The Experts below are selected from a list of 186 Experts worldwide ranked by ideXlab platform
Taelim Choi - One of the best experts on this subject based on the ideXlab platform.
-
highly β selective cyclopolymerization of 1 6 heptadiynes and ring closing enyne metathesis reaction using grubbs z selective catalyst unprecedented regioselectivity for ru based catalysts
Journal of the American Chemical Society, 2016Co-Authors: Kijung Jung, Eunhye Kang, Jeonghun Sohn, Taelim ChoiAbstract:It is well-known that Ru-based Grubbs catalysts undergo a highly selective α-addition to alkynes to promote Exo-Cyclization during ring-closing enyne metathesis (RCEYM) or to produce conjugated polyenes containing five-membered rings during the cyclopolymerization (CP) of 1,6-heptadiynes. There are a few reports of β-selective addition to alkynes using Schrock catalysts based on Mo but none for readily accessible and easy-to-use Ru-based catalysts. We report the first example of β-selective addition to alkynes using Grubbs Z-selective catalyst, which produces only endo products during the RCEYM reaction of terminal enynes and promotes the CP of 1,6-heptadiyne derivatives to give conjugated polyenes containing a six-membered ring as a major repeat unit. This unique preference for β-selectivity originated from the side-bound approach of alkynes to the catalyst, where the steric hindrance between the chelating N-heterocyclic carbene ligand of the catalyst and the alkynes disfavored α-addition. To enhance the...
-
Highly β‑Selective Cyclopolymerization of 1,6-Heptadiynes and Ring-Closing Enyne Metathesis Reaction Using Grubbs Z‑Selective Catalyst: Unprecedented Regioselectivity for Ru-Based Catalysts
2016Co-Authors: Kijung Jung, Eunhye Kang, Jeonghun Sohn, Taelim ChoiAbstract:It is well-known that Ru-based Grubbs catalysts undergo a highly selective α-addition to alkynes to promote Exo-Cyclization during ring-closing enyne metathesis (RCEYM) or to produce conjugated polyenes containing five-membered rings during the cyclopolymerization (CP) of 1,6-heptadiynes. There are a few reports of β-selective addition to alkynes using Schrock catalysts based on Mo but none for readily accessible and easy-to-use Ru-based catalysts. We report the first example of β-selective addition to alkynes using Grubbs Z-selective catalyst, which produces only endo products during the RCEYM reaction of terminal enynes and promotes the CP of 1,6-heptadiyne derivatives to give conjugated polyenes containing a six-membered ring as a major repeat unit. This unique preference for β-selectivity originated from the side-bound approach of alkynes to the catalyst, where the steric hindrance between the chelating N-heterocyclic carbene ligand of the catalyst and the alkynes disfavored α-addition. To enhance the β-selectivity for CP further, one could increase the size of the substrates on the monomers and lower the reaction temperature to obtain conjugated polyenes containing up to 95% six-membered rings. Moreover, the physical properties of the resulting polymer were analyzed in detail and compared with those of the conjugated polyenes containing only five-membered rings prepared from the same monomer but with a conventional Grubbs catalyst
Kijung Jung - One of the best experts on this subject based on the ideXlab platform.
-
highly β selective cyclopolymerization of 1 6 heptadiynes and ring closing enyne metathesis reaction using grubbs z selective catalyst unprecedented regioselectivity for ru based catalysts
Journal of the American Chemical Society, 2016Co-Authors: Kijung Jung, Eunhye Kang, Jeonghun Sohn, Taelim ChoiAbstract:It is well-known that Ru-based Grubbs catalysts undergo a highly selective α-addition to alkynes to promote Exo-Cyclization during ring-closing enyne metathesis (RCEYM) or to produce conjugated polyenes containing five-membered rings during the cyclopolymerization (CP) of 1,6-heptadiynes. There are a few reports of β-selective addition to alkynes using Schrock catalysts based on Mo but none for readily accessible and easy-to-use Ru-based catalysts. We report the first example of β-selective addition to alkynes using Grubbs Z-selective catalyst, which produces only endo products during the RCEYM reaction of terminal enynes and promotes the CP of 1,6-heptadiyne derivatives to give conjugated polyenes containing a six-membered ring as a major repeat unit. This unique preference for β-selectivity originated from the side-bound approach of alkynes to the catalyst, where the steric hindrance between the chelating N-heterocyclic carbene ligand of the catalyst and the alkynes disfavored α-addition. To enhance the...
