Half-​Sandwich Ruthenium Carbene Complexes Link trans-​Hydrogenation and gem-​Hydrogenation of Internal Alkynes

The hydrogenation of internal alkynes with [Cp*Ru]-based catalysts is distinguished by an unorthodox stereochemical course in that E-alkenes are formed by trans-delivery of the two H atoms of H2. A combined experimental and computational study now provides a comprehensive mechanistic picture: a metallacyclopropene (η2-vinyl complex) is primarily formed, which either evolves into the E-alkene via a concerted process or reacts to give a half-sandwich ruthenium carbene; in this case, one of the C atoms of the starting alkyne is converted into a methylene group. This transformation represents a formal gem-hydrogenation of a π-bond, which has hardly any precedent. The barriers for trans-hydrogenation and gem-hydrogenation are similar: whereas DFT predicts a preference for trans-hydrogenation, CCSD(T) finds gem-hydrogenation slightly more facile. The carbene, once formed, will bind a second H2 molecule and evolve to the desired E-alkene, a positional alkene isomer or the corresponding alkane; this associative pathway explains why double bond isomerization and over-reduction compete with trans-hydrogenation. The computed scenario concurs with para-hydrogen-induced polarization transfer (PHIP) NMR data, which confirm direct trans-delivery of H2, the formation of carbene intermediates by gem-hydrogenation, and their evolution into product and side products alike. Propargylic −OR (R = H, Me) groups exert a strong directing and stabilizing effect, such that several carbene intermediates could be isolated and characterized by X-ray diffraction. The gathered information spurred significant preparative advances: specifically, highly selective trans-hydrogenations of propargylic alcohols are reported, which are compatible with many other reducible functional groups. Moreover, the ability to generate metal carbenes by gem-hydrogenation paved the way for noncanonical hydrogenative cyclopropanations, ring expansions, and cycloadditions

Computational Investigations into the Mechanisms of Trans-Selective Hydrogenation and Hydrometalation of Alkynes

An overview on the mechanisms of the trans-selective hydrogenation and hydrometalation of alkynes using a CpRuL catalyst is provided. Unlike the more common mode of hydrogenation, syn, this select ruthenium catalyst system uniquely favors the products resulting from anti-addition across the alkyne π-system. The primary focus is summarizing the results of computational studies on the mechanism of these reactions, including key experimental supporting evidence. This chapter addresses hydrogenation, hydrosilylation, hydrostannation, and hydroboration. An overview of the mechanisms for all of these processes is highlighted

Further Investigation of the Tolerance and Mechanical Adjustability of the *Acri.Tec AR-1 PC/IOL in Rabbit Eyes: An Intraocular Lens with Reversibly Adjustable Optical Power

Purpose: To examine the tolerance and mechanical function of an adjustable intraocular lens (IOL) in rabbit eyes. Methods: Implantation of the * Acri.Tec AR-1 PC/IOL into 14 rabbit eyes. Manipulation of the lens 8 weeks after implantation in order to change the refractive power. Follow-up for up to 5 months. Histopathologic examination of the eyes. Results: Implantation and mechanical adjustment of the PC/IOL were possible. Eyes healed normally. No difference between eyes containing the * Acri.Tec AR-1 PC/IOL and eyes containing the control PC/IOL could be detected with respect to signs of inflammatory reaction, corneal transparency, intraocular pressure and histopathologic appearance. Histopathologic examination of the eyes showed that the * Acri.Tec AR-1 PC/ IOL did not cause any damage in rabbit eyes. Conclusion: The * Acri.Tec AR-1 PC/IOL is well tolerated in rabbit eyes for extended periods of time, suggesting that this PC/IOL should be well tolerated in the long run. Surgical adjustment of the adjusting element can be performed with little effort several weeks after implantation