26 votes
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How is exact racemization possible?

Racemization isn't "exact," but rather very very close to equality. It is just simple probability. Think of flipping a coin, p=probability for heads, and q=probability of tails. Now for a fair flip ...
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22 votes

Why do we get slightly more inversion product than retention product in SN1 reaction?

It is well known that SN1 reactions often give incomplete racemisation: Although many first-order substitutions do give complete racemization, many others do not. Typically there is 5–20% inversion, ...
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21 votes
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Why is the exo product formed in the Diels–Alder reaction between furan and maleic anhydride?

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 ...
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18 votes

Why do we get slightly more inversion product than retention product in SN1 reaction?

As we know, $\ce{S_N1}$ reaction is stereochemically non specific. This means, that it does not force the formation of one stereoisomer, as $\ce{S_N2}$ does. In $\ce{S_N1}$ reaction, presence of ...
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14 votes
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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 osmium tetroxide followed by sodium ...
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11 votes

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

Key The reaction is run in heptane, a non-polar solvent. A non-polar solvent will not stabilize or support ions as well as a polar solvent. Background When the $\ce{Cl2}$ begins to interact with ...
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10 votes
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Models for diastereoselectivity in nitronate additions

Dave Evans taught me that you should generally analyze these types of problems by drawing out allyl conformations. I've drawn the nitronate to minimize 1,3-allylic strain. Now, the top face of the ...
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10 votes
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Why do Grignard reagents add to propargyl alcohols?

The addition of Grignards to propargyl alcohols occurs via the alkoxide salt, after deprotonation:[1] Here: the reactivity may be explained by the stabilising influence of complexation; the ...
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10 votes
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Why do stabilised ylids lead to trans alkenes in the Wittig reaction?

TL;DR: With stabilised ylids, the trans oxaphosphetane is formed, under kinetic control. This is due to minimisation of dipole-dipole repulsions in the transition state for oxaphosphetane formation: ...
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9 votes
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Why does d/l-2,3-dibromobutane on reaction with NaI/acetone give cis-2-butene?

The process of elimination of two bromides which you are thinking is not the correct way how the reaction occurs in this case. First there will be a nucleophilic substitution (preferably SN2) by $\ce{...
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9 votes

Can a stereospecific / stereoselective reaction produce a racemic mixture of enantiomers?

You haven't mixed up your terms: a stereoselective reaction is defined as one which preferentially forms one stereoisomer over another. Likewise, all stereospecific reactions are also stereoselective, ...
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8 votes

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

1. The acetate aldol reaction Before looking at selective variants, its worth pointing out what an acetate aldol is. An acetate aldol (below), is the simplest kind of aldol in which the β-...
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8 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 ...
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8 votes

Stereoelectronics of the Houk model

1. Origins of the Houk model The initial idea behind the Houk model was infact provided by Kishi, who proposed that alkenes with a chiral centre on the adjacent carbon adopted a 'reactive conformation'...
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8 votes

How is exact racemization possible?

It's just theory vs. real life. When mixing components, you always have limitations with the purity of chemicals and the accuracy of the balance available. When you look at chemical reactions which ...
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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 ...
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6 votes
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Stereoselectivity of ring closure in intramolecular iodolactonisation

The results of the iodolactonization example presented in this question were published in 1982 by Bartlett, et al.1 While @Aniruddha Deb has captured the essence of the reaction, there is more than ...
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5 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 ...
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5 votes

Cram's rule for nucleophilic addition to carbonyl groups

Cram's rule, proposed by Donald Cram in 1952,[1] was the earliest model proposed for rationalising the stereoselectivity of nucleophilic additions to carbonyl groups with α-stereocentres. The model ...
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5 votes

Effect of protecting group on diastereoselectivity of LiAlH4 reduction of α-hydroxyketones

I think Jan did an excellent job of answering your question, however I am writing a slightly longer answer largely with more details concerning the Felkin–Anh rule, just so that someone who is not ...
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5 votes

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

Having the methyl group psuedo-axial means that when the nucleophile approaches, the transitiation state is chair-like. The energy penalty for having the methyl group initially in the less favourable ...
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5 votes
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Does syn-dihydroxylation form racemic mixture?

Oxidation of alkene to 1,2-syn-diol by cold basic $\ce{KMnO4}$ is a common classification test for alkene in undergraduate organic chemistry laboratory courses. OP had correctly recognized that this ...
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4 votes
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Origin of diastereoselectivity in Simmons–Smith cyclopropanation of allylic alcohol

Yes. Right after Clayden mentions this stereoselectivity, he proceeds to state that this is due to "coordination between the zinc atom and the hydroxyl group in the transition state" (pp. 1067-8, 1st ...
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4 votes

Is this photochemical [2+2] cycloaddition enantioselective?

If there is no source of chirality present in the reaction mixture, then there is no reason for the reaction to be enantioselective. In this case there are two possible sources of chirality: firstly ...
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4 votes

Effect of protecting group on diastereoselectivity of LiAlH4 reduction of α-hydroxyketones

The $\ce{TBDPS}$ group (tert-butyldiphenylsilyl) is a very bulky protecting group. A silicon atom is bonded to the oxygen, which already acts as a larger carbon atom. That is then substituted by three ...
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4 votes

Stereoselectivity of ring closure in intramolecular iodolactonisation

Without further details, I would say that both products are possible. The reaction that you have given here is an Iodolactonization reaction. You have correctly identified that the tert-butyl group ...
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3 votes

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

For most aldol reactions, the relative 1,2-diastereoselectivity is a consequence of the enolate geometry.* Both tin and titanium enolates are easily formed, and have been shown to exist predominantly ...
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3 votes

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

I have had great results for enantioselective ketone reduction using the Noyori Ru-TsDPEN system. This has advantages through being catalytic with low stoichiometry and doesn't require high pressure, ...
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3 votes

Stereo selective reaction products

Enyzmatic reduction (as would occur in yeast) could easily be a stereoselective process - probably via selective delivery of a hydride from NADH. As the other respondent notes, there is simply no way ...
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3 votes
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Stereochemical outcomes in opening of vinyl epoxides

While @orthocresol was busy finding the Carreira paper, I was busy constructing my own thoughts on the stereochemical consequences of the reactions of 1 (iii) and 5 (ii). Yes, it is an issue of ...
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