29,418 research outputs found
[2+2]-cycloadditions of alkenes with the triphenylallenyl cation
The triphenylallenyl cation (8), generated from triphenylpropynol (7) and FSO3H, reacts with alkenes to give the allyl cations 12, which may be deprotonated to yield the methyleneccylobutenes 14. Alternatively, 12 can be converted into the 2-vinyl-indenes 13 two subsequent electrocyclic reactions
Diastereoselective synthesis of β- and γ2-muurolene. A carbocationic pathway from mono- to sesquiterpenes
The Lewis acid catalysed addition of the piperityl chlorides 6 to isoprene yields adduct 7, which undergoes a cis-stereoselective cyclisation to give the diastereoisomeric muurolene monohydrochlorides 8. Treatment with potassium tert-butoxide affords β- and γ2- muurolene 2
Kinetic and Thermodynamic Studies of Carbenium Ion Additions Towards Alkenes
The ionisation (Ar2CHCl + BC13 v=± Ar2CH+BCi;) and dissociation (Ar2CH+BCi;
^=*Ar2CH+ + BCl^) equilibria of diarylmethyl chlorides i n boron trichloride/
dichloromethane solution have been studied by conductimetry, photometry
and *H NMR spectroscopy. Small differences i n the UV-vis spectra of diarylcarbenium
tetrachloroborates, which have been observed i n solutions of low
and high tetrachloroborate concentration, can be attributed to the formation
of 1 :1 ion-pairs i n the more concentrated solutions. Low temperature calorimetry
was used to determine the heats of addition of diarylcarbenium tetrachloroborates
to 2~methyl-1 -pentene (Ar2CH+BCi; + H2C=CRRT ArjCH-CHj-CRR'Cl
+ BC13), and it is estimated that the standard free enthalpy of t h i s reaction
is greater than 0 for systems with pKR+ > -2.6. Kinetic studies have shown
that paired and unpaired diarylcarbenium tetrachloroborates exhibit identical
reactivity towards alkenes. A rationalisation for the different situation
in carbocationic and carbanionic polymerisation i s presented. The rate
constants for the initiation of isobutene, styrene and isoprene polymerisation
by diarylcarbenium ions have been determined, and it is shown, how propagation
rate constants can be estimated from linear free energy relationships
Steric control of regiochemistry in the reactions of methyl substituted pentadienyl cations with isobutene
The dienylic chlorides 1–5 react with isobutene in the presence of zinc chloride/ether in dichloromethane to give the acyclic adducts 6–10 in 64–91% yield. The orientation effects are in contrast to the predictions of perturbational molecular orbital theory and can be explained on the basis of steric effects
Synthesis of 1,1,2,2,3,3-hexamethyl-4,5-bis(methylene)cyclopentane
An efficient synthesis of the title compound 14 is reported, which employs the ZnCl2/Et2O catalyzed addition of acetyl chloride (4) to 2-methyl-2-butene (5) and the [3+ + 2] cycloaddition of the 1,1,2,3-tetramethylallyl cation to 2,3-dimethyl-2-butene as key steps
Conformal Field Theories on K3 and Three-Dimensional Gauge Theories
According to a recent conjecture, the moduli space of the heterotic conformal
field theory on a ADE singularity of an ALE space is equivalent to
the moduli space of a pure \cx N=4 supersymmetric three-dimensional gauge
theory with gauge group G. We establish this relation using geometric
engineering of heterotic strings and generalize it to theories with non-trivial
matter content. A similar equivalence is found between the moduli of heterotic
CFT on isolated Calabi--Yau 3-fold singularities and two-dimensional
Kazama-Suzuki coset theories.Comment: 11pp, harvmac, v2: a relation between heterotic CFT on Calabi-Yau
3-fold singularities and Kazama-Suzuki models has been adde
N=1 Mirror Symmetry and Open/Closed String Duality
We show that the exact N=1 superpotential of a class of 4d string
compactifications is computed by the closed topological string compactified to
two dimensions. A relation to the open topological string is used to define a
special geometry for N=1 mirror symmetry. Flat coordinates, an N=1 mirror map
for chiral multiplets and the exact instanton corrected superpotential are
obtained from the periods of a system of differential equations. The result
points to a new class of open/closed string dualities which map individual
string world-sheets with boundary to ones without. It predicts an
mathematically unexpected coincidence of the closed string Gromov-Witten
invariants of one Calabi-Yau geometry with the open string invariants of the
dual Calabi-Yau.Comment: harvmac, 29 pages (b), 3 figures; v2: references adde
Summing up Open String Instantons and N=1 String Amplitudes
We compute the instanton expansions of the holomorphic couplings in the
effective action of certain \cx N=1 supersymmetric four-dimensional open
string vacua. These include the superpotential , the gauge kinetic
function and a series of other holomorphic couplings which are known
to be related to amplitudes of topological open strings at higher world-sheet
topologies. The results are in full agreement with the interpretation of the
holomorphic couplings as counting functions of BPS domain walls. Similar
techniques are used to compute genus one partition function for the closed
topological string on Calabi--Yau 4-fold which gives rise to a theory with the
same number of supercharges in two dimensions.Comment: 29 pages harvmac(b); v2: references adde
- …