Addition Reactions: Cycloaddition
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This Chapter Contains Sections titled: 2 + 2-Cycloaddition 2 + 3-Cycloaddition 2 + 4-Cycloaddition Miscellaneous Cycloadditions References A review of advances in Nickel-catalysed cycloaddition reactions directed towards the synthesis of carbocycles and heterocycles has been presented. The discussion includes the development and mechanistic studies of the Ni/NHC catalysts that couple diynes and nitriles to form pyridines. The use of vinyl cyclopropanes, aldehydes, ketones, tropones, 3-azetidinones, and 3-oxetenones as substrates in new Ni-catalysed cycloaddition reactions is also discussed. A second extensive review concerning Ni-catalysed cycloaddition reactions that have been studied since 2004 has been published. 1,3-Dipolar cycloadditions, Diels-Alder cycloadditions, 2+2+2- cycloadditions, and 3+3-cycloadditions are discussed. NHC-stabilized silylene monohydride reacted with diphenylacetylene to yield a 2,3,4,5-tetraphenyl-1-(tri-t-butylsilyl)-1H-silole via a 2+2+1-cycloaddition. DFT calculations indicate that the reaction mechanism did not include a silirene, the typical 2+1-cycloaddition product. The Rhodium-catalysed enantioselective 2+2+2-cycloaddition of silicon-containing prochiral triynes and internal alkynes yielded silicon-stereogenic dibenzosiloles with high enantoselectivities and high yields.Keywords:
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Diphenylacetylene
Abstract The reactions of substituted acetophenones with diphenylacetylene in the presence of [{RuCl 2 ( p ‐cymene)} 2 ] (2 mol‐%), AgSbF 6 (8 mol‐%), and Cu(OAc) 2 · H 2 O (25 mol‐%) in 1,2‐dichloroethane at 120 °C for 10 h provided substituted indenol derivatives in good‐to‐excellent yields. Under similar reaction conditions, unsymmetrical alkynes such as 1‐phenyl‐1‐propyne, 1‐phenyl‐1‐butyne, 1‐phenyl‐2‐(trimethylsilyl)acetylene, and a substituted enyne also reacted efficiently with substituted acetophenones to afford the corresponding indenol derivatives in a highly regioselective manner. The amount of silver salt plays a key role in the reaction. When the amount of silver salt exceeded more than 8 mol‐% in the presence of 2 mol‐% [{RuCl 2 ( p ‐cymene)} 2 ], a different type of dehydration product, namely a benzofulvene derivative, started to appear. In the presence of 20 mol‐% AgSbF 6 , substituted acetophenones readily reacted with alkynes in the presence of [{RuCl 2 ( p ‐cymene)} 2 ] (2 mol‐%) to give benzofulvene derivatives in excellent yields. A plausible reaction mechanism is proposed to account for the cyclization reaction.
Diphenylacetylene
Propyne
Trimethylsilyl
Alkyne
Enyne
Acetylene
Paraformaldehyde
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Molar ratio
Chlorine atom
Proton NMR
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C–H functionalization has been established as an efficient way to generate molecular complexity. The formation of stereogenic carbon atoms by asymmetric C–H functionalization has seen tremendous progress over the past decade. More recently, the direct catalytic modification of C–H bonds has been powerfully applied to the formation of noncarbon stereogenic centers, which constitute a key design element of biologically active molecules and chiral ligands for asymmetric catalysis. This area was opened by a seminal report describing enantioselective C–H functionalization for the formation of a silicon stereocenter. It rapidly expanded with advances in the enantioselective formation of phosphorus(V) centers. Moreover, enantioselective routes to chiral sulfur atoms in the oxidation states IV (sulfoxides) and VI (sulfoximines) have been disclosed. Herein, we discuss methods of using selective functionalization of C–H bonds to generate a remote heteroatom stereogenic center via an inner-sphere C–H activation mechanism.
