Electrocatalytic Semihydrogenation of Terminal Alkynes Using Ligand-Based Transfer of Protons and Electrons
作者:Maia E. Czaikowski、Sophie W. Anferov、Alex P. Tascher、John S. Anderson
DOI:10.1021/jacs.3c09885
日期:2024.1.10
Alkynesemihydrogenation is a broadly important transformation in chemical synthesis. Here, we introduce an electrochemical method for the selective semihydrogenation of terminal alkynes using a dihydrazonopyrrole Ni complex capable of storing an H2 equivalent (2H+ + 2e–) on the ligand backbone. This method is chemoselective for the semihydrogenation of terminal alkynes over internal alkynes or alkenes
炔烃半氢化是化学合成中广泛重要的转化。在这里,我们介绍了一种使用能够在配体主链上存储 H 2当量 (2H + + 2e – ) 的二氢吡咯 Ni 配合物对末端炔烃进行选择性半氢化的电化学方法。该方法对于末端炔烃的半氢化比内部炔烃或烯烃具有化学选择性。机理研究表明,这种转变是协调的和 Z 选择性的。计算结果支持基于配体的氢原子转移途径,而不是过渡金属加氢催化剂通常采用的氢化物机制。所提出的中间体的合成表明,催化机制是通过还原形式 Ni(I) 物质进行的。在没有过度还原或低聚的情况下,末端烯烃产物的高产率是所有均相催化剂中报道最好的。此外,该系统实现的金属-配体协同氢转移将H原子当量的有效流动引导至炔烃还原而不是析氢,为将类似策略应用于广泛的电还原转化提供了蓝图。
Diastereoselective cycloisomerizations of enediynes via palladium catalysis
作者:Barry M. Trost、Yian Shi
DOI:10.1021/ja00079a033
日期:1993.12
Considerations of atom economy drive a search for reactions that are simple additions which, performed intramolecularly, are cycloisomerizations. Exposure of acyclic enediynes to a catalyst generated by mixing a Pd(O) complex with acetic acid normally in the presence of a phosphine ligand creates [5.6.51 and [6.6.51 tricycles with extraordinary diastereoselectivity of remote stereogenic centers. Effects of substituents on the tethers as well as the olefinic and acetylenic bonds reveal a wide tolerance of functionality. While geminal substitution facilitates the cycloisomerization, it is not required. Allylic oxygen plays a role as a regioselectivity control element. Cycloisomerization dominates over allylic ionization in such cases by proper tuning of the ligand. The mechanism of this reaction appears to vary depending upon the structure of the substrate. In the normal cases, the process involves three stages, initiation, propagation, and termination. Chemoselective initiation at the acetylenic linkage closest to one of the chain's termini occurs by hydropalladation. Propagation entails intramolecular carbametalations. Termination by beta-hydrogen elimination generates a hexatriene that undergoes high rotoselectivity in its disrotatory cyclization to generate the final product. Blocking formation of the hexatriene shuts down reaction. With substrates bearing a gamma-siloxypropiolate as the acetylenic initiator, cycloisomerization forms a tricycle with different positions of the double bonds. In contrast' to the case of the other substrates, blocking formation of a hexatriene does not shut down cycloisomerization. Invoking a novel intramolecular Diels-Alder reaction of a dienylpalladium intermediate derived from the diyne moiety with the olefin, likely assisted by coordination to palladium, accounts for our observations. The ease of availability of the acyclic substrates because of the versatility of the acetylenes combined with the high chemo-, regio-, and diastereoselectivity makes this atom-economical reaction a very practical approach for the construction of polycycles.
Douglas fir tussock moth pheromone: identification of a diene analog of the principal attractant and synthesis of stereochemically defined 1,6-, 2,6-, and 3,6-heneicosadien-11-ones
作者:Lawrence M. Smith、Ronald G. Smith、Thomas M. Loehr、G. Doyle Daves、Gary E. Daterman、Robert H. Wohleb
DOI:10.1021/jo00406a012
日期:1978.6
Effect of coordinating ligands on the Pauson-Khand cycloaddition: trapping of an intermediate
作者:Marie E. Krafft、Ian L. Scott、Romulo H. Romero、Sabine Feibelmann、Craig E. Van Pelt
DOI:10.1021/ja00069a017
日期:1993.8
reaction, including reports that tertiary amineN-oxides greatly accelerate the rate of cycloaddition in both intra- and some intermolecular reactions. To date, there has been no direct evidence to support the mechanistic hypothese for either the thermal or amine oxide promoted reactions beyond complete identification of the hexacarbonyl alkyne complex. While the amine oxide promoted reaction is normally