The best alternatives to enamines for conjugate addition of enols of aldehyde, ketone, and carboxylic acid derivatives are silyl enol ethers. These stable neutral nucleophiles react very well with Michael acceptors either spontaneously or with Lewis acid catalysts such as TiCl₄ at low temperature. If the 1,5-dicarbonyl compound is required, then an aqueous work-up with either acid or base cleaves the silicon–oxygen bond in the product.

Addition of the silyl enol ether derived from acetophenone (PhCOMe) to a disubstituted enone promoted by titanium tetrachloride is very rapid and gives the diketone product in good yield even though a quaternary carbon atom is created in the conjugate addition. This is a typical example of this very powerful class of conjugate addition reactions.

It is even possible to use a silyl enol ether to create a new C–C bond that joins two new quaternary centres. Silyl ketene acetals (the silyl enol ethers of esters) are more nucleophilic than ordinary silyl enol ethers, and in this example the silyl ketene acetal undergoes conju gate addition to an unsaturated ketone catalysed by the usual Lewis acid (TiCl₄) for such reactions.

Ketene acetals

 Because enol ethers of esters have two identical OR groups joined to the same end of the same double bond, you will see them called ‘ketene acetals’ or, here, ‘silyl ketene acetals’. This is a reasonable description as you can imagine the carbonyl group of a ketene forming an acetal in the same way as an aldehyde. In fact, they cannot be made this way.

In these reactions, the electrophile coordinates to the TiCl4 Lewis acid first, producing an activated enone that is attacked by the silylated nucleophile. It is difficult to determine at what stage the trimethylsilyl group moves from its original position and whether it is trans ferred intramolecularly to the product. In many cases the anion liberated from the Lewis acid (Cl⁻, RO⁻, Br⁻) is a good nucleophile for silicon so it is reasonable to assume that there is a free trimethylsilyl species (Me3SiX) that captures the titanium enolate:

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

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

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