Prochirality

Take another look at the reactions in the chapter so far—in particular those that give single diastereoisomers (rather than single enantiomers or geometrical isomers), in other words, those that are diastereoselective. They all involve the creation of a new, tetrahedral stereogenic centre at a carbon that was planar and trigonal. This leads us to our first new definition. Trigonal car bons that aren’t stereogenic (or chiral) centres but can be made into them are called prochiral.

we introduced stereochemistry by thinking about the reactions of two sorts of carbonyl compounds. They are shown again here: the fi rst has a prochiral carbonyl group. The second, on the other hand, is not prochiral because no stereogenic centre is created when the compound reacts.

A tetrahedral carbon atom can be prochiral too—if it carries two identical groups (and so is not a chiral centre) but replacement of one of them leads to a new chiral centre, then the car bon is prochiral.

Glycine is the only common α amino acid without a chiral centre, but replacing one of the two protons on the central carbon with, say, deuterium creates one: the CH2 carbon is prochiral. Similarly, converting a malonate derivate into its monoester makes a chiral centre where there was none: the central C is prochiral. Now, does this ring any bells? It should remind you very much of the definitions of enantiotopic and diastereotopic in connection with NMR spectra. Replacing one of two enantiotopic groups with another group leads to one of two enantiomers; replacing one of two diastereotopic groups with another group leads to one of two diastereoisomers. Diastereotopic groups are chemically different; enantiotopic groups are chemically identical. Exactly the same things are true for the faces of a prochiral carbonyl group or double bond. If reaction on one of two faces of the prochiral group generates one of two enantiomers, the faces are enantiotopic; if the reaction generates one of two diastereoisomers, the faces are diastereotopic. We will now apply this thinking to the fi rst few reactions in this chapter: they are shown again below. The two examples in the top row have prochiral C=C or C=O bonds with diastereotopic faces: choosing which face of the double bond or carbonyl group to react on amounts to choosing which diastereoisomer to form. In the third example, the faces of the prochiral carbonyl group are enantiotopic: choosing which face to attack amounts to choosing which enantiomer to form. In the fourth example, the two faces of C=O are homotopic: an identical product is formed whichever face is attacked.

Knowing this throws some new light on the last chapter. Almost without exception, every stereoselective reaction there involved a double bond (usually C=C, sometimes C=O) with diastereotopic faces. The diastereotopic faces were distinguished by steric hindrance, or by a nearby hydrogen-bonding group, and so were able to react differently with an incoming reagent.

Just like diastereotopic signals in an NMR spectrum, diastereotopic faces are always different in principle, but sometimes not so in practice.

: this C=O group has two diastereotopic faces, which, due to free rotation about single bonds, average out to about the same reactivity, so we cannot expect any reasonable level of Dia stereoselectivity.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم رعاية الصحن الشريف ينهي استعداداته الخاصة بزيارة الأربعين

قسم الإعلام ينظم دورة تخصصية في تقنيات الذكاء الاصطناعي لعددٍ من منتسبيه

القسم النسوي بمجمع أبي الفضل العباس (عليه السلام) ينهي الاستعدادات لاستقبال زائرات الأربعين

قسم الشؤون الفكرية ينظّم ورشة تدريبية عن الالتزام والانضباط في الخدمة لفتية الكفيل

المزيد

الآخبار الصحية

علامات فارقة بين النوبة أو السكتة القلبية.. ماهي؟

الفلفل الأحمر.. سعرات حرارية منخفضة وفوائد صحية عديدة

8 أعراض إذا شعرت بها.. اذهب للطوارئ فورا !

تعرف على أكثر 8 أماكن اتساخا في المنزل

المزيد

الآخبار التكنلوجية

إنتاج الهيدروجين الأخضر في البحر.. هذه أحدث الدراسات

"ناسا" تخطط لبناء مفاعل نووي على سطح القمر !

ابتكار ومواهب وأموال.. مارك زوكربيرغ يعلن الحرب على أبل

عودة التعاون الفضائي بين روسيا وأميركا.. وبحث ملفات "هامة"

المزيد

مواضيع ذات صلة

Non-linear Hammett plots

A complete picture of the transition state from Hammett plots

Using the Hammett ρ values to uncover mechanisms

Reactions with small Hammett ρ values

Reactions with negative Hammett ρ values

Reactions with positive Hammett ρ values

Equilibria with positive Hammett ρ values

The Hammett substituent constant σσ A

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms