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Questions tagged [stereoselectivity]

For questions about reactions that favour the formation of one or more stereoisomers.

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Why does d/l-2,3-dibromobutane on reaction with NaI/acetone give cis-2-butene?

This is what I have read/know about the dehalogenation of vicinal dihalides by NaI/acetone: The two halides which are leaving must be on opposite sides (from the mechanism). In case they are present ...
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0answers
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Carrying on a stereospecific 1,4 conjugate addition on a terminal alkyne

I am looking for a way to attack a propiolate ester with a triazole and get the Z isomer of the resulting acrylate selectively. All literature survey can come up with is big nitrogen containing ...
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0answers
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Enantioselective alkylation of Aldehydes by Dialkylzinc reagents

Enantioselective catalysis is one of the important reactions in Organic chemistry which has huge applications in practical basis for synthesis of optically pure natural compounds. In this respect, I ...
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Rationalising diastereoselectivity of hydroboration using stereoelectronics

Using bulky reagents such as 9-BBN, hydroboration is often highly diastereoselective, giving rise to the 1,2-anti diastereomer: This is easily explained using a steric argument, with the major ...
1
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1answer
56 views

Is one syn aldol product preferentially formed over the other?

In Carruthers' Modern Methods of Organic Synthesis, the following aldol reactions of two (Z)-enolates with benzaldehyde are shown: The two transition states shown seem to be different, and lead to ...
4
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1answer
65 views

Stereoselectivity of Roush crotylboration

One can use a tartrate derived boronate to enact enantioselective crotyl transfer. Eg in scheme below: I do not understand how the stereoselectivity arises. We have a chiral boronate, true, and we ...
4
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1answer
135 views

Explaining stereochemistry in a sigmatropic 1,3-alkyl migration

I have quite a bit of difficulty in explaining sigmatropic rearrangement reactions. Consider the following reaction: How do I explain the formation of the shown product? When simply moving the ...
3
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1answer
165 views

Elimination mechanism in alkenes

Do E2 elimination reactions occur in alkenes? If not, what mechanism is used? If they do, are they limited by any stereochemical rules as alkanes do (antiperiplanar)? My question is pernitent to the ...
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247 views

Why are lithium (Z)-enolates more stereoselective than (E)-enolates in aldol reactions?

In aldol reactions, it is apparently a general rule of thumb that (Z)-enolates give higher stereoselectivity (for the syn aldol product) than (E)-enolates do for the anti product. Quoting Carey & ...
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0answers
265 views

How can an SN1 reaction give non-racemic products?

I'm in organic 1 right now and we assume that SN1 reaction will always have racemic products for the sake of simplicity. But then our professor have a question for us that these solvolysis reactions ...
1
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1answer
348 views

How to Predict the Formation of Racemic Mixtures or Single Products from a Diels Alder Reaction?

So far, I understand the mechanism of the Diels Alder reaction and am able to correctly predict the product of some slightly complex examples. However, it seems that in some examples, a racemic ...
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1answer
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Why do stabilised ylids lead to trans alkenes in the Wittig reaction?

Why do stabilised ylids (such as $\ce{Ph3P=CHCO2Me}$) lead to (E)-alkenes in the Wittig reaction, in contrast to unstabilised ylids which are (Z)-selective? The (E)-alkene is formed from ...
21
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3answers
264 views

Why don't alkynes undergo preferential anti bromination to the degree that alkenes do?

Alkenes, for the most part (unless there is a phenyl group or highly ionising solvent) undergo almost exclusively anti addition of bromine. However, under similar conditions alkynes, although undergo ...
9
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1answer
153 views

Stereoelectronic effects in torquoselectivity of 4π electrocyclic opening

Thermal 4π electrocyclic ring openings of cyclobutenes to form butadienes are known to proceed in a conrotatory manner, consistent with the Woodward–Hoffmann rules. However, there are two different ...
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Stereoelectronics of the Houk model

In Clayden's Organic Chemistry it gives the Houk model as a way for predicting selectivity for alkene reactions where an existing stereocentre is present (diastereoselectivity). Unlike the ...
6
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1answer
118 views

Chelation-controlled aldol: why is the α substituent pseudo-axial?

In this Mukaiyama aldol reaction, taken from a 1999 paper by D.A. Evans,1 when $\ce{TiCl4}$ was used as the Lewis acid catalyst, the aldehyde 4a formed a chelated intermediate whereas 4b did not (due ...
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1answer
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Is this photochemical [2+2] cycloaddition enantioselective?

