14 research outputs found
Metal‐Assisted Cyclizations: Cascade and Domino Reactions
No full textDomino and cascade cyclizations provide one of the most elegant and economic accesses to complex carbon frameworks. In particular, transition metal-mediated and -catalyzed reactions offer an almost endless number of synthetic pathways from simple precursors to elaborate molecules. In the following overview emphasis is placed on some of the more impressive developments in this field within the last decade, spanning a circle from the very recent developments of rhodium-catalyzed cycloaddition reactions over the almost classical palladium-catalyzed oligocyclizations to the multifold reaction possibilities of Fischer carbene complexes
Metal-Assisted Cyclizations: Cascade and Domino Reactions
No full textDomino and cascade cyclizations provide one of the most elegant and economic accesses to complex carbon frameworks. In particular, transition metal-mediated and -catalyzed reactions offer an almost endless number of synthetic pathways from simple precursors to elaborate molecules. In the following overview emphasis is placed on some of the more impressive developments in this field within the last decade, spanning a circle from the very recent developments of rhodium-catalyzed cycloaddition reactions over the almost classical palladium-catalyzed oligocyclizations to the multifold reaction possibilities of Fischer carbene complexes
Facile Construction of Spirocyclopropanated Bi‐, Tri‐ and Tetracyclic Skeletons by Novel Cascades Involving Intra‐ and Intermolecular Heck Reactions of 2‐Bromo‐1,6‐enynes and Bicyclopropylidene
No full textAcyclic 2-bromo-1,6-enynes 5-R, 9-R and 11-R with bulky substituents at the acetylenic terminus were co-cyclized with the highly strained bicyclopropylidene (12) under palladium catalysis at 80 degreesC to give the cross-conjugated tetraenes 13-R, 18-R and 19-R in moderate-to-good yields (34-71%). Only the co-cyclization of 5-Ph gave rise to an additional product, which was identified as the 11-membered ring 20. At elevated temperatures (120-140 degreesC) the initially formed tetraenes underwent 6pi-electrocyclization to give spiro[cyclopropane-1,4'-bicylo[4.3.0]-1(6),2-dienes] 21-R, 22-R and 23-R. This novel class of spirocyclopropanated oligocycles is also accessible by a one-pot protocol. The highest yields for both the tetraenes and bicyclo[4.3.0]nonadiene and its heteroanalogues were obtained with bulky substituents at the alkyne terminus of the precursors. Heteroatom-containing precursors 9-R and 11-R gave lower yields than their all-carbon analogues 5-R. The acyclic 2-bromo-1,8-dien-6-ynes 28a,b,c upon palladium-catalyzed co-cyclization with bicyclopropylidene (12) at 110 degreesC gave spirocyclopropanated tricycles 31a,b and 32, respectively, in moderate yields (14, 31 and 32%). These products were formed by two consecutive 6pi-electrocyclizations. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
ChemInform Abstract: The Dichotomy Between Benzene and Fulvene Formation in Palladium-Catalyzed Domino Tricyclizations of 2-Bromotetradec-1-ene-7,13-diynes.
No full textPalladium‐Catalyzed Cross‐Coupling Reactions and Electrocyclizations—Efficient Combinations for New Cascade Reactions.
No full text)‐Mediated Intramolecular [2+2+2] Cocyclizations of (Methylenecyclopropyl)diynes as an Easy Access to Cyclopropanated Oligocycles
No full textEnediynes with methylenecyclopropane moieties attached either through the three-membered ring or the double bond smoothly under-went cobalt-mediated [2+2+2] cocyclizations to give cyclopropane-fused and spirocyclopropanated cobalt-complexed tricyclo[7.3.0.0(2,6)]dodeca-1,6-dienes (1,2,3,3a,4, 6,7,8-octahydro-as-indacenes) under relatively mild conditions in yields ranging from 31 to 94% (6 examples). Electron-withdrawing substituents at the acetylenic terminus of the precursor are essential for a successful oligocyclization. While diphenyloxyphosphinyl-substituted enediynes provided tricyclo[7.3.0.0(2,6)]dodeca-1,6-diene complexes as mixtures of two diastereomers in high yields, the analogous methoxycarbonyl-substituted precursors gave only one diastereomer. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005
The dichotomy between benzene and fulvene formation in palladium-catalyzed domino tricyclizations of 2-bromotetradec-1-ene-7,13-diynes
No full text2-Bromotetradec-1-ene-7,13-diynes 1, 5, 12 with different substituents at the acetylenic terminus under palladium catalysis have been found to either yield an angularly bisannelated benzene derivative 3 (79%), or fulvene derivatives of type 5 (52-74%) as well as monoannelated fulvenes 13b-e (32-48%). The mechanisms for all three reaction modes most probably involve intramolecular 5-exo-trig carbopalladations of an intermediate 1,3,5-hexatrienyl-6-palladium bromide intermediate
