Highly regio- and stereoselective hydrometallation reactions of fluorine-containing internal alkynes: Novel approaches to fluoroalkylated alkenes

Hydroalumination, hydrocupration, and hydroboration reactions of various fluorine-containing alkynes were investigated. The alkyne reacted smoothly with 2.0 equiv. of Red-Al at −78 °C to give the hydroaluminated adduct in a highly regio- and stereoselective manner, which was treated with iodine, the corresponding vinyliodide being produced in moderate yield. Hydrocupration of the alkynes also took place, but the resulting vinylmetal reacted with various electrophiles sluggishly. In sharp contrast, the reaction with dicyclohexylborane proceeded smoothly to afford the cis-addition products preferentially, which were subjected to Suzuki-Miyaura cross-coupling reaction, leading to trisubstituted alkenes in high yields.     

锆氢催化的酰胺硼氢化合成胺类化合物研究:机理、使用范围和应用

发展温和条件下胺类化合物的高效合成方法是催化与合成领域长期研究的课题.其中,酰胺还原因其原料来源广、易于合成而广受关注.酰胺还原到胺需要选择性断裂C=O键,因此该反应具有很大的挑战性.传统酰胺还原方法需要使用当量的强还原试剂,如四氢铝锂、硼氢化钠等,且官能团兼容性较差.使用氢气还原原子经济性最高,也最有吸引力;然而,目前已报道的体系大都在高温(>120℃)或高压(>40 bar H2)的条件下进行.虽然催化硼氢化可以在温和的条件下将羰基化合物还原,但由于酰胺化合物惰性比较高,其选择性的催化硼氢化研究则相对较少,而且在温和条件下对三级、二级、一级酰胺都适用的例子依然非常有限.本文采用前过渡金属锆氢催化剂实现了室温条件下酰胺选择性硼氢化制备胺类化合物,并进行了详细的机理研究.原位红外监测到反应过程中酰胺和硼烷逐渐减少,目标产物逐渐增多;但并未给出其他反应中间体的信息.核磁研究以及对照实验结果表明,反应中有苯甲醛的生成,可能是反应中间体.因此推测,该催化体系经历了锆氢介导的酰胺C?N键断裂、重组、C?O键断裂这一特殊的酰胺键活化转化过程.DFT计算也证实了上述反应历程的可行性.除一些常见官能团外,本方法对羧酸酯、氰基、硝基、烯烃和炔烃这些可能被硼氢化的官能团同样具有兼容性.而且本文体系对一些生物、药物分子衍生酰胺的硼氢化也可以顺利进行.可见,本文发展了一种温和条件下使用廉价催化剂和原料选择性合成胺类化合物的方法.     

trans‐Selective Insertional Dihydroboration of a cis‐Diborene: Synthesis of Linear sp3‐sp2‐sp3‐Triboranes and Subsequent Cationization

The reaction of aryl‐ and amino(dihydro)boranes with dibora[2]ferrocenophane 1 leads to the formation 1,3‐trans‐dihydrotriboranes by formal hydrogenation and insertion of a borylene unit into the B=B bond. The aryltriborane derivatives undergo reversible photoisomerization to the cis‐1,2‐μ‐H‐3‐hydrotriboranes, while hydride abstraction affords cationic triboranes, which represent the first doubly base‐stabilized B₃H₄⁺ analogues.     

Total synthesis of (+)-pseudohygroline

A simple and efficient total synthesis of five-membered pyrrolidine, (+)-pseudohygroline is described. The key steps involved in this synthesis are highly stereoselective Prins cyclisation followed by reductive ring opening and hydroboration.     

Synthesis of Lignans Based on a Borate-mediated One-pot Sequential Suzuki-Miyaura Coupling of Cyclic Boranes

Lignans are a group of polyphenolic phytochemicals that possess a large spectrum of chemical structures and biological activities. Here the syntheses of lignans – anwulignan, burseran, dehydroxycubebin, ruburisandrin B, and sesamin – are achieved based on a borate-mediated one-pot sequential Suzuki-Miyaura coupling of cis- and trans-fused bicyclic boranes, which were prepared by diastereoselective cyclic hydroboration of exo-cyclic diene with cyclopentyl- and thexylboranes, respectively. A one-pot sequential Suzuki-Miyaura coupling of each cyclic borate with various aryl bromides initiated by activation of the cyclic borane with the carbon nucleophile provided 2,3-dibenzylbutane derivatives with different aromatic substituents. Finally, the syntheses of naturally occurring lignans were accomplished in several steps from the products of Suzuki-Miyaura coupling.     

