Allenes synthesis with insertion

Allenes from ethylene derivatiyes Synthesis with insertion of 1 C-atom One-step procedure Ring expansion... 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

The amino derivatives of rare-earth metals follow a distinct mechanism involving direct insertion of the allene or alkene into the metal-nitrogen bond (Scheme 6.73). With the less-reactive lanthanum complex 6.215, amine 6.214 underwent insertion of only the allene, giving a pyrrolidine product 6.216 as the trans isomer. On the other hand, using the more-reactive samarium complex 6.217, tandem allene and alkene insertion occurred giving, after hydrogenation, the pyrrolizidine alkaloid, xenovenine 6.189, A closely related synthesis of this alkaloid is in Scheme 6.59. Other syntheses may be found in Scheme 6.65, and Scheme 9.46. 

A one-pot synthesis of 3,3-disubstituted indolines was achieved by taking advantage of a sequential carbopalladation of allene, nucleophile attack, intramolecular insertion of an olefm and termination with NaBPh4 (Scheme 16.6) [10]. First, a Pd(0) species reacts with iodothiophene selectively to afford ArPdl, probably because the oxidative addition step is facilitated by coordination with the adjacent sulfur atom. Second, the ArPdl adds to allene, giving a Jt-allylpalladium complex, which is captured by a 2-iodoaniline derivative to afford an isolable allylic compound. Under more severe conditions, the oxidative addition of iodide to Pd(0) followed by the insertion of an internal olefm takes place to give an alkylpalladium complex, which is transmetallated with NaBPh4 to release the product. 

Palladium chemistry has been used in the synthesis of tetrahydroisoquinolines. Different combinations of iodoaryl-amine-alkene can be used in these multicomponent reactions. For example, the metal-mediated o-alkylated/alkenyl-ation and intramolecular aza-Michael reaction (Scheme 109) give moderate yields of heterocycle <2004TL6903>, whereas the palladium-catalyzed allene insertion-nucleophilic incorporation-Michael addition cascade (Equation 172) produces good yields of tetrahydroisoquinolines in 15 examples <2003TL7445> with further examples producing tetrahydroquinolines (Scheme 110) <2000TL7125>. 

A new method for the synthesis of furan-substituted allenes via Cu(I)-catalyzed coupling of conjugated ene-yne-ketones with terminal alkynes was developed. A copper carbene migratory insertion was proposed as the key step in this transformation, with conjugated ene-yne-ketones as carbene precursors (14OL4082). 

The well known palladium-catalysed insertion of carbon monoxide into substituted aryl halides has been elaborated to provide a useful synthesis of benzo-diazepinones, and the reaction of 1,2-diamines with allenic nitriles has been... 

Synthesis.—A usually reliable method of preparing allenes is by the insertion of cyclopropylidene carbenes (carbenoids). However, treatment of tetrasubstituted ew-dibromocyclopropanes with methyl-lithium gives bicyclobutanes with allenes as minor products. The dibromocyclopropane (155) yields only the bicyclobutane (156) and no allene is detectable, whereas the isomeric dibromocyclopropane (157) yields, in addition to the bicyclobutane (158), appreciable amounts of allene (159). This is attributed to a lengthening of the C-2—C-3 bond in an unsymmetrical transition state with positive charge better accommodated on a carbon bearing two phenyl groups. 

Allene insertion/nucleophile incorporation can also be combined with 1,3-dipole cycloaddition as an efficient protocol for the synthesis of various fused polycycles. In 2005, Dondas et al. used this strategy to synthesize various nitrogen- or oxygen-containing fused polycycles such as 323 through intermediate azomethine ylides, azomethine imines, and nitrones [112] (Scheme 6.85). 

Chiacchio et al. (2007) have also demonstrated the synthesis of methylene-isoxazolidine nucleoside analogs by microwave-assisted nitrone cycloaddition. The insertion of a methylene isoxazolidine spacer unit between the nucleobase and the hydroxyl-methyl group in the N, O nucleoside analogs could control the conformational mobility of the system. The cycloaddition between a proper N-methylated nitrone and a suitably functionahzed allene, (often thymallene) in CCl or EtOH at 70°C for 10 min generated the methylene-isoxazolidine nucleoside ankogs with good yields (45-72%) and purity. The reaction rate was enhanced, when the reactions were ran under microwave irradiation, in comparison to conventional reaction conditions. 

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