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21 votes
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Theoretical basis behind orbital correlation diagram for pericyclic reactions

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 ...
orthocresol's user avatar
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14 votes
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Reaction of substituted furan with methyl vinyl ketone with subsequent rearomatisation

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 ...
orthocresol's user avatar
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10 votes
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Retro Diels–Alder reaction in synthesis of corallopyronin A

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 ...
user55119's user avatar
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9 votes
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Why is the endo product the major product in a Diels-Alder reaction?

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 ...
S R Maiti's user avatar
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8 votes
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Treatment of 2-vinylbicyclo[2.2.2]oct-5-en-2-ol with potassium hydride

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 ...
NotEvans.'s user avatar
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8 votes

Why can't we use orbital correlation diagrams for sigmatropic rearrangements?

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 ...
NotEvans.'s user avatar
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8 votes

Claisen rearrangement in substituted ring

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-...
Dennis Cao's user avatar
7 votes
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Explaining stereochemistry in a sigmatropic 1,3-alkyl migration

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 ...
orthocresol's user avatar
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7 votes
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Identification of endo and exo products in this Diels-Alder reaction

The product you have drawn looks like an exo-product. Conformation 1 leads to an exo-transition state that produces exo 2 as the enantiomer shown. With an achiral catalyst ent-exo 2 would also be ...
user55119's user avatar
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6 votes
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Stereoelectronic effects in torquoselectivity of 4π electrocyclic opening

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 ...
orthocresol's user avatar
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6 votes

Identification of endo and exo products in this Diels-Alder reaction

For me, the most foolproof way to identify the endo and exo products is to look at the stereochemistry in the product. Consider first a standard intermolecular Diels–Alder reaction: I labelled the ...
orthocresol's user avatar
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5 votes

Claisen rearrangement in substituted ring

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 ...
Zhe's user avatar
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5 votes

Pericyclic Reaction Mechanism

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.
user55119's user avatar
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5 votes

Do eta-4 benzene ligands participate in Diels Alder or other cycloadditions?

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 ...
orthocresol's user avatar
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5 votes
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Can vinyl alcohol react as a dienophile in Diels-Alder reactions?

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 ...
orthocresol's user avatar
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5 votes
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Does the reaction of 2‐hydroxy‐6‐methyl‐5‐nitrocyclohepta‐2,4,6‐trien‐1‐one with NaOH involve ring contraction?

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 ...
Waylander's user avatar
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4 votes

Reaction between 1,2-dicyanoethylene and 2-phenylbuta-1,3-diene in toluene and heat

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-...
orthocresol's user avatar
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4 votes
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Kinetic isotope experiment used to distinguish radical and concerted mechanisms for the Diels Alder reaction

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 ...
NotEvans.'s user avatar
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4 votes

Why is a racemic mixture formed in the Diels-Alder cycloaddition?

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 ...
Martin - マーチン's user avatar
4 votes

Can the Smirnov-Zamkow reaction be described as a pericyclic process?

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, ...
Zhe's user avatar
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3 votes

How fast is the interconversion triplet → singlet in organic molecules?

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 ...
KG3's user avatar
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3 votes
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Direction of arrows in electrocyclic reactions

The answer is rather bland and probably not what you want to hear. Electrocyclic reactions are called electrocyclic because they are cyclic. (Please don’t see cyclic circular reasoning in there.) They ...
Jan's user avatar
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3 votes

What is the difference between Fukui FMO approach and Woodward-Hoffmann rules?

The FMO approach, based on perturbation theory, focuses on analyzing the relative energies and populations of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (...
Alex's user avatar
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3 votes

How to classify the hydride shift in the following diazacyclopenta[cd]pentalene system?

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 ...
Jan's user avatar
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3 votes

Application of the Woodward-Hoffman rules to a [14+2] cycloaddition

General rules for cycloadditions Before getting into the actual Woodward Hoffman 'analysis', one can quickly check any proposed cycloaddition reaction against the following table of general results. ...
NotEvans.'s user avatar
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3 votes
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Are alkenes or alkynes reduced first with diimide?

Your given answer to the textbook question is seemingly correct. I said seemingly, because this reaction is not well understood to date, even though its first introduction was given at time as far as ...
Mathew Mahindaratne's user avatar
3 votes
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Cis, trans diene reacting with trans dienophile in D-A reaction

Macrophage: I thought you did a fine job in analyzing the Diels-Alder reaction of the E,Z-pentadiene with fumaronitrile. To complement my comment above, I offer an illustration of the transition ...
user55119's user avatar
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2 votes
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Application of the Woodward-Hoffman rules to a [14+2] cycloaddition (follow up)

For reactions in the excited state, a modified statement of the Woodward Hoffman rule applies: A pericyclic change in the first excited state is symmetry allowed when the total number of (4q+2)s ...
NotEvans.'s user avatar
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2 votes
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What is the driving force behind sigmatropic reactions?

Most of these reactions are indeed equilibrium reactions, meaning that they go forwards and backwards all the time and at a given point in time there will be a certain ratio of reactants to products ...
Jan's user avatar
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2 votes

Why can anthracene, but not phenanthrene, take part in Diels–Alder reactions?

HOMOs and LUMOs don't explain everything. Polyaromatic hydrocarbons also have their reactivities explained by Clar's aromatic sextet rule. There is an excellent reference here from the Journal of ...
Mark Banash's user avatar

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