21

Great question! It turns out that the rate of formation of the "expected" endo product is actually ~500 times faster than the rate of formation of the exo product. However, the Diels–Alder is a reversible reaction. In this case, the exo product is thermodynamically favored over the endo product by about $\pu{1.9 kcal/mol}$. So, even though the predicted ...


20

Introduction Instead of the usual cycloadditions (which has some extra complications in terms of the reacting orbitals), let's consider the concerted reaction $\ce{H2 + D2 -> 2HD}$. We'll come back to the cycloadditions at the end. occurring via a square planar transition state. This can be analysed with the W–H rules and you'll find that it's a ...


18

I believe that you are asking if there are any reactions that are essentially "non-polar". There are two classes that immediately come to mind: (1) radical reactions and (2) pericyclic reactions. A common radical reaction studied in introductory organic chemistry is the radical halogenation of alkanes. Instead of cationic and anionic reactants/intermediates/...


14

Your Diels–Alder adduct is pretty much correct. Without any further evidence we cannot definitely say whether the exo or the endo adduct is formed (you have drawn the exo adduct, which generally arises via thermodynamic control). However, the stereochemistry of this adduct does not affect the final product, so it does not matter which one is formed. By the ...


14

Normal dioxygen ($\ce{O2}$) exists as a ground state, triplet biradical. This is an example of a molecule that, thermodynamically should be quite reactive, yet is kinetically unreactive - once again, a case of kinetics vs. thermodynamics. Because spin must be conserved in a reaction, if ground state, triplet $\ce{O2}$ were to react with something, it ...


11

Have a look at the reaction mechanism of the Diels-Alder Reaction, e.g. at Wikipedia. We have talked about this reaction before on this site, and concluded that a sufficient explanation is only possible with the help of molecular orbitals: How accurate is this polar mechanism for the Diels-Alder reaction? Borrowing from Wikipedia,[1] here is the most ...


11

To provide a reason for the observed regioselectivity, it is helpful to draw anthracene's aromatic π-electron system in alternance of single and double bonds. In this instance, it is more beneficial than "the ring" symbolizing the delocalised electron system, as this helps you to account for the precise number of π-electrons before the reaction (starting ...


9

What is the type of the ring opening; is it a concerted or radical based C-C bond cleavage? This is a chelotropic reaction, a subset of cycloaddition\fragmentation reactions. As to whether it proceeds by a concerted or radical mechanism, well, you haven't presented any experimental data that can help us decide. For example, if the reaction had been run ...


9

Here is my assessment of the mechanism. You are correct about the Dess-Martin periodinane (DMP) oxidation of 1 to 2. The resulting ketone 2 can also exist as the enols (E)-3 and (Z)-4. Any one of the three can undergo a retro-Diels-Alder-like thermolysis, but I have chosen (Z)-4 for the convenience of cyclizing ketene 5, which has lost acetone in the ...


8

The reaction in question is one of the early examples of an oxy-Cope rearrangement [1] in which a 3-hydroxy-hexadiene undergoes a [3,3]-sigmatropic rearrangement to afford a ketone. The general mechanism is shown below: Source: Organic Chemistry, Oxford University Press, 2 ed. Clayden, Warren and Greeves In a general sense, the reaction is a [3,3]-...


8

The reason that comes up again and again is the one below, taken from Orbital Interaction Theory of Organic Chemistry. Orbital correlation diagrams are useful for cycloadditions and electrocyclic reactions but not for sigmatropic rearrangements since no element of symmetry is preserved. Source: Orbital Interaction Theory of Organic Chemistry. Arvi ...


7

As Ben Norris mentioned in his comment, ozonolysis is better described as a pericyclic reaction. Pericyclic reactions fall outside of the usual nucleophile-electrophile paradigm. This reaction is an example of a dipolar cycloaddition, because ozone is a 1,3-dipole. Chemists have observed that electron-rich alkenes react faster with ozone than electron-poor ...


7

Electrons are concentrated on the two terminal oxygens in ozone but remember, ozone exhibits resonance. The electrons can be delocalized across each oxygen through p-orbitals. So yes, at first if you think about the reaction in purely Coulombic terms then it doesn't make much sense. Why would electrons go to a region that already has a relative surfeit of ...


7

Another great example of how it's important to look at actual evidence. There have been various studies on Claisen rearrangement selectivity, here's a combined experimental/theoretical one on meta-substituted ethers specifically: Gozzo, F. C.; Fernandes, S. A.; Rodrigues, D. C.; Eberlin, M. N.; Marsaioli, A. J. Regioselectivity in Aromatic Claisen ...