-
Highly β‑Selective Cyclopolymerization of 1,6-Heptadiynes and Ring-Closing Enyne Metathesis Reaction Using Grubbs Z‑Selective Catalyst: Unprecedented Regioselectivity for Ru-Based Catalysts
2016Co-Authors: Kijung Jung, Eunhye Kang, Jeonghun Sohn, Taelim ChoiAbstract:It is well-known that Ru-based Grubbs catalysts undergo a highly selective α-addition to alkynes to promote Exo-Cyclization during ring-closing enyne metathesis (RCEYM) or to produce conjugated polyenes containing five-membered rings during the cyclopolymerization (CP) of 1,6-heptadiynes. There are a few reports of β-selective addition to alkynes using Schrock catalysts based on Mo but none for readily accessible and easy-to-use Ru-based catalysts. We report the first example of β-selective addition to alkynes using Grubbs Z-selective catalyst, which produces only endo products during the RCEYM reaction of terminal enynes and promotes the CP of 1,6-heptadiyne derivatives to give conjugated polyenes containing a six-membered ring as a major repeat unit. This unique preference for β-selectivity originated from the side-bound approach of alkynes to the catalyst, where the steric hindrance between the chelating N-heterocyclic carbene ligand of the catalyst and the alkynes disfavored α-addition. To enhance the β-selectivity for CP further, one could increase the size of the substrates on the monomers and lower the reaction temperature to obtain conjugated polyenes containing up to 95% six-membered rings. Moreover, the physical properties of the resulting polymer were analyzed in detail and compared with those of the conjugated polyenes containing only five-membered rings prepared from the same monomer but with a conventional Grubbs catalyst
Hiroyuki Ishibashi - One of the best experts on this subject based on the ideXlab platform.
-
controlling the regiochemistry of radical Cyclizations
ChemInform, 2006Co-Authors: Hiroyuki IshibashiAbstract:This review describes the results of our recent studies on the control of the regiochemistry of radical Cyclizations. N-vinylic α-chloroacetamides generally cyclized in a 5-endo-trig manner to give five-membered lactams, whereas 4-Exo-trig Cyclization occurred when the cyclized radical intermediates were highly stabilized by an adjacent phenyl or phenylthio group to afford β-lactams. The 5-Exo or 6-Exo Cyclization of aryl radicals onto the alkenic bond of enamides could be shifted to the corresponding 6-endo or 7-endo mode of Cyclization by a positional change of the carbonyl group of enamides. The 6-endo- and 7-endo-selective aryl radical Cyclizations were applied to radical cascades for the synthesis of alkaloids such as phenanthroindolizidine, cephalotaxine skeleton, and lennoxamine. The 5-Exo-trig Cyclization of an alkyl radical onto the alkenyl bond of enamides could also be shifted to the 6-endo mode by a positional change of the carbonyl group of enamides. The 6-endo- selective Cyclization was applied to the radical cascade to afford a cylindricine skeleton. Other examples of controlling the regiochemistry of radical Cyclizations and their applications to the synthesis of natural products are also discussed. © 2006 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 6: 23–31; 2006: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20069
-
7 endo selective aryl radical Cyclization onto enamides leading to 3 benzazepines concise construction of a cephalotaxine skeleton
Journal of Organic Chemistry, 2005Co-Authors: Tsuyoshi Taniguchi, Atsuko Ishita, Masahiko Uchiyama, Osamu Tamura, Osamu Muraoka, Genzoh Tanabe, Hiroyuki IshibashiAbstract:Bu3SnH-mediated radical Cyclizations of 2-(2-bromophenyl)-N-ethenylacetamide gave 6-Exo Cyclization product 15 as the major product, whereas N-[2-(2-bromophenyl)ethyl]-N-ethenylamides gave almost exclusively 7-endo Cyclization products. These results indicated that the position of the carbonyl group on enamide played an important role in deciding the course of the Cyclization. The 7-endo selective Cyclization was applied to concise construction of a cephalotaxine skeleton.
