Silver-catalyzed formal inverse electron-demand Diels-Alder reaction of 1,2-diazines and siloxy alkynes

A highly effective silver-catalyzed formal inverse electron-demand Diels-Alder reaction of 1,2-diazines and siloxy alkynes has been developed. The reactions provide ready access to a wide range of siloxy naphthalenes and anthracenes, which are formed in good to high yields, under mild reaction conditions, using low catalyst loadings.     

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.     

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.     

Sulfoxide-directed thermal intramolecular [4 + 2] cycloadditions between 2-sulfinyl butadienes and unactivated alkynes

2-Sulfinyl butadienes tethered to unactivated alkynes undergo a facile thermal intramolecular Diels-Alder cycloaddition, often at room temperature, to produce cyclohexa-1,4-dienes with good selectivities, in high yields, and preserving the valuable vinyl sulfoxide functionality.     

Intramolecular ketenimine-ketenimine [2 + 2] and [4 + 2] cycloadditions

Bis(ketenimines), in which the two heterocumulenic functions are placed in close proximity on a carbon skeleton to allow their mutual interaction, show a rich and not easily predictable chemistry. Intramolecular [2 + 2] or [4 + 2] cycloadditions are, respectively, observed when both ketenimine functions are supported on either ortho-benzylic or 2,2'-biphenylenic scaffolds. In addition, nitrogen-to-carbon [1,3] and [1,5] shifts of arylmethyl groups in N-arylmethyl-C,C-diphenyl ketenimines are also disclosed.     

First cobalt(I)-catalyzed [6 + 2] cycloadditions of cycloheptatriene with alkynes

[reaction: see text] The CoI2(dppe)/Zn/ZnI2 system effectively catalyzes the [6 + 2] cycloaddition of cycloheptatriene with terminal alkynes to afford 7-alkyl-bicyclo[4.2.1]nona-2,4,7-trienes in fair to excellent yields. The catalyst proved to be tolerant toward functional groups such as ketone, sulfone, ester, ketal, ether, alcohol, imide, and nitrile. The enantioselective cycloaddition of CHT and phenylacetylene with BINOL phosphoramidite ligand 4 afforded the cycloadduct 3a (R = Ph) in 91% yield and 74% ee.     

CpRuCl- and CpCo-catalyzed or mediated cyclotrimerizations of alkynes and [2+2+2] cycloadditions of alkynes to alkenes: A comparative DFT study

Computational methods have been used to better understand both the CpRuCl- and CpCo-catalyzed and mediated cyclotrimerization of alkynes and the [2+2+2] cycloaddition of alkynes to alkenes. These studies have allowed more accurate mechanisms to be proposed for each of these transformations and for some previously proposed intermediates to be discarded. The mechanistic rationale of these processes depends on the nature of the metal, ligands and substrate partners.     

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]     

Rhodium catalysed [2+2+2] cycloadditions of acetylenes

Wilkinson's catalyst RhCl(PPh₃)₃ is an effective catalyst (0.5 – 2 mole %) for the rapid intermolecular trimerisation of 1,6-heptadiynes with monoacetylenes under mild conditions.     

Frontier-orbital analyses of ketene [2+2] cycloadditions

Twelve kinds of ketene [2+2] cycloadditions have been investigated by ab initio calculations. They are composed of four ketenes (Y–HC=C=O, Y=H, NH₂, Cl, and CN) and three isoelectronic ketenophiles (ethylene, methylenimine, and formaldehyde). All the transition state geometries obtained here are not different significantly, but the extent of formation of two covalent bonds differs appreciably. The difference is attributable to the degree of the charge transfer interactions. One is the interaction from the π orbital and/or the lone pair orbital of a ketenophile to the LUMO of a ketene (dominant charge transfer, CT1). The other is that from the HOMO of the ketene to the π* orbital of the ketenophile (second dominant charge transfer, ct1). CT1 contributes to the formation of only one covalent bond, and ct1 does to the formation of the other. This independent function is characteristic of ketene [2+2] cycloadditions. They are not concerned with the orbital phase. We also have examined Fukui's postulate that the deformation of particular frontier orbitals causes the reaction progress. The role has been verified both by configuration analyses along the intrinsic reaction coordinate of the ketene-ethylene reaction and by the examination of distortions of frontier-orbital shapes along the low-frequency vibrational modes. Received: 25 June 1998 / Accepted: 28 August 1998 / Published online: 11 November 1998     

NHC-catalyzed enantioselective [2 + 2] and [2 + 2 + 2] cycloadditions of ketenes with isothiocyanates

The enantioselective N-heterocyclic carbene-catalyzed formal [2 + 2] and [2 + 2 + 2] cycloaddition of ketenes and isothiocyanates were developed. Reaction with N-aryl isothiocyanates at room temperature favors the [2 + 2] cycloaddition, while reaction with N-benzoyl isothiocyanates at -40 °C favors the [2 + 2 + 2] cycloaddition.     

