Title: ETH_Sem6 / OCIV

What is the difference between supra and antara?

With the help of a scheme, descirbe the meaning of the terms conrotatory and disrotatory:

How can you very quickly determine whether a reaction will proceed in a conrotatory or a disrotatory fashion?

What is the three step proceedure to count the electrons participating in a pericyclic reaction?

Q: Torqueselectivity, what is the usual observation if steric effects dominate?
A: The larger substituent ends up trans to the alkenyl chain.

Q: What does the term contrasteric mean?
A: Example for a contrasteric pericyclic reaction:

How do donor substituents vs. acceptor substituents behave in electrocyclic ring opening in terms of Torqueselectivity?

MOs of the cyclopentadienyl anion:

Q: What is a special important case for which Ovchinnikov's rule breaks down?
A: If the carbon framework includes heteroatoms. This can lift degeneracy and thus interfere with the predictions of this rule.

Orbitals and coefficients (qualitatively) of butadiene:

What is the character table/symmetry labels and symmetry elements of the C_2v point group?

What is the character table/symmetry elements and symmetry labels of the C_s point group?

What is the character table/symmetry elements and symmetry labels of the C_2 point group?

Pi orbitals of benzene with energetic ordering and orbital coefficients:

What can happen if you only choose a subset of symmetry elements in the OCD?

Q: What does it mean if a reaction is thermally allowed according to an OCD?
A: If all the occupied orbitals in the lowest electronic configuration of the reactants correlate to all the occupied orbitals in the lowest electronic configuration of the products.

State Kasha's rule:

What is the procedure to get to the SCD?

How are zeroth order correlation lines assigned?

Explain the phenomena of avoided crossing and pericyclic minimum for zeroth order linnes in a SCD:

When can there be an ambiguity in the statement "photochemically allowed/forbidden"?

Definition of the quantum yield:

How do you figure out, how many orbitals you should consider in a OCD?

Important exam tips for drawing an OCD:

Q: In which case does FMO not work?
A: For excited states!

What is an important consideration when choosing a model system for a reaction?

Q: What are the MOs and the relative orbital coefficients of the allyl system?
A: Here: Allyl vs. ethylene

Q: Relevant orbitals of cyclobutene:
A: Here in the context of electrocyclic ring opening:

pi orbitals and relative orbital coefficients of butadiene:

Q: MOs and corresponding orbital coefficients of pentadienyl radical:
A: Orderec energetically from bottom to top with the top one having the highest energy:

What are the Walsh orbitals of cylopropane and what are problems??

Q: To what do avoided crossings lead?
A: To so-called symmetry imposed barriers.

Q: What kind of interactions contribute to an activation barrier even for "allowed" reactions?
A: - harmonic distortion of bond lengths and angles

Q: What is a Jablonski-Diagram and what are the typical lifetimes of the different processes?
A: Shows the different types of transitions for a given system. Here a very general example:

Qualitative potential hypersurface for a diabatic reaction:

Qualitative potential hypersurface for an adiabatic reaction:

What is a "hot molecule" reaction and how does the qualitative potential energy surface look like?

What is chemoluminescence?

Q: What is the qualitative potential energy surface for a chemoluminescence reaction and what are the conditions for that?
A: The last condition for chemoluminescence, which limits this phenomenon to only a few systems is: There must be accessible orbitals orthogonal to the bonds being made or broken so that spin-orbit coupling can connect the ground-state to low-lying triplets. There must be intersystem crossing and spin-orbit coupling is the mechanism to do that.

Why is the cyclopropylcarbinyl cation exceptionally stable?

What are the so-called Coulomb, resonance and overlap integral and what are their respective signs?

Q: What is important about the  orbital coefficients in the LCAO that can be really important for determining them?
A: They are normalised!

Q: What does the square of a given orbital coefficient signify?
A: For a single coefficient, its square is the probability that an electron occupying the LCAO MO is at that atomic site.

What are the approximations of the Hückel method?

Q: What needs to be considered about the choice of a model system for f.ex. a pericyclic reaction? (How far are we allowed to simplify it?)
A: - don't break the conjugated system if there is a larger one!
- f.ex. if you have a pericyclic reaction involving a benzene you shouldn't break the conjugation of the benzene ring, even if only a "butadiene" part of it reacts, but if it is f.ex. substituted with an aldehyde you can disregard aldehyde but not the benzene ring
- disregard some substituents (even some that might potentially conjugate) to keep some symmetry but don't break the conjugation of the symmetric framework

How do you set up the Hückel determinant?

