Asymmetric cyclopentannelation. Chiral auxiliary on the allene

An enantioselective variant of the synthesis of cross-conjugated cyclopentenones, based on D-glucose-derived chiral auxiliaries, is described. Minor modification of the method makes it applicable to the preparation of both enantiomeric series of products. Both enantiomers of the key intermediate in our roseophilin synthesis have been prepared.     

Theoretical studies on the electrocyclic reactions of bis(allene) and vinylallene. Role of allene group

The reaction mechanisms of the electrocyclic ring closure of bis(allene) and vinylallene were studied by ab initio MO methods. The conrotatory and disrotatory pathways of the electrocyclic reactions from bis(allene) to bis(methylene)cyclobutene were determined by a CASSCF method. The transition state on the conrotatory pathway is 26.8 kcal/mol above bis(allene) and about 23 kcal/mol lower than that on the disrotatory pathway at a MRMP calculation level. The activation energy on the conrotatory pathway is lower by 23 kcal/mol than that of the electrocyclic reaction of butadiene. This lower energy barrier comes from the interactions of the "side pi orbitals" of the allene group. The interaction of the "vertical pi orbitals" of the allene group is predominant at the early stage of the reaction. The activation energy of the electrocyclic reaction of vinylallene is about 8.5 kcal/mol higher than that on the conrotatory pathway of bis(allene).     

The infrared fundamental intensities and polar tensor of allene.

The polar tensor of allene was calculated from the infrared fundamental band intensities of C3H4 and C3D4. The ambiguities in the signs of the dipole moment derivatives with respect to their normal coordinates were resolved by comparison of tensor elements with ab initio calculations at the B3LYP, MP2(FC) and CCD(FC) levels with a 6/311 + + G(3d,3p) basis set. The results are similar to those previously obtained by Koga and co-workers except for the choice of an average of two sign combinations for the E symmetry elements. The values of the mean dipole moment derivatives for the sp and sp2 carbon atoms obtained in this work, 0.032 and -0.133 e, respectively, are in good agreement with the CCD(FC)/6-311 + + G(3d,3p), 0.061 and -0.128 e, and MP2(FC)/6-311 + + G(3d,3p), 0.072 and -0.153 e, theoretical results. The mean dipole moment derivatives are shown to be consistent with potential models relating 1s electron ionization energies and atomic charges.     

Aluminum carbenoids in allene cyclopropanation

The reactions of aluminum carbenoids with alkyl- and phenyl-substituted allenes and cyclic allenes are studied. An efficient method for the synthesis of substituted spiropentanes was developed. 1,2-Cyclononadiene was selectively converted into bicyclo[7.1.0]dec-1-ene. An unusual transformation of α-methylphenylallene into a spiroindane derivative under the action of Et₃Al-CH₂I₂ was found.     

Radical transformations for allene synthesis

Allenes are valuable organic molecules that feature unique physical and chemical properties. They are not only often found in natural products, but also act as versatile building blocks for the access of complex molecular targets, such as natural products, pharmaceuticals, and functional materials. Therefore, many remarkable and elegant methodologies have been established for the synthesis of allenes. Recently, more and more methods for radical synthesis of allenes have been developed, clearly emphasizing the associated great synthetic values. In this perspective, we will discuss recent important advances in the synthesis of allenes via radical intermediates by categorizing them into different types of substrates as well as distinct catalytic systems. The mechanistic studies and synthetic challenges will be highlighted.

Recent important advances in the synthesis of allenes via radical strategies are highlighted.     

The excited states of allene

Excited states of allene are calculated using virtual orbitals from a Gaussian SCF calculation on the ground state. The ground state ionization potential is predicted to be 10.14 eV. Although the calculated transition energies are too high, the spectrum is predicted to consist of 3 weak transitions followed by a strong transition at shorter wavelength, in good qualitative agreement with experiment. States of the flattened (D _2h) molecule were also studied. The rotation barrier of the ground state is predicted to be 3.12 eV. The lowest triplet state is predicted to be planar.