DFT study on [4+2] and [2+2] cycloadditions to [60] fullerene

The functionalisation of C₆₀ fullerene with 2,3-dimethylene-1,4-dioxane (I) and 2,5-dioxabicyclo [4.2.0]octa-1(8),6-diene (II) was investigated by the use of density functional theory calculations in terms of its energetic, structural, field emission, and electronic properties. The functionalisation of C₆₀ with I was previously reported experimentally. The I and II molecules are preferentially attached to a C—C bond shared and located between two hexagons of C₆₀ via [4+2] and [2+2] cycloadditions bearing reaction energies of ?15.9 kcal mol^?1 and ?72.4 kcal mol^?1, respectively. The HOMO-LUMO energy gap and work function of C₆₀ are significantly reduced following completion of the reactions. The field electron emission current of the C₆₀ surface will increase after functionalisation of either the I or II molecule.     

Intramolecular [2 + 2]cycloadditions of dialkylketenes with alkenes. Regiochemistry of intramolecular [2+2]cycloadditions of ketenes with alkenes

Unsaturated dialkylketenes 7a, 7b and 7c undergo intramolecular [2 + 2]cycloadditions to give 8a (45%), 9b (23%) and 9c (45%). Intramolecular cycloadditions of dialkylketenes give higher yields than intramolecular cycloadditions of monoalkylketenes, even though dialkylketenes are less reactive than monoalkylketenes. An intramolecular competition experiment with ketene 17 establishes that trans-alkenes are approximately 33 times more reactive than cis-alkenes in intramolecular cycloadditions. Ketene 36 furnishes 22% of the expected bicyclo[3.2.0]heptanone 37 and 28% of bicyclo[3.1.1]heptanone 38.     

Multicomponent cycloadditions: the four-component [5+1+2+1] cycloaddition of vinylcyclopropanes, alkynes, and CO

Prompted by the view that intermediates of transition metal-catalyzed reactions could be intercepted by one or more additional components, studies in our laboratory have led to the design and development of new three-component [5+2+1], [4+2+1], and [2+2+1] cycloadditions. These continuing studies have now led to the identification of a fundamentally new four-component [5+1+2+1] cycloaddition reaction of vinylcyclopropanes, alkynes and CO, yielding hydroxyindanone products in generally good yields. Terminal alkynes bearing aryl or alkyl groups are tolerated well. Substitution at any position of the VCP leads predictably to substituted hydroxyindanone products. Using a bis-alkynyl substrate, the reaction can be carried out bi-directionally, forming 10 C-C bonds and four new rings from seven components in a single, operationally simple process.     

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.     

Synthesis of macrocycles via cobalt-mediated [2 + 2 + 2] cycloadditions

We investigated the formation of macrocycles from alpha,omega-diynes in cobalt-mediated co-cyclotrimerization reactions. Long-chain alpha,omega-diynes underwent metal-mediated [2 + 2 + 2] cycloadditions with nitriles, cyanamides, or isocyanates in the presence of CpCo(CO)2 (Cp = cyclopentadienide) to yield pyridine-containing macrocycles, i.e., meta- and para-pyridinophanes, such as 5m/5p, 35m/35p, and 41m/41p. The regioselectivity of these reactions was affected by the length and type of linker unit between the alkyne groups, as well as by certain stereoelectronic factors. An analogous alpha,omega-cyano-alkyne, 28, combined with an alkyne to yield two isomeric meta-pyridinophanes, such as 5m and 29m, and an ortho cycloadduct (benzannulation product), such as 29o. We developed a reaction protocol for these cobalt-based [2 + 2 + 2] cycloadditions that involves markedly improved conditions such that this process offers a convenient, flexible synthetic approach to macrocyclic pyridine-containing compounds. For example, diyne 6 reacted with p-tolunitrile in 1,4-dioxane to give 7p and 7m (7:1 ratio) in 87% yield at a moderate temperature of ca. 100 degrees C in 24 h without photoirradiation or syringe-pump addition. Isocyanates were also effective reactants, as exemplified by the formation of 44p almost exclusively (44p:44m > 50:1) in 64% yield from diyne 8 and 2-phenylethylisocyanate. By using this improved protocol we were able to co-cyclotrimerize long-chain alpha,omega-diynes with alkynes in certain cases to demonstrate a successful macrocyclic variant of the Vollhardt reaction. For instance, diyne 6 reacted with dipropylacetylene to give paracyclophane 57p and benzannulene 57o (2:1 ratio) in 29% yield.