Q: What do you get from solving the Hückel determinant and how do you get the orbital coefficients from this?
A: F.ex. for a 2x2 system (ethylene) you get the orbital coefficients from solving the following equation using the determined energy values:

Q: For what exactly is Hückel theory very useful and what is an important limitation?
A: The Hückel model is very good at getting the shapes and order of the orbitals right, but it is inaccurate in calculating the energies.

Q: What is TMM and what are the MOs?
A: - trimethylenemethane: a biradical

What is an electronic configuration?

Q: What is degeneracy?
A: If two states, orbitals or configurations have the same energy, they are said to be degenerate.

What is meant with alternant or non-alternant when discussing the Hückel regularities?

Q: Which molecules are always alternant?
A: Linear unbranched polyenes.

Alternant

Non-alternant

Benzylic, odd alternant

What does disjoint vs. non-disjoint mean when discussing the Hückel regularities?

What are some regularities observed for all alternant systems and when do they not apply?

How are the Hückel orbitals of all alternant systems split?

What is a regularity of the orbital coefficients of all alternant systems?

What are the regularities for linear, unbranched polyenes?

What is a Frost-Muslin diagram?

What is Möbius overlap vs. Hückel overlap?

What is the Möbius overlap analog of a Frost-Muslin diagram?

Aromaticity vs. antiaromaticity in the Möbius overlap:

What is Ovchinnikovs rule and that is the corresponding formula?

What is the relationship between hybridisation and geometry?

Q: What is the MO rationalisation behind the unexpected stability of the cyclopropane?
A: If we consider the Walsh orbitals of cyclopropane, we see, that all bonding orbitals are filled. This is reminiscent of the picture of benzene!

What are the limitations of Hückel theory?

Q: What is perturbation theory?
A: In this theory, the interaction between two moieties introduces a perturbation in their wavefuntions, which can be described by a correction to their original wavefunctions (zeroth-order functions).

What is the term for the first-order perturbation correction of the energies?

What are the first order perturbation changes to the wavefunction?

What are three general conclusions that can be made from the first order perturbation treatment?

What is a symmetry operation?

What is a symmetry element?

What are five common symmetry elements used in chemistry?

What is a representation of a group?

What are the naming conventions for the irreducible representations?

Q: What is important when you try to assign orbital symmetries?
A: Always specify your coordinate system and stick to it so that there is no ambiguity as to which axis or plane is being used.

How is the total symmetry of an electronic configuration determined and what are important considerations for it?

How are the symmetries of orbitals or electronic configurations denoted?

How can we use symmetry to evaluate whether or not a value/integral is zero or not?

What are the considerations of energetic splittings of two interacting orbitals?

What is FMO theory?

What are important conclustions on interactions between orbitals depending on whether they are filled or not?

What are two important caveats when we look at HOMO-LUMO interactions?

What is hyperconjugation?

Q: For what kind of reaction can hyperconjugation be very important?
A: For reactions involving carbocations.

Why do we observe stabilising interactions?

Q: What is the so-called anomeric effect?
A: - stereoelectronic effect
- belongs to the class of negative hyperconjugation, in which a filled p or pi orbital interacts with empty sigma* orbitals of neighbouring bonds

Q: What is the approximate value of stabilisation of the anomeric effect?
A: Approximatel 1.4 kcal/mol

What is the so-called exo-anomeric effect?

Q: What is the so-called gauche effect?
A: - it's actually an anomeric effect

Q: What is the general rationale, when we look for stabilising interactions?
A: Look for filled-unfilled interactions.

Why do we need symmetry adapted orbitals?

Q: What are the two kinds of interactions between remote chromophores?
A: 1. Through-space interactions: what we normally think of but it is rather short ranged
2. Throught-bond effects: can be quite far reaching, as far as over eight bonds

What regularity did Paddon-Row observe for through-space and through-bond interactions?

Q: What needs to be considered when looking at Grob fragmentation and what are the results from this?
A: - Grob fragmentation is only possible, if there is enough through-bond interaction through correct geometric arrangements
- we need antiperiplanar geometry for all four components
- this model not only predicts, which compounds will undergo the fragmentation, but it will also predict the resulting stereochemistry

What are the FMOs for a carbene fragment?

Q: What is a pericyclic reaction?
A: A pericyclic reaction is a reaction, for which there is continuous, cyclic orbital overlap in the TS. There is however no specifiaction to whether this overlap is Hückel or Möbius type.

