole forming reaction, the rapid formation of an iodoalkyne is shown to dictate the selectivity of an iodotriazole over a protiotriazole. To ensure the exclusive production of iodotriazole, the complete conversion of an alkyne to an iodoalkyne has to, and can be, achieved at the early stage of the reaction.

The NHC-catalyzed transformation of unsaturated aldehydes into saturated esters through an organocatalytic homoenolate process has been thoroughly studied. Leveraging a unique "Umpolung"-mediated β-protonation, this process has evolved from a test bed for homoenolate reactivity to a broader platform for asymmetric catalysis. Inspired by our success in using the β-protonation process to generate enals from ynals with good E/Z selectivity, our early studies found that an asymmetric variation of this reaction was not only feasible, but also adaptable to a kinetic resolution of secondary alcohols through NHC-catalyzed acylation. In-depth analysis of this process determined that careful catalyst and solvent pairing is critical for optimal yield and selectivity; proper choice of nonpolar solvent provided improved yield through suppression of an oxidative side reaction, while employment of a cooperative catalytic approach through inclusion of a hydrogen bond donor cocatalyst significantly improved enantioselectivity.

An improved method has been developed for the preparation of 5-iodo-1,2,3-triazoles directly from organic azides and terminal alkynes by a reaction mediated by copper(I) and iodinating agents generated in situ. The major methodological advance of the current procedure is that it provides a high conversion and good iodo/proto selectivity with a broad range of substrates without using an excess of the alkyne, which was required in the previous method. The use of an accelerating ligand is essential to the success of reactions involving unreactive azides or alkynes. New mechanistic insights are provided, including the confirmation that a 1-iodoalkyne is formed as a key intermediate under the established conditions for the reaction.

Synthesis of 5-Iodo-1,2,3-triazoles from Organic Azides and Terminal Alkynes: Ligand Acceleration Effect, Substrate Scope, and Mechanistic Insights. -The improved method for the selective formation of 5-iodo-1,2,3-triazoles proceeds via an iodoalkyne intermediate under mild conditions. The use of an accelerating tris-triazolyl ligand is essential for the success of reactions involving unreactive azides or alkynes. -(BARSOUM, D. N.; BRASSARD, C. J.; DEEB, J. H. A.; OKASHAH, N.; SREENATH, K.; SIMMONS, J. T.; ZHU*, L.; Synthesis 45 (2013) 17, 2372-2386, http://dx.