New amine-stabilized deuterated borane-tetrahydrofuran complex (BD3-THF): convenient reagent for deuterium incorporations

Convenient methods for the preparation of BD₃-THF complex were developed. Certain amines stabilize the BD₃-THF for long-term storage. Regioselectivity studies were carried out with the new amine-stabilized BD₃-THF with representative olefins. Hydroboration of olefins provides a convenient tool for making corresponding deuterated alcohols after oxidation.     

Bis(enamino-Cp) Group 4 metal complex chemistry: developing a Mannich-type carbon-carbon coupling reaction at the bent metallocene famework

Treatment of 6-methyl-6-dimethylaminofulvene (2a) with methyl lithium in ether results in a deprotonation reaction to yield the enamino-substituted lithium cyclopentadienide reagent 3a. Its reaction with zirconium tetrachloride (0.5 molar equivalents) results in the formation of the [1-dimethylaminobutadien-1,3-diyl-bis(cyclopentadienyl)]zirconium dichloride product 4a. The unsaturated ansa-metallocene is probably formed via a Lewis acid-catalyzed Mannich-type carbon-carbon coupling reaction of an in situ generated bis[(dimethylaminoethenyl)cyclopentadienyl]ZrCl₂ intermediate. A number of related examples of this condensation reaction at the intact Group 4 bent metallocene framework is described, using different amino-substituents and Ti, Zr and Hf systems. Three examples of the unsaturated ansa-metallocene systems were characterized by X-ray diffraction. In a few cases it was possible to isolate the open (enamino-Cp)₂ZrCl₂ complexes. In the case of, e.g., (piperidinoethenylcyclopentadienyl)₂ZrCl₂ (13c) treatment with a catalytic quantity of the Lewis acid ZrCl₄ or the Brønsted acid resulted in a clean conversion to the respective Mannich condensation product, here the ansa-metallocene complex 4c. The strongly electrophilic borane HB(C₆F₅)₂ did not catalyze the CC-coupling reaction under the applied reaction conditions but added cleanly to the enamino-CC double bond of, e.g., 13c to yield a cyclic nitrogen donor-stabilized hydroboration product (19).     

Heavier Tetrylenes as Single Site Catalysts

An overview of the development of compounds with heavier low-valent group 14 elements (known as tetrylenes) as single component catalyst for organic transformation has been provided. Compounds with heavier group 14 elements possess stereochemically active lone pairs and energetically accessible π-antibonding orbitals, thereby resembling the electronic configuration of transition-metal compounds. Such compounds with low-valent group 14 elements has been known for small molecule activation since Power's report of dihydrogen activation by a digermyne, but their utilization in catalysis remained as a “Holy Grail” in main group chemistry. In recent years, numerous methodologies have been discovered epitomizing the use of Si(II), Ge(II) and Sn(II) compounds as single site catalysts for hydroboration of aldehydes, ketones, pyridines, cyanosilylation of aldehydes and ketones, N-formylations aromatic amines, dehydrocoupling reactions. This mini-review highlights these significant developments with an emphasis on the mechanistic investigation.     

Ni-catalyzed hydroboration and hydrosilylation of olefins with diboron and silylborane

Herein we report a Ni-catalyzed formal hydroboration of olefins, which afforded anti-Markovnikov-type alkylboranes with B₂pin₂ and a stoichiometric amount of water. Formal hydrosilylation using air- and moisture-sensitive silylboranes also proceeded under optimized conditions. The reaction with trans-stilbene and D₂O resulted in 1,2-H migration, which suggested that the reaction proceeded via β-hydride elimination and reinsertion mechanisms.     

Direct,high-yielding,one-step synthesis of vic-diols from aryl alkynes

An unprecedented, high yielding, direct, one-step synthesis of vic-diols from alkynes has been developed via metal-free, open-to-air dihydroboration with ammonia borane. The electronics of the alkyne and the reaction stoichiometry are critical for obtaining optimal yields of the 1,2-diol.