A Bio-Inspired, CatalyticE→ZIsomerization of Activated Olefins
摘要:
Herein, Nature's flavin-mediated activation of complex (poly)enes has been translated to a small molecule paradigm culminating in a highly (Z)-selective, catalytic isomerization of activated olefins using (-)-ribo-flavin (up to 99:1 Z/E). In contrast to the prominent Z -> E isomerization of the natural system, it was possible to invert the directionality of the isomerization (E -> Z) by simultaneously truncating the retinal scaffold, and introducing a third olefin substituent to augment A1,3-strain upon isomerization. Consequently, conjugation is reduced in the product chromophore leading to a substrate/product combination with discrete photophysical signatures. The operationally simple isomerization protocol has been applied to a variety of enone-derived substrates and showcased in the preparation of the medically relevant 4-substituted coumarin scaffold. A correlation of sensitizer triplet energy (ET) and reaction efficiency, together with the study of additive effects and mechanistic probes, is consistent with a triplet energy transfer mechanism.
3-phenyl-2-cyclopentenyl methylketones (15–18, 30, 31) undergo a 1,3-acetyl shift on direct irradiation, and the oxa-di-π-methane rearrangement to photochemically non-interconverting endo and exo bicyclo-[2.1.0]pentyl methylketones on tripletsensitization. Exceptions include the 2-methyl-3-phenyl-2-cyclopentenyl methylketone 32 and the 1-phenyl-2-cyclo-pentenyl methylketone 44 which are unreactive
Catalytic hydrogenation of α,β-unsaturated carboxylic acid derivatives using copper(<scp>i</scp>)/N-heterocyclic carbene complexes
作者:Birte M. Zimmermann、Sarah C. K. Kobosil、Johannes F. Teichert
DOI:10.1039/c8cc09853k
日期:——
air-stable copper(I)/N-heterocyclic carbene complex enables the catalytic hydrogenation of enoates and enamides, hitherto unreactive substrates employing homogeneous copper catalysis and H2 as a terminal reducing agent. This atom economic transformation replaces commonly employed hydrosilanes and can also be carried out in an asymmetric fashion.
Selective Synthesis of
<i>Z</i>
‐Cinnamyl Ethers and Cinnamyl Alcohols through Visible Light‐Promoted Photocatalytic
<i>E</i>
to
<i>Z</i>
Isomerization
A photocatalytic E to Z isomerization of alkenes using an iridium photosensitizer under mild reaction conditions is disclosed. This method provides scalable and efficient access to Z-cinnamyl ether and allylic alcohol derivatives in high yields with excellent stereoselectivity. Importantly, this method also provides a powerful strategy for the selective synthesis of Z-magnolol and honokiol derivatives
Copper(<scp>i</scp>)-catalysed asymmetric allylic reductions with hydrosilanes
作者:T. N. Thanh Nguyen、Niklas O. Thiel、Johannes F. Teichert
DOI:10.1039/c7cc07008j
日期:——
copper(I)-catalysed asymmetric allylic reduction enables a regio- and stereoselective transfer of a hydride nucleophile in an SN2′-fashion onto allylic bromides. This transformation represents a conceptually orthogonal approach to allylic substitution reactions with carbon nucleophiles. A copper(I) complex based upon a chiral N-heterocyclic carbene (NHC) ligand allows for stereoselectivity reaching 99% ee
铜(I)催化的不对称烯丙基还原可实现S N 2'形式的氢化物亲核试剂的区域和立体选择性转移到烯丙基溴上。这种转变代表了与碳亲核试剂进行烯丙基取代反应的概念上正交的方法。基于手性N-杂环卡宾(NHC)配体的铜(I)配合物可使立体选择性达到99%ee。不管起始材料的双键构型如何,该催化剂都能进行立体收敛反应。
Copper-Catalyzed Trifluoromethylation of Trisubstituted Allylic and Homoallylic Alcohols
An efficient copper‐catalyzed trifluoromethylation of trisubstituted allylic and homoallylicalcohols with Togni’s reagent has been developed. This strategy, accompanied by a double‐bond migration, leads to various branched CF3‐substituted alcohols by using readily available trisubstituted cyclic/acyclic alcohols as substrates. Moreover, for alcohols in which β‐H elimination is prohibited, CF3‐containing