Since allenes are versatile building blocks both for organic synthesis [1] and in transition-metal-promoted carbon±carbon bond-forming reactions, [2] development of practical methods for their preparation is of great interest. Allene moieties can be prepared by alkylation of allenylmethyl halides with an appropriate carbanionic species, [1e] by S N 2'-type selective displacement of propargyl alcohol derivatives with organocopper reagents, [1e, 3] and by intramolecular regio-and stereoselective reduction of alkynes. [4] The synthesis of allenes by metal-catalyzed cross-coupling reactions is an alternative method; [5] however, some disadvantages are the inconvenient preparation of the reagents, limited substrate scope, and incompatibility of sensitive groups to the reaction conditions. [6] Recently, several reports have described the synthesis of allenes with transition metal complexes. [5] Despite this recent progress a method for the highly efficient and catalytic synthesis of substituted allenes is still needed. We have realized this goal (Scheme 1) as part of our continuing studies directed toward the development of efficient indium-mediated reactions. [7] , [8] The catalytic activity of several palladium complexes was initially examined in the reaction of 1-iodonaphthalene with allenylindium. The best results were obtained with 4 mol % [Pd(PPh 3 ) 4 ] in the presence of 3 equivalents of LiI in DMF at 100 8C under a nitrogen atmosphere (see Experimental Section), and 1-allenylnaphthalene was produced in quantitative yield (98 %) with complete regioselectivity (Table 1, entry 1). The allenylindium reagent generated in situ from the reaction of 1 equivalent of indium with 1.5 equivalents of propargyl bromide gave the best results as a coupling partner. [9] To demonstrate the efficiency and scope of the present method, we applied this catalytic system to a variety of propargyl halides and organic electrophiles. For the propargyl halides as coupling partners, the presence of various alkyl substituents at the a and g positions had little effect on either the reaction rate or the product yield (Table 1). Under the optimized conditions, treatment of iodobenzene with 3bromo-1-phenyl-1-butyne and indium gave selectively trisubstituted allene 2 in 80 % yield (entry 2). Varying the electron demand of the substituents on the arene did not diminish the efficiency and selectivity (entries 3, 7±9). It is noteworthy that protection of an oxo group and a hydroxy group on substrates is not necessary, as demonstrated by the reactions of 4iodoacetophenone (entry 5) and 3-iodophenol (entry 6), respectively. Hetero substituents turned out to be compatible with the reaction conditions (entry 10). The method worked equally well with vinyl halides (entries 11 and 12) and imidoyl bromide (entry 13). Treatment of iodophenylacetylene with allenylindium produced 3-methyl-1-phenyl-3,4-pentadien-1yne (14) in 91 % yield (entry 14). In the case of vinyl triflate, the desired product 15 was obtained in 90 % yield (entry 15). Surprisingly...
Rhodium-catalyzed
oxidative [4 + 2] cyclization reactions through
the C–H activation of azulene carboxylic acids as nonbenzenoid
aromatic compounds with symmetrical and unsymmetrical alkynes were
developed under aerobic conditions, which produced azulenolactone
derivatives with a wide substrate scope and excellent functional group
tolerance. Interestingly, azulenic acids in reaction with alkynes
underwent iridium-catalyzed [2 + 2 + 2] cyclization accompanied by
decarboxylation to afford tetra(aryl)-substituted benzoazulene derivatives.
The reactivity order for C–H activation reaction is greater
toward azulene-6-carboxylic acid, azulene-1-carboxylic acid, and azulene-2-carboxylic
acid. For the first time, the expansion of azulenes having directing
group as nonbenzenoid aromatic compounds for C–H activation
was successful, indicating that nonbenzenoid aromatic compounds can
be used as good substrates for the C–H activation reaction.
Therefore, the research area of C–H activation will certainly
expand to nonbenzenoid aromatic compounds in future.
Tetraorganoindates, which were prepared easily from the reaction of 1 equiv of InCl(3) with 4 equiv of organometallics, could be employed as effective nucleophilic cross-coupling partners in Pd-catalyzed carbonylative cross-coupling reactions with a variety of organic electrophiles. The present method gave unsymmetrical ketones and 1,4-diacylbenzenes in good yields with highly efficient transfer of almost all the organic groups attached to the indium under a carbon monoxide atmosphere in THF at 60 degrees C.
An efficient method for the bismuth‐catalyzed regioselective synthesis of pyranocoumarins and furocoumarins has been developed from the reaction of 4‐hydroxycoumarins with propargyl alcohols. The reaction proceeds through sequential propargylation and intramolecular cyclization reactions catalyzed by bismuth(III) triflate.
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