Despite the fact that halogenation of alkenes has been known for centuries, enantioselective variants of this reaction have only recently been developed. In the past three years, catalytic enantioselective versions of halofunctionalizations with the four common halogens have appeared and although important breakthroughs, they represent just the very beginnings of a nascent field. This Minireview provides a critical analysis of the challenges that accompany the development of general and highly enantioselective halofunctionalization reactions. Moreover, the focus herein, diverges from previous reviews of the field by identifying the various modes of catalysis and the different strategies implemented for asymmetric induction.
Observed pseudo-first-order rate constants (k(obs)) of the hydride-transfer reactions from isopropyl alcohol (i-PrOH) to two NAD(+) analogues, 9-phenylxanthylium ion (PhXn(+)) and 10-methylacridinium ion (MA(+)), were determined at temperatures ranging from 49 to 82 degrees C in i-PrOH containing various amounts of AN or water. Formations of the alcohol-cation ether adducts (ROPr-i) were observed as side equilibria. The equilibrium constants for the conversion of PhXn(+) to PhXnOPr-i in i-PrOH/AN (v/v = 1) were determined, and the equilibrium isotope effect (EIE = K(i-PrOH)/K(i-PrOD)) at 62 degrees C was calculated to be 2.67. The k(H) of the hydride-transfer step for both reactions were calculated on the basis of the k(obs) and K. The corresponding deuterium kinetic isotope effects (e.g., KIE(OD)(H) = k(H)(i-PrOH)/k(H)(i-PrOD) and KIE(beta-D6)(H) = k(obs)(i-PrOH)/k(obs)((CD3)2CHOH)), as well as the activation parameters, were derived. For the reaction of PhXn(+) (62 degrees C) and MA(+) (67 degrees C), primary KIE(alpha-D)(H) (4.4 and 2.1, respectively) as well as secondary KIE(OD)(H) (1.07 and 1.18) and KIE(beta-D6)(H) (1.1 and 1.5) were observed. The observed EIE and KIE(OD)(H) were explained in terms of the fractionation factors for deuterium between OH and OH(+)(OH(delta+)) sites. The observed inverse kinetic solvent isotope effect for the reaction of PhXn(+) (k(obs)(i-PrOH)/k(obs)(i-PrOD) = 0.39) is consistent with the intermolecular hydride-transfer mechanism. The dramatic reduction of the reaction rate for MA(+), when the water or i-PrOH cosolvent was replaced by AN, suggests that the hydride-transfer T.S. is stabilized by H-bonding between O of the solvent OH and the substrate alcohol OH(delta+). This result suggests an H-bonding stabilization effect on the T.S. of the alcohol dehydrogenase reactions.
Obwohl die Halogenierung von Alkenen seit Jahrhunderten bekannt ist, wurden enantioselektive Varianten dieser Reaktion erst vor kurzem entwickelt. Die in den vergangenen drei Jahren beschriebenen katalytischen enantioselektiven Varianten von Halogenfunktionalisierungen mit den vier gängigen Halogenen sind wichtige Durchbrüche, stellen aber erst die Anfänge eines im Entstehen begriffenen Gebiets dar. Dieser Kurzaufsatz bietet eine kritische Betrachtung der Herausforderungen, die mit der Entwicklung allgemein anwendbarer und hoch enantioselektiver Halogenfunktionalisierungen einhergehen. Zudem liegt der Schwerpunkt hier anders als bei früheren Übersichten zu diesem Gebiet darauf, die verschiedenen Katalysearten und Strategien zur asymmetrischen Induktion aufzuzeigen.
1991 cyclopentane derivatives cyclopentane derivatives Q 0030 10 -111 2-Pentylcyclopent-2-en-1-one by Catalytic Pauson-Khand Reaction.-First a Co2(CO)6-alkyne complex is created which reacts regioselectively with (II) and (III) to yield the title compound (IV) as major product. The formation of by-products (cyclopentadienones, cyclohexenes and spiro compounds) depends on the pressure of (II) and ( III) and on the concentration of (I).-(RAUTENSTRAUCH, V.; MEGARD, P.; CONESA, J.; KUESTER, W.; Angew.
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