Questions tagged [stereoselectivity]

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

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Diastereoselectivity in the zinc-mediated Barbier allylation

Penumati & Kommu (2014) report the stereoselective synthesis of (+)-$\alpha$-conhydrine from R-(+)-glyceraldehyde. In their synthesis, the imine 6 underwent a zinc-mediated Barbier allylation to ...
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216 views

Why does only the cis-isomer of potassium dioxalatodiaquochromate(III) form?

A friend of mine performed a standard synthesis reaction of potassium dioxalatodiaquochromate(III) by adding potassium dichromate to ethandioic acid dihydrate and a little water. The synthesis ...
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79 views

Stereochemical outcomes in opening of vinyl epoxides

Explain the outcomes of the following reactions. In each case the major product is shown. Firstly, I don't understand why the chloride always attacks on the right hand side of the epoxide. Also,...
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57 views

Chugaev and Selenoxide Elimination giving different products

In class we are going over intramolecular elimination. In this question I will provide two similar reactants that provide a different mixture of products. The chugaev elimination gives a 50/50 ...
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How does chelation control result in rate acceleration?

In the chapter on diastereoselectivity in Clayden, Warren & Greeves (2012), the Cram-Chelate model is discussed. Most significantly, under chelation control, the diastereoselectivity of the ...
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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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59 views

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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31 views

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 ...
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1answer
59 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 ...
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1answer
93 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 ...
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1answer
226 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 ...
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1answer
250 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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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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345 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 ...
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1answer
495 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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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 ...
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3answers
341 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 ...
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230 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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1answer
2k views

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 ...
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153 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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77 views

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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178 views

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, ...
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526 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 ...
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2k 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
2k 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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477 views

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
3k views

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
92 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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336 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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957 views

Why does the chlorination of indene occur with syn selectivity in heptane?

In the chlorination of indene to form 1,2-dichloro-2,3-dihydro-1​H-indene in heptane as solvent, the syn dichloride is formed as the major product, as reported in J. Org. Chem. 1980, 45 (25), 5150–...
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272 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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422 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 ...
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79 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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802 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 ...
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124 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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27k views

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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18k views

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 ...
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353 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 ...