Facial Synthesis of o‐Carborane‐Substituted Alkenes and Allenes by a Regioselective Ene Reaction of 1,3‐Dehydro‐o‐carborane

1,3‐Dehydro‐o‐carborane is a useful synthon for selective cage boron functionalization of o‐carboranes. It reacts readily with alkenes or alkynes to give a variety of cage B(3)‐alkenyl/allenyl o‐carboranes by ene reactions in very high yields and excellent regioselectivity. This can be ascribed to the highly polarized cage C?B multiple bond, which lowers the activation barriers of the ene reaction.     

Consecutive Rh(I)-catalyzed Alder-ene/Diels-Alder/Diels-Alder reaction sequence affording rapid entry to polycyclic compounds

Conversion of acyclic allenynes to polycyclic compounds using consecutive transition metal catalyzed carbon-carbon bond forming reactions in a single chemical operation is described. Reaction of an allenyne with a Rh(I) catalyst affords a cross-conjugated triene via a formal Alder-ene reaction. The triene then participates in a Rh(I)-catalyzed intramolecular [4+2] cycloaddition reaction to generate a new conjugated diene. An external dienophile is added to this diene which then undergoes a second [4+2] cycloaddition reaction to afford a complex polycyclic ring system. This reaction sequence demonstrates the synthetic potential of allenynes and cross conjugated trienes and highlights the rapid increases in molecular complexity that are possible by one-pot sequential transition metal catalyzed carbon-carbon bond forming reactions.     

Stereodivergent ruthenium-catalyzed transfer semihydrogenation of diaryl alkynes

[Ru(3)(CO)(12)]-catalyzed transfer semihydrogenation of various functionalized diaryl alkynes with N,N-dimethylformamide (DMF) and water as hydrogen source affords cis- and trans-stilbenes. The stereodivergent approach can be switched by the use of acetic (HOAc) or trifluoroacetic (TFA) acid as additives. The catalytic processes can be applied to the synthesis of analogues of natural products such as cis-combretastatin A-4 and trans-resveratrol.     

An Alternative Mechanism for the Cobalt‐Catalyzed Isomerization of Terminal Alkenes to (Z)‐2‐Alkenes

The cobalt‐catalyzed selective isomerization of terminal alkenes to the thermodynamically less‐stable (Z)‐2‐alkenes at ambient temperatures takes place by a new mechanism involving the transfer of a hydrogen atom from a Ph₂PH ligand to the starting material and the formation of a phosphenium complex, which recycles the Ph₂PH complex through a 1,2‐H shift.     

Alder-ene reaction of aryne with olefins

A novel intermolecular Alder-ene reaction based on aryne and olefins was developed. We performed this transformation under mild conditions such as at room temperature, and this reaction displayed high selectivity and good yields only in the presence of CsF. Hence, the intermolecular Alder-ene reaction of aryne with olefins provides an effective route to synthesize derivatives of olefins.     

A study of (binap)(enyne)tetracarbonyldicobalt(0) complexes

Four (binap)(enyne)tetracarbonyldicobalt(0) complexes have been synthesised and their reactivity monitored by variable temperature (31)P NMR spectroscopy. Formation of (binap)dicarbonylhydridocobalt(-1) 12 occurred at temperatures between 35 and 55 degrees C, depending on the nature of the alkene and alkyne components of the enyne. The structure of 12 was determined by X-ray crystallography, and its presence under Pauson-Khand reaction conditions was verified by NMR spectroscopy.     

Ligand-Controlled Diastereoselective Cobalt-Catalysed Hydroalkynylation of Terminal Alkynes to E- or Z-1,3-Enynes

A diastereoselective hydroalkynylation of terminal alkynes to form the head-to-head dimerization products by two different cobalt-phosphine catalyst system is reported. The use of the bidentate ligand dppp and additional triphenylphosphine led to the selective formation of the (E)-1,3-enynes (E:Z>99:1) in good to excellent yields, while the tridentate ligand TriPhos led to the corresponding (Z)-1,3-enynes in moderate to good yields with excellent stereoselectivities (up to E:Z=1:99). Both pre-catalysts are easy to handle, because of their stability under atmospheric conditions. The optimized reaction conditions were identified by the Design of Experiments (DoE) approach, which has not been used before in cobalt-catalysed reaction optimisation. DoE decreased the number of required reactions to a minimum.     

Alder-ene reaction: aromaticity and activation-strain analysis

We have computationally explored the trend in reactivity of the Alder-ene reactions between propene and a series of seven enophiles using density functional theory at M06-2X/def2-TZVPP. The reaction barrier decreases along the enophiles in the order H₂CCH₂ > HCCH > H₂CNH > H₂CCH(COOCH₃) > H₂CO > H₂CPH > H₂CS. Thus, barriers drop in particular, if third-period atoms become involved in the double bond of the enophile. Activation-strain analyses show that this trend in reactivity correlates with the activation strain associated with deforming reactants from their equilibrium structure to the geometry they adopt in the transition state. We discuss the origin of this trend and its relationship with the extent of synchronicity between H transfer from ene to enophile and the formation of the new C C bond. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011     

Phytochemicals from Echinacea and Hypericum: A Direct Synthesis of Isoligularone

Reaction of trienes with α,β‐unsaturated aldehydes produces bicyclic products via a tandem Diels–Alder/ene reaction. The adduct from tiglic aldehyde was converted into isoligularone by conversion to a furan followed by benzylic oxidation.     

Highly stereoselective one-pot synthesis of tetrasubstituted alkenes via carbopalladation reaction of fluorine-containing acetylene derivatives

Treatment of fluoroalkylated alkynes with various aryl halides and arylboronic acids in the presence of Pd(0) in DMF/H₂O at 100 °C for 2 h led to the smooth three-component coupling reaction, the corresponding tetrasubstituted alkenes being obtained in high yields stereoselectively.     

Cobalt-catalyzed benzylic C-H amination via dehydrogenative-coupling reaction

An efficient direct benzylic C-H amination via dehydrogenative-coupling by using an inexpensive catalyst/oxidant (CoBr₂/^tBuOO^tBu) system is described. Various unmodified amides including primary or secondary sulfonamides, carboxamides, and carbamates preformed well with benzylic hydrocarbons with moderate to good yields.