The simplest and best Michael acceptors are those α,β-unsaturated carbonyl compounds with exposed unsaturated β carbon atoms, such as exo-methylene ketones, lactones, and vinyl ketones. However, their extreme reactivity can make than hard to handle (they polymerize readily), you will meet a method that makes them in situ to circumvent these problems.

One trick to persuade a stubborn enolate to do conjugate rather than direct substitution is to add an extra anion-stabilizing substituent in the α position. The margin shows a selection of reagents that do this. In each case the extra group (CO₂Et, SPh, SOPh, SO₂Ph, SiMe₃, and Br) can be removed after the conjugate addition is complete. Unsaturated esters are good Michael acceptors because they are not very electrophilic. Unsaturated amides are even less electrophilic and (provided they are tertiary and have no acidic NH protons) will even give conjugate addition products with lithium enolates.

The nitrile group is not as reactive towards direct attack by nucleophiles as its carbonyl cousins but is equally able to stabilize an adjacent negative charge. Alkenes conjugated with nitriles are thus activated towards nucleophilic attack without the complications of competing direct addition to the activating group.

With base, methyl benzyl ketone forms its more stable enolate, which undergoes smooth and rapid conjugate addition to acrylonitrile. Acrylonitrile is one of the best Michael acceptors for enolates.

The cyanide group can also act as an anion-stabilizing group in the nucleophile. In combi nation with an ester group, the enolizable proton is acidified to such an extent that potassium hydroxide can be used as base.

The simplest amino acid, glycine, would be an ideal starting material for the synthesis of more complicated amino acids but it does not easily form enols or enolates. By conversion to the methyl ester of its benzaldehyde imine, two electron-withdrawing groups are introduced to help stabilization of the enolate and conjugate addition of acrylonitrile is now possible. The base used was solid potassium carbonate. Simple hydrolysis of the alkylated product leads to the extended amino acid.

You saw on p. 606 how two ester groups in fumarate diesters encourage conjugate addition, but what if there are two different groups at the ends of the Michael acceptor? Then you must make a judgement as to which is more electron-withdrawing. One case is clear-cut. The nitro group is worth two carbonyl groups so that conjugate addition occurs β to the nitro group in this case.

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

Nitroalkanes are superb nucleophiles for conjugate addition

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