Q: What are three main types of pericyclic reactions?
A: 1. Cycloadditions
2. Electrocyclisations
3. Sigmatropic rearrangements

Q: What are the typical bond "losses" and "gains" in a cycloaddition?
A: -2 pi bonds and +2 sigma bonds

Q: What are the typical bond "losses" and "gains" in a electrocyclisations?
A: -1 pi bond and +1 sigma bond (for ring closing and vice versa for ring opening)

Q: What are the typical bond "losses" and "gains" in a sigmatropic rearrangements?
A: No change in the overall count of pi or sigma bonds

What do the terms "allowed" and "forbidden" exactly mean for pericyclic reactions?

What does it mean, if we say a concerted pericyclic reaction?

What is meant by "allowed" or "forbidden" in pericyclic reactions?

What is meant by a synchronous or asynchronous mechanism in a pericyclic reaction?

Q: What is a cycloaddition reaction?
A: A reaction in which a cyclic compound is formed from two acyclic reactants through a cyclic TS.

Q: What is the Diels-Alder reaction?
A: Archetypical cycloaddition reaction between a diene and an olefin. Its a [Pi4_s+Pi2_s] cycloaddition.

What is the recipe for analysing the selectivity for a Diels-Alder reaction?

Q: Which one is the dominant interaction if we draw two reacting fragments on an energy diagram?
A: - same symmetry
- filled-unfilled
- smallest energy gap

Q: Why is the [Pi2_a+Pi2_s] not observed, even though it would be symmetry allowed?
A: It has a huge barrier imposed by geometrical constraints.

Q: How do we indicate electrons vs. atoms in cycloadditions?
A: For counting the electrons we use square brackets [n+m] and for atoms we use parentheses (n+m).

What is aromatic transition state theory (ATST)?

Q: What is the endo rule?
A: There are two possible [4+2] allowed products, endo and exo. The endo rule refers to the fact that in this type of reaction, only the endo product is observed, despite it being the thermodynamically less stable product.

What is meant by the endo vs. exo product of Diels-Alder reactions?

Q: Why is only the endo product observed in Diels-Alder reactions?
A: The TS has additional stabilising interactions (secondary orbital interactions) which override any adverse steric effects.

Q: What are substituent effects on cycloadditions?
A: - they can affect the rate of the cycloaddition
- in cases where both the diene and the dienophile are substituted unsymmetrically there is a regioselectivity explained by the effects of the substituents on the orbital coefficients

What is normal and inverse electron demand in Diels Alder reactions?

Q: What would be the most extreme case of a donor on an ethylene molecule and how could we model this?
A: The most extreme case would be an allyl anion and thus the orbitals and the respective coefficients can often be approximated by this.

Q: What would be the most extreme case of an acceptor on an ethylene molecule and how could we model this?
A: The allyl cation would be the most extreme case and it can often be used to model an approximate such cases to predict the orbital energies and coefficients.

In a diagram, show the effects of the different types of substituents (X, Z and C) on an ethylene molecule:

Q: What is the so-called ortho rule for Diels Alder reactions?
A: We preferentially observe the ortho substituted product for a Diels Alder reaction over the meta substituted product if we have a reaction between an electron rich diene and an electron poor dienophile.

Q: What effects do Lewis Acids have on Diels Alder reactions?
A: Lewis Acids are good Diels Alder catalysts. They increase the rate of the reaction and also the ortho specificity. For an electron deficient dienophile, coordination of the Lewis Acid to the electron withdrawing substituent further lowers the LUMO and further decreases the LUMO orbital coefficient at the substitution site. Selectivity by the ortho and endo rules is also enhances by Lewis acid catalysis.

What are very common DA-analogous reactions?

Which are typical 4Pi electron species that perform (3+2) DA-like cycloadditions?

Q: What are cheletropic reactions?
A: They are a subset of cycloadditions. The difference is that the two bonds being made or broken occur at the same atom.

Linear vs. non-linear approach for cheletropic reactions:

What are electrocyclic reactions?

Q: About what aspects are we particularly interested for electrocyclic reactions?
A: - stereochemistry
- the direction of rotation during the ring opening/closing

What do we have to be careful about when we do a HOMO/LUMO analysis for electrocyclic reactions?

What are the rotation considerations for the electrocyclic ring opening of cyclobutene?

What are the "rules" on the number of pi electrons involved in a electrocyclic reaction and the rotation?

What is torquoselectivity?

How do you choose a good model system?

Q: For a given starting material, what is the fastest reaction?
A: The one with the lowest TS.