How is it deduced that one enantiomer is preferentially formed in the reaction? When I think of the MOs of the reactants, both enantiomers are viable as products.
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1answer
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Models for diastereoselectivity in nitronate additions

In the scheme below (taken from the referenced Chem. Eur. J. paper on the synthesis of Tamiflu), there is a step in which a nitronate anion (the anion derived from 5) is added into an electrophile 6, ...
11
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1answer
396 views

Why do Grignard reagents add to propargyl alcohols?

In Phil Baran's lectures on heterocyclic chemistry at Scripps, one of the lecture problems is concerned with this synthesis of Vioxx: The first step supposedly involves an anti addition of the aryl ...
13
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1answer
918 views

Why is the endo product the major product in a Diels-Alder reaction?

I understand that the endo product is, at least in all cases in my textbook, the major product in a Diels-Alder reaction. However, I don't understand why this is the case. The explanation in my ...
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1answer
1k views

Cram's rule for nucleophilic addition to carbonyl groups

I recently heard about Cram's rule for predicting the stereoselectivity in nucleophilic additions to carbonyl groups. Can anybody explain what this is and whether it is an accurate rule?
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3answers
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Effect of protecting group on diastereoselectivity of LiAlH4 reduction of α-hydroxyketones

In the reactions above, taken from Tetrahedron Lett. 1982, 23 (23), 2355–2358, the choice of protecting group on the α-hydroxyl group has a large influence on the diastereoselectivity of reduction. ...
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1answer
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Stereoselectivity of dihydroxylation with osmium tetroxide

Dihydroxylation responds to steric effects and proceeds at less hindered positions. What will be the major product in the following reaction if the reagent used is $\ce{OsO4}$ followed by $\ce{NaHSO3}$...
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1answer
85 views

Stereo selective reaction products

Pyruvic acid on reduction with yeast gives (-) lactic acid while with $\ce{NaBH4}$ it reduces to (+) lactic acid. How? I know this has something to do with stereoselectivity. But I don't know which ...
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1answer
316 views

Origin of diastereoselectivity in Simmons–Smith cyclopropanation of allylic alcohol

In Organic Chemistry by Clayden et al., the following reaction is shown: The cyclopropane ring is formed on the same side as the alcohol group. Is this because of the interaction between Zn and the ...
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2answers
223 views

Why do Zr, Sn, and Ti enolates form syn aldols selectively?

In Organic Chemistry (2nd ed.) by Clayden, Greeves, and Warren, it is stated that (p 869) [...] the enolates of some metals [Sn(II), Zr, Ti] give syn aldols regardless of enolate geometry. There ...
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1answer
348 views

How can a ketone be enantioselectively reduced, in the presence of an ester?

I'd like to reduce enantioselectively a ketone in presence of an ester (which should NOT be reduced to an aldehyde and further to an alcohol). Adding $\ce{LiAlH4}$ would also reduce the ester and ...
7
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0answers
62 views

Stereoselectivity of 1,3-dipolar cycloadditions

How do the 1,3-dipolar cycloadditions follow the endo rule as in the reaction below, where the anti diastereoisomer is formed predominantly?
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1answer
665 views

Why does acetate aldol with Evans auxiliary give almost no diasteroselectivity?

The following acetate aldol reaction gives no diasteroselectivity: Because I don't know what the basis of above statement is (I heard it in a lecture and read it on some internet web pages), I expect ...
2
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0answers
118 views

Does the amine used to generate the enolate affect the selectivity of an Evans aldol reaction?

In an Evans aldol reaction, the enolate is typically generated by addition of a boron Lewis acid (e.g. $\ce{(n-Bu)2BOTf}$) and an amine, which acts as a weak base. I'd like to know whether there's any ...
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1answer
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Why is the exo product formed in the Diels–Alder reaction between furan and maleic anhydride?

For most Diels–Alder reactions, the major product is endo because there are favourable interactions between the newly forming pi bond and the electron withdrawing groups of the dienophile. Why is the ...
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1answer
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Why does the unstabilised Wittig reaction selectively form cis alkenes?

This question is meant for a simple unstabilised ylide. The mechanism of the Wittig reaction, as given on ChemTube3d, involves a concerted formation of the oxaphosphetane (this is generally favoured ...
13
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1answer
342 views

The Evans auxiliary: rotational freedom of the benzyl group - stereoselectivity still guaranteed?

I created the L-Phenylalanine based Evans auxiliary bearing a benzyl group with help of a molecular model set and got into some questions. The structural formula will do: As I suppose a low ...