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Piperonal | C8H6O3 - ChemSynthesis

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A:
I will change the phosphorane to the ylide in the notes and re-post them. I think that the reactivity is better understood using the ylide form. This is a very good point.

The position of the alkene from a Wittig reaction is completely defined by the reactants and is an important feature of the Wittig. It forms between thecarbonyl carbon and the anion of the ylide. If you look at the mechanism on slide 159, the alkene forms between the green carbon (the anion of the ylide) and the blue carbon (the carbonyl carbon)

Piperonal - chemical structural formula, chemical names, chemical properties, synthesis references

A: The polyhalogenation is more of a problem wih the base induced alpha-halogenation. These reactions are done under very basic conditions, so the enol formation is very rapid.

The reactions that we learn are fine on paper. Sometime there are disconnects in textbook; as I noted today the HVZ reaction is over 100 years old. There are more modern reagents and reaction conditions for many of these reactions that are vastly improved from the 1890's vintage and can control which side of the carbonyl is the site for substitution and eliminated problems of multiple reactions. The alpha-bromination followed by elimination to give a a,b-unsaturated ketone is fine paper chemistry, but it is no longer the first thing that comes to mind if one really wanted to do that reaction.

The descrete formation of an enolate eliminates the issue of multiple reactions because the carbonyl is treated with an equal molar equiv. of the base to generate the enolate. Once the enolate reacts with an electrophile, there is no longer any base around to form a new enolate. So the reaction is controlled by stoichiometry.

Piperonal as electrophile in the Baylis-Hillman reaction



A: If you use LDA to generate the enolate, you must use a non-protic solvent because a protic solvent will be deprotonated by LDA. So if you use LDA in ethanol, what you really have is lithium ethoxide, not LDA.

Direct alkylation as described at the end of the chapter is not conceptionally different from the base catalyzed enolate formation described at the beginning of section 22.8, except for the reaction conditions are considerable different. When the enolate is generated with NaOEt in EtOH, the concentration of the enolate is related to the relative pKa's of the base and carbonyl compound. So enolate concentration is fairly low. For direct alkylation with LDA, the enolate is generated descretely and quantitatively before the electrophile is introduced. These are much more reactive conditions. We will see the value of this more in the next Chapter.

A: Enolates are much more reactive than enols so they give higher yields and shorter reaction time. When one forms the enolate specifically, the problem of products derived from multiple alkylation is eliminated. We will see in the next chapter that forming descrete enolates is critical for what is know as a cross-aldol condensation. One does not isolate the enolate, it is generated discretely, then the electrophile is introduced to the reaction. This is referred to in situ.

A synthesis of hydroxy-β-piperonyl-γ-butyrolactone derivative

What did you mean by: "When one forms the enolate specifically, the problem of products derived from multiple alkylation is eliminated." How does an enol give multiple alkylation products whereas an enolate ion gives but one.

This is an issue of stoichiometry. Once the enolate is formed, the base is consumed. After the enolate reacts, subsequent enolate formation can not occur because there is no longer any base present. Under the catalytic conditions, the product can be enolized again and react with the electrophile again, although the rate of the second alkylation reaction is usually slower than the first.


A: Yes, that is the correct reasoning. The dipole pulls electron density from the alpha C-H bond and makes deprotonation easier. This is the inductive effect component. The resonance effect also plays a large role. Since an ester O or amide N is involved in resonance interactions with the carbonyl, the degree to which the enolate can be stabilized through resonance is decreased. This is probably the larger effect.

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Microscale Organic Laboratory 4th Revised edition, …



A: This is something of a model or a prototype reaction for the nucleophilic addition to carbonyls. Hydrates of ketones and aldehyde are intermediates for some reactions, i.e., Jones oxidation. Gem-diols really can not be synthesized. You put and aldehyde or ketone in water and you get an equilibrium with the hydrate. You remove the water and you get the carbonyl back.

Synthesis of Bridged Oligophenylenes from Fluorene. …


A: This is really beyond the scope right now and the deeper reasons are well beyond my understanding. Almost all textbooks described NMR the way it was done 30 years ago. The NMR instruments do not work that way anymore and the physics and math required to fully understand NMR are bewildering (very advanced, at least for me).

The book very briefly mentions FT-NMR (Chapter 13.4) which is the way NMR instruments work. The dipole of the nuclei does't play a significant role in NMR. NMR is related to the interaction of the magnetic moment of a nuclei with an external magnetic field. Not all nuclei are NMR active. In any event, the magnetization of the nuclei is perturbed by a pulse of EM energy. The nuclei react to this perturbation and this gives rise to the NMR signal. In FT- (or pulsed or time-domain) NMR this perturbation is done many times and a spectrum is collected each time. The signals are then averaged (this is called signal averaging) and converted into the spectrum. After the nuclei is perturbed, it relaxes back to it original state; this information is collected and is one spectrum; it is then perturbed again after a time delay to allow for the relaxation and this is how multiple spectra are collected and averaged. All protons relax quickly and over a fairly uniform time range, in a few milliseconds or less, so the spectra can be collected very quickly. Carbons however relax slowly relative to protons and over a large range of the relaxation rates. Most carbons relax within a second or so, but a carbonyl can take several seconds or longer to relax and this is difficult to predict. So while the data is being collected, the different rates of relaxation cause the intensities of the carbon signals to differ. The delay between pulses is often not set sufficiently long enough to allow full relaxation and this added to weak signal of the carbonyl. So carbon intensities do not reflect the relative number of nuclei giving rise to that signal and therefore can not be reliable integrated without special techniques. Because carbon is such an insensitive NMR nuclei, the intensity is sacrificed in order to obtain as many pulses as possible. While the data for a proton spectrum can usually be collected in less than a minute, carbon spectra can take hours to collect.

Microscale Organic Laboratory with Multistep and Multiscale ..


A: This is a good example of the basicity versus nucleophilicity. These parameters must also be measure in the contexts of the rest of the system. Acid chlorides and anhydride are generallt so reactive toward nucleophilic acyl substitution, that it would be very difficult to find a base that would rather act as a base and do an a-deprotonation over acting as a nucleophile and adding to the carbonyl. Although I can recall a very few exampled of enolate formation and anhydride, these are not compounds that enolates are generally formed from.

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