Q: What is subjacent orbital control?
A: If f.ex. the energetically most favourable orbital interaction is constrained by geometry, the alternative, next-best interaction becomes the only viable option. This kind of case has been termed subjacent orbital control, because the orbital interaction determining the outcome is not the primary one.

What are sigmatropic rearrangements?

What is the nomenclature for sigmatropic rearrangements?

[1,5]-supra H shift:

[1,7]-antara H shift:

What is the correct nomenclature for a Cope rearrangement?

What is the nomenclature for the Claisen RAR?

What is the correct nomenclature for the Wittig RAR?

What information can be gleaned from the exact name of a sigmatropic RAR?

Which are in theory the possible "models" to describe a sigmatropic RAR and which ones should be used?

What greatly simplifies the sigmatropic RARs involving a hydrogen as the migrating group?

[1,3] vs. [1,5] vs. [1,7] H shifts:

What regularities for sigmatropic RARs can we glean from (j+i)?

Q: What is usually observed and why, if both [1,5] and [1,7] H shifts can potentially happen?
A: When both possibillities exist, the [1,7] is preferred, due to orbital coefficients!

[1,5] vs. [1,7] H shifts in seven membered rings:

What changes, if we consider the migrating group in a sigmatropic RAR to be a carbon group?

Inversion vs. retention for migrating carbon groups:

Summarised table of rules for the migration of carbon groups in sigmatropic RARs:

What is the Ene-Reaction?

What is different about the Oxy-Cope RAR compared to the normal Cope?

Q: What is the competing reaction for Oxy-Cope RAR and how was the issue solved?
A: The issue was solved by the discovery of the anionic oxy-cope RAR, which can take place at RT.

What is a retro-Diels Alder reaction and what is a classical example with a high driving force?

Q: To what other functional groups can cyclopropane rings on a carbon sceleton behave?
A: They can behave very similarly to a double bond:

Q: What can be an important consideration when a double bond is "substituted" with a cyclopropane ring?
A: The two "sides" of the pi system are not equivalent anymore. So by making this substitution, a reaction which showed little to no stereoselection is changed into a highly stereoselective one.
This effect is goverend by different orbital overlap.

What is a key point for [3,3] sigmatropic RARs catalysed vs. uncatalysed?

Q: What is a "dissociative" TS?
A: A TS, where in a concerted reaction one bond is broken slightly more than the new one is already forming.

Q: What is the TS for [3,3] sigmatropic RARs for both catalysed and uncatalysed reactions?
A: A chair!

What is the very general procedure for OCDs?

What is the rational behind making an OCD?

What is the general recipe for making an OCD?

Q: What is the geometry of the DA reaction and what is thus the conserved symmetry element?
A: It's boat-like so the conserved symmetry element is a mirror plane:

What is the most common type of damage to DNA by UV light?

What are the four caveats of OCDs?

What is a better explanation for "allowed" and "forbidden"?

What is the rationale behind SCDs?

What is an important consideration for the construction of an OCD in terms of symmetry?

In a diagram show explicitly a pericyclic minimum, an avoided crossing, internal conversion and a symmetry imposed barrier:

What is the explanation, why most photoreactions yield ground-state products?

Q: What can be done to get a higher quantum yield for a photochemical reaction?
A: Make a very exothermic photochemical reaction:

Q: What is incredibly important and should not be forgotten about the FMOs of the Benzyne?
A: They are orthogonal to the aromatic ring and thus rather composed of sp^2 than p orbitals --> worse overlap as they are slightly pointing away from each other.
This also results in a slightly different approach f.ex. in a DA reaction.

Q: Where is the selectivity in the following reaction determined and what does this indicate about the intermediate 9?
A: Cycloaddition reactions with benzyne are irreversible!

Q: How could the deprotection of a Boc protecting group in theory be modeled under strongly acidic conditions?
A: 1. The first step could be modelled as a retro-Ene reaction or just as a simple loss of the tert-butyl cation (like in an S_N1 reaction) followed by deprotonation to yield isobutene
2. The loss of CO_2 is a simple elimination reaction

Q: Only for which orbitals does substitution of an atom in the framework have an effect?
A: Only for orbitals with non-zero coefficients at that position. This is very important when we look at degenerate orbitals since in that case often only one of the orbitals has a change in energy and thus degeneracy is lifted.

Q: What should you consider/do when you have a Berson-Willcott rearrangement and you have the product given and
more than one possible direction of migration?
A: Try out several and see which one gives the correct product. Always be on the lookout for other possible migration patterns if you don't see it directly. Stick to your predicted rules of retention and inversion (maybe check twice you're correct but afterwards rather look for alternative migration patterns).