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Surface Modification
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Diphenylacetylene
Acetylene
Reactivity
Phenylacetylene
Hydrogen chloride
Diethylamine
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Abstract Enantioselective organocatalytic synthesis of tetrahydropyridines bearing a chiral tetrasubstituted carbon stereogenic center has been achieved. The spiro‐type monoaryl phosphine catalyst, ( R )‐SITCP, was found to promote the formal [4+2] cycloaddition of saccharin‐derived ketimines and α‐methyl allenoate to afford the corresponding six‐membered N‐heterocycles in high yields and excellent regioselectivities with up to 93% ee .
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Die Umsetzung von PtL3 (4) (L = RN = PNRR′, R = tBu, R′ = SiMe3) mit den Alkenen C2Cl3H, C2Cl4, C2H2(CN)2 und C2(CN)4 ergibt die Platin-Phosphazen-Komplexe 1a, 1, 5a und b; mit Diphenylacetylen entsteht 6. Wasser addiert sich an die P = N-Bindungen von 1 und 5 unter Bildung der Element-Organic Amine/Imine Compounds, XXIV. Tris[(tert-butylimino)[tert-butyl(trimethylsilyl)amino]phosphane}platinum(0) – a Useful Synthesis Building Block The reaction of PtL3 (4) (L = RN = PNRR′, R = tBu, R′ = SiMe3) with the alkenes C2Cl3H, C2Cl4, C2H2(CN)2, and C2(CN)4 affords the platinum phosphazene complexes 1a, 1, 5a, and b; with diphenylacetylene 6 is formed. Water adds to the P = N bonds of 1 and 5 with formation of the
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Trimethylsilyl
Phosphazene
Imine
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Abstract The enantioselective synthesis of P ‐stereogenic chiral organophosphines under organocatalysis is a challenging research field, and reports that use this approach are rare. Herein, we have developed the enantioselective synthesis of P ‐stereogenic chiral oxazaphospholidines by using a bicyclic thiazole as the organocatalyst in the P–N and P–O bond‐forming reaction. The P ‐chiral products were prepared in high yields with moderate enantioselectivities. The base that was used in this process had a significant influence on the enantioselectivity of the reaction and in some cases led to the opposite configuration of the P ‐chiral center.
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Thiazole
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Asymmetric catalysis is one of the most attractive and efficient approaches toward synthesis of chiral enantioenriched compounds using achiral compounds as substrates and reagents. Compared with the well-established enantioselective preparation of carbon-stereogenic compounds, the corresponding preparation of silicon-stereogenic compounds is much less explored. In fact, most of the available methods rely on the use of stoichiometric amounts of chiral reagents, and much less progress has been made on catalytic enantioselective synthesis from achiral compounds. This review article mainly focuses on our recent achievements in this research area through the use of a synthetic strategy that employs transition-metal-catalyzed enantioselective desymmetrization reactions of prochiral tetraorganosilanes.
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Abstract The enantioselective construction of all‐carbon quaternary stereocenters is one of the most challenging fields in asymmetric synthesis. An asymmetric desymmetrization strategy offers an indirect and efficient method for the formation of all‐carbon stereocenters. An enantioselective formation of cyano‐bearing all‐carbon quaternary stereocenters in 1,2,3,4,‐tetrahydroquinolines and 2,3,4,5‐tetrahydro‐1H‐benzo[ b ]azepines by copper‐catalyzed desymmetric N‐arylation is demonstrated. The cyano group at the prochiral center plays a key role for the high enantioselectivity and works as an important functional group for further transformations. DFT studies provide a model which successfully accounts for the origin of enantioselectivity.
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Abstract Asymmetric catalysis is one of the most attractive and efficient ways of synthesizing chiral enantioenriched compounds from achiral precursors. Compared with the well‐investigated enantioselective preparation of carbon‐stereogenic compounds, the corresponding preparation of silicon‐stereogenic compounds is much less established. In particular, the available methods typically rely on the use of stoichiometric amounts of chiral reagents, and little progress has been made on catalytic enantioselective synthesis using prochiral organosilanes until recently. This Focus Review highlights recent advances in the catalytic enantioselective preparation of silicon‐stereogenic organosilanes under transition‐metal catalysis through desymmetrization of prochiral organosilanes including diorganodihydrosilanes and tetraorganosilanes.
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