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Zusammenfassung Aus einer SCF Rechnung mit Gauß-Funktionen für den Grundzustand des Allens werden die virtuellen Orbitale für die Berechnung angeregter Zustände übernommen. Das Ionisationspotential des Grundzustands ergibt sich dabei zu 10,14 eV. Die Rechnungen zeigen, daß das Spektrum aus drei schwachen Übergängen und einem starken Übergang kürzerer Wellenlänge besteht. Dies steht in guter Übereinstimmung mit dem experimentellen Befund, obwohl die Übergangsenergien zu hoch berechnet werden. Die Zustände des Moleküls mit der Symmetrie D _2h werden studiert. Die Energieschranke für die Rotation wird zu 3,12 eV berechnet und der tiefste Triplett-Zustand als eben vorausgesagt.

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Résumé Calcul des états excités de l'aliène à l'aide des orbitales virtuelles fournies par un calcul SCF en gaussiennes sur l'état fondamental. Le calcul prédit un potentiel d'ionisation de 10.14 eV. Les énergies de transition caculées sont trop hautes mais le spectre calculé consiste en 3 transitions peu intenses suivies par une transition intense à plus courte longuer d'onde, ce qui est en bon accord qualitatif avec l'expérience. On a aussi étudié les états de la molécule «aplatie» (D _2h). La barrière de rotation est de 3.12 eV dans l'état fondamental. L'état triplet le plus bas devrait être plan.

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Work supported by the U.S. Army Research Office —Durham.     

Isomerization and desulfurization of allene episulfides

Allene episulfides unevenly substituted underwent a thermal and acid catalyzed isomerization and a desulfurization, which were rationalized by the tautomeric recyclization and sulfur elimination of thioxyallyl intermediate.     

On the question of allene formation from tricyclic cyclopropylidenes

The carbenoid or carbene derived from $\text{8}$ undergoes dimerization or trapping with DPIBF. The allene mechanism previously proposed was made doubtful by the finding that stereoisomeric carbenes

and

give stereoisomeric products without crossover.     

Traceless chiral auxiliaries for the allene ether Nazarov cyclization

The key stereochemical factors that determine transfer of asymmetry from the chiral auxiliary to the cyclopentenone in the allene ether version of the Nazarov reaction have been elucidated. On the basis of the new insights into the mechanism, two highly effective chiral auxiliaries were designed and prepared.     

Reaction between ozone and allene in the gas phase

The kinetics of the reaction between ozone and allene (A) were studied in the range of 226 to 325°K in the gas phase. Initial O₃ pressures varied from 0.01 to 0.7 torr and allene pressures varied from 0.05 to 6 torr. At the higher initial O₃ pressures the most important product was O₂ followed by CO, H₂O, CO₂, and C₂H₄. Oxygen balances averaging about 110% were obtained, which implies that no important oxygenated products were missed. However, carbon balances were only about 50% and hydrogen balances were even less, so that unidentified hydrocarbons were presumably formed. The rate law found was ? d[O₃]/dt = k₁[O₃][A] + k_2a[O₃]²[A]/[O₃]₀ where log k₁(M^?1sec^?1) = 6.0 ± 0.7 ? (5500±1000)/2.30RT and log k_2a(M^?1sec^?1) = 6.9 ± 0.7 ? (6200 ± 800/2.30RT). A mechanism is proposed which accounts for the rate law and the observed stoichiometry of O₂ formed–O₃ used. This involves a heterogeneous catalyzed decomposition of O₃. The rate constant k₁ is identified with the primary addition reaction A + O₃ → AO₃, and this rate constant is compared with those from other O₃ addition reactions.     

Synthesis and structure of the chromiumcarbonyl complexed phenyl allene

The chromiumtricarbonyl complexed phenyl allene 4 can be readily synthesized by palladium catalyzed hydride reduction or dimethyl cuprate mediated reduction of suitable propargyl ester derivatives 3 and 5. This parent compound of chromiumcarbonyl complexed aryl allenes is fairly stable and is unambiguously characterized by an X-ray structure analysis. Thiolate adds to 4 to give a 3.5:1 distribution between the nonconjugated and the conjugated addition products 7 and 8 in good yield.     

From allene to allene: a palladium-catalyzed approach to beta-allenyl butenolides and their application to the synthesis of polysubstituted benzene derivatives

An allene to allene protocol for the synthesis of beta-allenyl butenolides in moderate to high yields from 2,3-allenoic acids and propargylic carbonates catalyzed by Pd(OAc)2-TFP has been developed; the products were applied successfully to the Diels-Alder reaction with electron-deficient alkynes to afford polysubstituted benzene derivatives with an excellent regioselectivity.