Ruthenium-catalyzed regio- and stereoselective addition of carboxylic acids to aryl and trifluoromethyl group substituted unsymmetrical internal alkynes

The regio- and stereoselective addition of carboxylic acids to aryl and trifluoromethyl group substituted unsymmetrical internal alkynes has been accomplished: the Ru(3)(CO)(12)/3PPh(3) catalyst system has effectively catalyzed the reaction to afford the trifluoromethyl group substituted (E)-enol esters with high regio- and stereoselectivities.     

Ruthenium-catalyzed [2+2] cycloadditions between substituted alkynes and norbornadiene: a theoretical study

Theoretical predictions have been made using density functional theory for the reaction paths of a series of substituted alkynes undergoing a ruthenium-catalyzed cycloaddition with norbornadiene. Substituents on the alkynes have been varied in order to probe electronic and steric effects and the role of an intramolecular hydrogen bond. Strong electron-withdrawing groups activate the alkyne and decrease the reaction barrier leading to an increased rate. Bulkier substituents are predicted to lead to higher barriers and slower rates. The hydroxyl group on the alkyne hydrogen bonds to the chlorine stabilizes the transition state and increases the reaction rate. Generally good agreement is found with the trends in recently reported experimental relative rates of reaction of substituted alkynes with norbornadiene.     

Nickel-mediated regioselective [2 + 2 + 2] cycloaddition of carboryne with alkynes

Transition metal-benzyne complexes have found many applications in organic synthesis, mechanistic studies, and the synthesis of functional materials. In sharp contrast, the chemistry of transition metal-carboryne complexes, especially late transition metal complexes, is virtually unknown. This communication reports a novel nickel-mediated regioselective [2 + 2 + 2] cycloaddition reaction of carboryne with alkynes via the Ni-carboryne intermediate (eta2-C2B10H10)Ni(PPh3)2. Because of the bulkiness of the carborane moiety, a high regioselectivity was achieved in the reactions involving unsymmetrical alkynes. This work furnishes a novel method for the preparation of highly substituted benzocarboranes which are difficult to obtain by other methods.     

Cobalt-catalyzed regioselective [3+2] annulation of ortho-formyl and acetyl substituted phenylboronic acids with alkynes

Treatment of alkynes with ortho-formyl and acetyl phenylboronic acids in the presence of a cobalt catalyst resulted in the formation of 2,3-disubstituted indenols in good yields. When aryl silyl alkynes were used, 2-aryl-3-silyl indenols were obtained regioselectively in good yields.     

Ruthenium-catalyzed [2+2] cycloadditions between C1-substituted 7-oxanorbornadienes and alkynes

Ruthenium-catalyzed [2+2] cycloadditions between C1-substituted 7-oxanorbornadienes and alkynes were investigated. Most of the cycloadditions occurred smoothly at 65 °C, giving the cyclobutene cycloadducts in moderate to good yields. The C1 substituent showed strong effect on the regioselectivity (up to 110:1) of the cycloadditions.     

Pyridines via solid-supported [2 + 2 + 2] cyclotrimerization

The formation of pyridines via a crossed [2 + 2 + 2] cycloaddition has been achieved on a solid-support for the first time.     

Ruthenium-catalyzed [2 + 2] cycloadditions of ynamides

Ruthenium-catalyzed [2 + 2] cycloadditions between norbornene and ynamides were investigated. The ynamide moiety was found to be compatible with the ruthenium-catalyzed cycloaddition conditions, giving the corresponding cyclobutene cycloadducts in moderate to good yields (up to 97%). [reaction: see text]     

Phenanthridine synthesis via [2+2+2] cyclotrimerization reactions

A concise synthesis of phenanthridines via a microwave-assisted [2+2+2] cyclotrimerization reaction has been developed.     

Synthesis of indanones via solid-supported [2+2+2] cyclotrimerization

A new facile approach toward natural and unnatural indanones has been developed, featuring a solid-supported [2+2+2] cyclotrimerization as the key step. This strategy has been applied to the chemo- and regioselective assembly of indanone arrays and to the total synthesis of a recently isolated indanone marine natural product.     

Ruthenium-catalyzed [2 + 2] cycloadditions between 7-substituted norbornadienes and alkynes: an experimental and theoretical study

The ruthenium-catalyzed [2 + 2] cycloadditions of 7-substituted norbornadienes with an alkyne have been investigated. The cycloadditions were found to be highly regio- and stereoselective, giving only the anti-exo cycloadducts as the single regio- and stereoisomers in good yields. The results on the relative rate of different 7-substituted norbornadienes in the Ru-catalyzed [2 + 2] cycloadditions with an alkyne indicated that the reactivity of the alkene component decreases dramatically as the alkene becomes more electron deficient. Ab initio computational studies on the ruthenium-catalyzed [2 + 2] cycloadditions provided important information about the geometries and the arrangements of the four different groups on the Ru in the initial Ru-alkene-alkyne pi-complex, 14, and in the metallacyclopentene 15. Based on our computational studies, we also found that the first carbon-carbon bond formed in the [2 + 2] cycloaddition is between the C(5) of the alkene and the C(b) (the acetylenic carbon attached to the ester group) of the alkyne 8. Our computational studies on the potential energy profiles of the cycloadditions showed that the activation energy relative to the reactants for the oxidative addition step is in the range of 9.3-9.8 kcal/mol. The activation energy relative to the metallacyclopentene for the reductive elimination step is much higher than for the oxidative addition step (in the range of 25.9-27.6 kcal/mol).     