New transition metal-mediated reactions for the efficient construction of cyclopropanated oligocycles
No full textZiel dieser Arbeit ist die Entwicklung neuer Übergangsmetall-vermittelter Reaktionen zum Aufbau von cyclopropanierten Oligocyclen. Im ersten Teil der Arbeit führt die Palladium-katalysierte Reaktion von 2-Brom-1,6-eninen mit dem hochgespannten Alken Bicyclopropyliden zu neuartigen kreuzkonjugierten Tetraenen. Diverse Enine mit variablen Substituenten am Alkinterminus sowie mit Heteroatomen in der Kette können zur Reaktion gebracht werden. Ein Cyclisierungsvorläufer zeigt ein abweichendes Reaktionsverhalten und liefert ein Elfring-Derivat als Nebenprodukt. Die Reaktion der kreuzkonjugierten Tetraene mit verschiedenen Dienophilen liefert die korrespondierenden Diels-Alder-Addukte unter thermischen Bedingungen bzw. hohem Druck. Einige dieser Produkte reagieren in einer nachfolgenden thermischen 6p-Elektrocyclisierung zu linear-anellierten, tetracyclischen Cyclohexadienen mit Cyclopropaneinheit. Mit hochreaktiven Dieno- und Heterodienophilen zeigen die Tetraene ein alternatives Reaktionsverhalten und unterlaufen eine En-Reaktion an der exocyclischen Doppelbindung des Tetraens. Die Struktur der En-Addukte wird durch eine Röntgenstrukturanalyse bestätigt. Die thermische 6p-Elektrocyclisierung der Tetraene liefert Spirocyclopropan-bicyclononadiene. Durch die Erhöhung der Reaktionstemperatur bei der Palladium-katalysierten Reaktion von 2-Brom-1,6-eninen mit Bicyclopropyliden gelingt die Synthese dieser Tricyclen im Eintopfverfahren. Die Einführung eines konjugierten Alkenylsubstituenten an der Dreifachbindung der Vorläufer ermöglicht die Palladium-katalysierte Kaskadenreaktion der resultierenden Bromendiine mit Bicyclopropyliden zu spirocyclopropanierten Tri- bzw. Tetracyclen. Im zweiten Teil der Arbeit wird die Reaktivität von linearen Methylencyclopropandiinen in Cobalt-vermittelten [2+2+2]-Cycloadditionen untersucht. Die Cobalt-vermittelte Transformation der synthetisierten Endiine variabler Kettenlänge liefert sowohl ring- als auch spiroanellierte Tricyclen in hohen Ausbeuten. Die Cycloadditionen zeigen dabei eine gewisse Diastereoselektivität in Abhängigkeit von der zu bildenden Ringgrösse.The objective of this thesis is the development of new transition metal-mediated reactions for the construction of cyclopropanated oligocycles. In the first part of the thesis, the palladium-catalyzed reaction of 2-bromo-1,6-enynes with the highly strained alkene bicyclopropylidene yields a novel kind of cross-conjugated tetraenes. Various enynes with different substituents on the alkyne terminus as well with hetero atoms in the chain are able to undergo this reaction. One cyclization precursor shows a different reaction behaviour, giving rise to an eleven-membered ring derivative as the sideproduct. Reaktion of the cross-conjugated tetraenes with various dienophiles yield the corresponding Diels-Alder adducts under thermal conditions or high pressure, respectively. Some of these products react in a subsequent 6p-electrocyclisation to give linear-annelated, tetracyclic cyclohexadiens with a cyclopropane moiety. The tetraenes show an alternative reaction pattern with highly reactive dienophiles and hetero-dienophiles by undergoing an ene-reaction at the exocyclic double-bond. The structure of these ene-adducts is confirmed by X-ray structure analysis. Thermal 6p-electrocyclization of the tetraenes gives rise to spirocyclopropan-bicyclononadienes. By raising the temperature of the palladium-catalyzed reaction of 2-bromo-1,6-enynes with bicyclopropylidene these tricycles become accessible in a one-pot procedure. Introduction of a conjugated alkenyl substituent at the triple bond of the precursor allows the palladium-catalyzed cascade reaction of the resulting bromodienynes to give spirocyclopropanated tri- or tetracycles. In the second part of the thesis, the reactivity of linear methylenecyclopropandiynes in cobalt-mediated [2+2+2] cycloadditions is investigated. The cobalt-mediated transformation of the synthesized endiynes with variable chain length gives rise to ring- and spiroannelated tricycles in high yield. In this process, the cycloadditions show a certain diastereoselectivity depending on the ring size to be formed
Stereoselective Cross‐Coupling Reaction of 2,4‐Diaryl‐1,1‐diboryl‐1,3‐butadienes: Stereocontrolled Approach to 1,3,4,6‐Tetraarylated 1,3,5‐Hexatrienes.
No full textStereoselective Cross‐Coupling Reaction of 1,1‐Diboryl‐1‐alkenes with Electrophiles: A Highly Stereocontrolled Approach to 1,1,2‐Triaryl‐1‐alkenes.
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