6

What is the endo-selectivity of Diels-Alder reactions? If the diene used in the Diels-Alder reaction has asymmetric substituents at the end carbons, and if the dienophile is unsymmetric, then two different isomers of the final adduct can form. The isomer, where the functional group(s) (usually carbonyl) on the alkene end up on the same side as the newly ...


6

The torquoselectivity of the above reactions is dictated by stereoelectronic effects, which (in this case) are significant enough to take precedence over steric effects. In general, electron-donating substituents (e.g. OMe, OTMS, ...) prefer to move outwards, and electron-accepting substituents (e.g. CHO) prefer to move inwards. These can be understood by ...


5

Coordinated benzene as a dienophile It would be logical to suggest that an η4-benzene ligand could behave as a dienophile. The reason why benzene is characteristically unreactive, both as a dienophile and as a diene, is because of its aromaticity; if aromaticity is lost, one would expect it to become more reactive. However, to the best of my knowledge (and ...


5

Drawing on the suggestion of @user55119 I propose the mechanism shown below. Whether the first intermediate bicyclic [4.1.0] heptanone arises from an intramolecular electrocyclic process or an intramolecular aldol is unclear but either seems possible. Once the highly strained bicyclic [4.1.0] heptanone intermediate is formed, the reaction proceeds with OH- ...


4

The concerted Diels-Alder reaction has in principle eight different possible transition states. Depending on the substrates some of them will lead to the same products. Below you can see an overview of the (non-optimised) transition state arrangements for the reaction of isoprene and propene. From these images you should be able to determine that the ...


4

I was going to write a comment, but I think this is good enough for an answer. The issue that symmetry rules forbid a [2+2] thermal cycloaddition if the addition is suprafacial-suprafacial. However, you can achieve a suprafacial-antarafacial [2+2] thermal cycloaddition. This is symmetry allowed because the antarafacial symmetry is exactly opposite ...


4

The presence of an electron-donating group on the diene, or an electron-withdrawing group on the dienophile, is not an absolute requirement for the Diels-Alder reaction to occur. There are many Diels-Alder reactions that don't have such features. In any case, the cyano groups on the dienophile are electron-withdrawing groups. So, it's still a pretty ...


4

Firstly, your Diels-Alder products are missing the double bond in the ring (formed between what was formerly C-2 and C-3 of the diene). You could always simply hydrogenate it down to the alkane, but I feel like this would present some selectivity issues in your professor's suggested synthetic route, especially since a terminal double bond is more likely to ...


4

Although you probably know the answer by now, here is the cyclic transition state. It is an oxy-ene reaction. The Lewis acid lowers the activation energy of the reaction.


4

What Fleming is likely referring to is work by Edward Thornton (J. Am. Chem. Soc. 1972, 94, 1168) in which he attempted to make use of secondary kinetic isotope effects (KIE) to prove whether the Diels Alder reactions being studied were indeed pericyclic, or occurred via a radical pathway.* The phrasing in your post (which I assume is from the book itself) ...


4

I'm definitely not an expert in organometallic chemistry, and I don't know which aspect ($\eta^{\mathbf{4}}$, any other hapticity, or benzene as a dienophile or diene) is more important for you. Anyway, here's a JACS paper, which describes the the Diels-Alder reaction of N-methyl maleimide with an $\eta^2$-rhenium complex of benzene as the diene.


4

Problem #1 is that if you interchange the methyl and hydrogen groups, you get diastereomers, not enantiomers. But this is a relatively minor point. I assume your question is about the diastereoselectivity, not enantioselectivity (as ron rightly pointed out there is no enantioselectivity in this reaction). The analysis you have drawn is somewhat similar to a ...


4

First, consider if this reaction reversible? If so, we'll just form the thermodynamically favorable product. But the temporarily breaking of aromaticity and tautomerization to reform aromaticity suggests that this process is not reversible. So that means we're looking at kinetic factors. There are a few key concepts I learned from Dave Evans' class that ...


4

It depends on the organic molecule. When spin-orbit coupling facilitates intersystem crossing, it can be fast: 100 femtoseconds for nitronapthalene (viz. Chem. Eur. J. 2018, 24, 5379 – 5387). The Wikipedia article on intersystem crossing cites a timescale for radiative decay (phosphorescence) of triplet to singlet of 10−8 to 10−3 s.


3

It took me a while to see what was actually happening. Initially, I was focused on a [1,5] suprafacial sigmatropic proton shift followed by a diaza-electrocyclic ring opening reaction. However, after drawing the corresponding orbitals I realised that I would have a eight-electron antiaromatic π system, in which a proton shift would need to occur ...


3

No, there is no general orbital symmetry rule for predicting which of the two possible disrotatory motions will be favored. However, by examining steric effects in the two possible transition states we can often make a good guess as to which product will predominate. There are always a few "special cases" like the one you referenced in your post, where ...


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