-
regiochemistry in aryl radical Cyclization onto methylenecycloalkanes
Journal of Organic Chemistry, 2000Co-Authors: Hiroyuki Ishibashi, Tetsuya Kobayashi, Sayaka Nakashima, Osamu TamuraAbstract:Bu(3)SnH-mediated aryl radical Cyclization onto methylenecycloalkanes having a phenylthio, an ester, or a nitrile group at the terminus of the alkenic bond provides exclusively Exo Cyclization products. The results are in sharp contrast to those reported for nonsubstituted methylenecycloalkanes, which give exclusively endo Cyclization products. Formation of endo Cyclization products has been suggested to be a result of a consecutive 5-Exo Cyclization of an aryl radical and neophyl rearrangement. The Exo-selective aryl radical Cyclization offers a new method for synthesizing fused aromatic compounds containing a benzylic quaternary carbon atom.
-
effect of temperature of 5 endo and 4 Exo trig radical Cyclizations of n vinylic α halo amides
Tetrahedron Letters, 1998Co-Authors: Hiroyuki Ishibashi, Masahiro Higuchi, Masashi Ohba, Masazumi IkedaAbstract:Abstract The radical 3a generated from N -vinylic trichloroacetamide 1a provided the 5- endo-trig Cyclization product 2a in boiling toluene, whereas, at room temperature, gave the 4- Exo-trig Cyclization product 9 . The results may be explained in terms of the reversibility of 4- Exo Cyclization. © 1997 Elsevier Science Ltd. All rights reserved.
Eiji Yamamoto - One of the best experts on this subject based on the ideXlab platform.
-
copper i catalyzed diastereoselective borylative Exo Cyclization of alkenyl aryl ketones
ChemInform, 2016Co-Authors: Eiji Yamamoto, Ryoto Kojima, Koji Kubota, Hajime ItoAbstract:Copper(I)-catalyzed diastereoselective borylative Exo-Cyclization of alkenyl ketones with bis(pinacolato)diboron is reported. The reaction of alkenyl aryl ketones under a CuCl/Xantphos catalyst system provides four- or five-membered-ring syn-2-(borylmethyl)cycloalkanol derivatives in good yields with high syn selectivities. The utility of this method is demonstrated by further transformations of the cyclic products.
-
copper i catalyzed borylative Exo Cyclization of alkenyl halides containing unactivated double bond
ChemInform, 2013Co-Authors: Koji Kubota, Eiji YamamotoAbstract:Several synthetic applications of the reaction products are demonstrated, including the synthesis of histamine H3 receptorligand (XV).
-
copper i catalyzed borylative Exo Cyclization of alkenyl halides containing unactivated double bond
Journal of the American Chemical Society, 2013Co-Authors: Koji Kubota, Eiji YamamotoAbstract:A borylative Exo-Cyclization of alkenyl halides has been reported. The reaction includes the regioselective addition of a borylcopper(I) intermediate to unactivated terminal alkenes, followed by the intramolecular substitution of the resulting alkylcopper(I) moiety for the halide leaving groups. Experimental and theoretical investigations of the reaction mechanism have also been described. This reaction provides a new method for the synthesis of alkylboronates containing strained cycloalkyl structures from simple starting materials.
Dennis P Curran - One of the best experts on this subject based on the ideXlab platform.
-
rate constants for reaction of a fluorous tin hydride reagent with primary alkyl radicals
Tetrahedron Letters, 1997Co-Authors: Dennis P Curran, John H Horner, Felix N Martinez, Martin NewcombAbstract:Abstract Rate constants for 6-Exo Cyclization of the 7,7-diphenyl-6-heptenyl radical (3) in benzotrifluoride (BTF, PhCF3) were determined by laser flash photolysis, and radical clock 3 was used in competition kinetic studies for determination of rate constants of reactions of the fluorous tin hydride reagent tris(2-(perfluorohexyl)ethyl)tin hydride with primary alkyl radicals.