Ruthenium-catalyzed [2 + 2] cycloadditions of 2-substituted norbornenes

The studies of remote substituent effects in controlling regio- and stereoselectivities in chemical reactions provide important information in understanding long-range stereoelectronic effects. The effect of remote substituents on ruthenium-catalyzed [2 + 2] cycloadditions of 2-substituted norbornenes has been investigated. The cycloadditions occurred at room temperature in excellent yields, and regioselectivities of 1.2:1 to 7.5:1 were observed with various 2-substituted norbornenes.     

New iridacyclohexadienes and iridabenzenes by [2+2+1] cyclotrimerization of alkynes and facile interconversion between iridacyclohexadienes and iridabenzenes

Iridabenzenes [Ir[=CHCH=CHCH=C(CH2R)](CH3CN)2(PPh3)2]2+ (R=Ph 4 a, R=p-C6H4CH3 4 b) are obtained from the reactions of H+ with iridacyclohexadienes [Ir[-CH=CHCH=CHC(=CH-p-C6H4R')](CO)(PPh3)2]+ (R'=H 3 a, R'=CH3 3 b), which are prepared from [2+2+1] cyclotrimerization of alkynes in the reactions of [Ir(CH3CN)(CO)(PPh3)2]+ with HC[triple chemical bond]CH and HC[triple chemical bond]CR. Iridabenzenes 4 react with CO and CH3CN in the presence of NEt3 to give iridacyclohexadienes [Ir[-CH=CHCH=CHC(=CHR)](CO)2(PPh3)2]+ (6) and [Ir[-CH=CHCH=CHC(=CHR)](CH3CN)2(PPh3)2]+ (7), respectively. Iridacyclohexadienes 6 and 7 also convert to iridabenzenes 4 by the reactions with H+ in the presence of CH3CN. Alkynyl iridacyclohexadienes [Ir[-CH=CHCH=CHC(=CH-p-C6H4R')](-C[triple chemical bond]CH)(PPh3)2] (8) undergo a cleavage of C[triple chemical bond]C bond by H+/H2O to produce [Ir[-CH=CHCH=CHC(=CH-p-C6H4R')](-CH3)(CO)(PPh3)2] (10) via facile inter-conversion between iridacyclohexadienes and iridabenzenes.     

Highly regioselective [2+2+2] cycloaddition of terminal alkynes catalyzed by titanium complexes of p-tert-butylthiacalix[4]arene

Mono- and dinuclear titanium complexes of p-tert-butylthiacalix[4]arene were applied as a catalyst for [2+2+2] cycloaddition of terminal alkynes. They showed high catalytic activity and regioselectivity toward 1,3,5-trisubstituted benzenes over 1,2,4-trisubstituted isomers. The regioselectivity was rationalized in terms of the steric effect of the thiacalixarene skeleton and the coordination of the bridging sulfur atom to the titanium center.     

Ruthenium-catalyzed [2 + 2 + 1] cocyclization of isocyanates, alkynes, and CO enables the rapid synthesis of polysubstituted maleimides

Intermolecular [2 + 2 + 1] cocyclization of isocyanates, alkynes, and CO (1 atm) proceeded smoothly in the presence of a catalytic amount of Ru3(CO)12 (3.3 mol %) in mesitylene at 130 degrees C for 3 approximately 42 h to give a variety of polysubstituted maleimides in excellent yields with high selectivity. The reaction may involve an azaruthenacyclopentenone intermediate derived from oxidative cyclization of an isocyanate and an alkyne on an active ruthenium species.     

Rhodium-catalyzed [2 + 2 + 2] cyclotrimerization in an aqueous-organic biphasic system

Macrocyclization via rhodium-catalyzed intramolecular [2 + 2 + 2] annulation of triynes has been explored in an aqueous-organic biphasic system. The biphasic system controls the concentration of hydrophobic substrates in the aqueous reaction phase and offers diluted reaction conditions without the use of a slow addition technique. The system also achieves selective cross-annulation between hydrophobic diynes and hydrophilic alkynes.     

Ruthenium-catalyzed [2+2] cycloadditions of bicyclic alkenes and ynamides

Ruthenium-catalyzed [2+2] cycloadditions between bicyclic alkenes and ynamides were investigated. The ynamide moiety was found to be compatible with the ruthenium-catalyzed cycloaddition conditions giving the corresponding cyclobutene cycloadducts in moderate to good yields (up to 97%). Diastereoselective cycloaddition utilizing chiral cyclic ynamides were also examined and a low to moderate level of asymmetric induction was observed.     

Silver-catalyzed [2 + 2] cycloadditions of siloxy alkynes

We have described the first [2 + 2] cycloadditions of siloxy alkynes with a range of unsaturated carbonyl compounds. The reactions are efficiently promoted by substoichiometric amount of silver trifluoromethanesulfonimide and display excellent regio- and diastereoselectivity combined with a broad substrate scope. Our studies have established unambiguously the stepwise mechanism of this process and provided evidence for a novel role of silver in the catalytic cycle of the reaction, which involves silver-based complexation and activation of siloxy alkyne toward the subsequent 1,4-addition.     

Ruthenium-catalyzed annulation of aromatic ketones with internal alkynes: A reliable route to substituted naphthalene derivatives

We report herein the preparation of polysubstituted naphthalene derivatives by the ruthenium-catalyzed benzannulation reaction of aromatic ketones with internal alkynes. This method (1) forms two new carbon-carbon bonds in one single step, (2) accommodates a variety of functional groups, and (3) undergoes mild reaction conditions.