β-Diesters (malonates and substituted derivatives, ) combine three useful features in conjugate addition reactions:

• they form stable enolate anions that undergo clean conjugate addition

• if required, one of the ester groups can be removed by hydrolysis and decarboxylation

• the remaining acid or ester is ideal for conversion into other functional groups.

Diethyl malonate adds to diethyl fumarate in a conjugate addition reaction promoted by sodium ethoxide in dry ethanol to give a tetraester. Diethyl fumarate is an excellent Michael acceptor because two ester groups withdraw electrons from the alkene. The mechanism involves deprotonation of the malonate, conjugate addition, and reprotonation of the product enolate by ethanol solvent. In this reaction two ester groups stabilize the enolate and two more promote conjugate addition.

The value of malonate esters is illustrated in this synthesis of a substituted cyclic anhydride by conjugate addition to ethyl crotonate, hydrolysis, and decarboxylation, followed by dehydration with acetic anhydride. This route is very general and could be used to make a range of anhydrides with different substituents simply by choosing an appropriate unsaturated ester.

If the nucleophile is sufficiently enolized under the reaction conditions then the enol itself is able to attack the unsaturated carbonyl compound. Enols are neutral and thus soft nucleophiles favouring conjugate attack. 1,3-Diketones are enolized to a significant extent, and under acidic conditions conjugate addition proceeds very efficiently even though there can be absolutely no base present. In this example methyl vinyl ketone (butenone) reacts with a cyclic β-diketone promoted by acetic acid to form a quaternary centre.

The mechanism involves acid-catalysed conversion of the keto form of the cyclic β-diketone into the enol form, which is able to attack the protonated enone. The mechanistic detail is precisely analogous to the attack of an enolate; the only difference is that both reactants are protonated. The product is the enol form of the triketone, which rapidly tautomerizes to the more stable keto form.

The thermodynamic control of conjugate addition allows even enals that are very electro philic at the carbonyl carbon to participate successfully. As you will see in the next chapter, an aldol reaction (direct addition to C=O) must be possible here, but it is reversible and 1,4-addition eventually wins out. Acrolein combines with this five-membered diketone under very mild conditions to give a quantitative yield of product.

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اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تفتتح تكية طرف باب السلالمة بعد إعادة بنائها

شعبة مدارس الكفيل الدينية النسوية تعقد جلسة تثقيفية ضمن فعاليات مخيم بنات العقيدة

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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 ؟

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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

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

Using Michael acceptors as electrophiles

Conjugate addition to enones gives enolates regiospecifically

Enones provide a solution to regioselectivity problems