In this chapter so far, you have seen that highly stabilized anions, such as those derived from β-dicarbonyl compounds, are particularly good at nucleophilic addition because their stability helps to reverse the unwanted alternative direct C=O addition (aldol) pathway, and facilitates proton transfer in the catalytic version of the reaction. The nitro group is so powerfully electron-withdrawing that just one is equivalent to two carbonyls in pKa terms. Thus if β-dicarbonyls are good for conjugate addition, you might expect nitroalkanes to undergo conjugate addition in just the same way. The good news is that they do, very well. The first stage is a base-catalysed conjugate addition.

The product enolate that is formed is much more basic than the anion of the nitro com pound so it removes a proton from the nitro compound and provides another molecule of anion for the second round of the reaction.

The acidifying effect of the nitro group is so profound that very mild bases can be used to catalyse the reaction. This enables selective removal of the proton next to the nitro group and helps to avoid side reactions of the carbonyl component. Common examples of mild bases include amines, quaternary ammonium hydroxides, and fluorides. Even basic alumina (a largely inert powder) is sufficient to catalyse virtually quantitative addition of this benzylic nitroalkane to cyclohexenone at room temperature!

Anions of nitro compounds form quaternary centres with ease in additions to α,β unsaturated mono- and diesters. The difference between the acidity of the protons next to a nitro group and those next to the esters in the products combined with the very mild basic conditions ensures that no unwanted side reactions occur.

The effectiveness of nitro compound conjugate addition makes it ideal for use in combination with other reactions in making several bonds in one pot. The next example combines conjugate addition and intramolecular conjugate addition to make a six-membered ring.

The base used for both steps is Cs₂CO₃. The large caesium cation forms fully ionic compounds so the uncomplexed carbonate ion can exert its full basicity. Deprotonation of the conjugate addition product next to the nitro group produces a second anion, which does an intra molecular SN₂ displacement of iodide to form a six-membered ring.

The nitro group can be converted into other useful functional groups following conjugate addition. Reduction gives primary amines while hydrolysis reveals ketones. The hydrolysis is known as the Nef reaction and used to be achieved by formation of the nitro-stabilized anion with a base such as sodium hydroxide followed by hydrolysis with sulfuric acid. These conditions are rather unforgiving for many substrates (and products) so milder methods have been developed. One of these involves reaction of the nitro ‘enolate’ with ozone (ozonolysis) at low temperature rather than treatment with acid. Base-catalysed conjugate addition of nitro propane to methyl vinyl ketone occurred smoothly to give the nitroketone. Formation of the salt with sodium methoxide was followed by oxidative cleavage of the C=N linkage with ozone. The product was a 1,4-diketone, which was isolated without further aldol reaction by this route.

This is a good general method for the synthesis of 1,4-diketones, which can otherwise be difficult to make, and additional substituents are easily accommodated on the enone—a characteristic of conjugate addition.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم الشؤون الفكرية ينظم مجلسًا عزائيًّا في دولة بوروندي الإفريقية

العتبة العباسية المقدسة تواصل إقامة مجالسها العزائية لإحياء شهر المحرَّم

سماحة السيد الصافي يدعو إلى التدبر في القرآن والاستفادة منه في مواجهة تحديات الحياة

ضمن برنامج التطوير الحسيني التاسع.. جمعية كشافة الكفيل تقيم مهرجان الكشاف الحسيني

المزيد

الآخبار الصحية

دراسة تكشف علاقة خطيرة بين عمر الأم وصحة مولودها

دراسة تحذر.. اضطراب نوم بسيط قد يسبب الموت المبكر ؟

الاستحمام بالماء الساخن.. راحة مؤقتة وأضرار دائمة للبشرة

ماذا يحدث للجسم عند تخطي وجبة الإفطار؟

المزيد

الآخبار التكنلوجية

كيف تساعد الصيانة الوقائية في الحفاظ على كفاءة التكييف في سيارتك ؟

في ظاهرة غريبة حيرت العلماء.. كوكبنا قد يسجل أقصر يوم في التاريخ هذا الصيف!

السد الصيني العملاق يعيق دوران الأرض ويغيّر شكلها ؟

السعودية تطلق 10 تجارب علمية إلى محطة الفضاء الدولية

المزيد

مواضيع ذات صلة

The Claisen ester condensation and other self-condensations

Problems with acylation at carbon

Cross-condensations

The aldol reaction

A variety of electrophilic alkenes will accept enol(ate) nucleophiles

Conjugate addition of silyl enol ethers leads to the silyl enol ether of the product

Enamines are convenient stable enol equivalents for conjugate addition

Alkali metal (especially Na, K) enolates can undergo conjugate addition

1,3-Dicarbonyl compounds undergo conjugate addition

Using Michael acceptors as electrophiles

Conjugate addition to enones gives enolates regiospecifically

Enones provide a solution to regioselectivity problems