The Z selectivity observed with simple alkyl R groups is nicely complementary to the E selectivity observed in the Julia olefi nation. This complementarity was exploited by some chemists who wanted to make isomers of capsaicin (the compound that gives chilli peppers their ‘hotness’) to follow up suggestions that capsaicin might be carcinogenic. The key inter mediates in the synthesis of the E and Z isomers of capsaicin were the E and Z unsaturated esters shown below. By using a Wittig reaction with an unstabilized ylid it was possible to make the Z isomer selectively, whilst the Julia olefi nation gave the E isomer.

How can the Z selectivity in Wittig reactions of unstabilized ylids be explained? We have a more complex situation in this reaction than we had for the other eliminations we considered because we have two separate processes to consider: formation of the oxaphosphetane and decomposition of the oxaphosphetane to the alkene. The elimination step is the easier one to explain—it is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state. Addition of the ylid to the aldehyde can, in principle, produce two diastereo isomers of the intermediate oxaphosphetane. Provided that this step is irreversible, then the stereospecificity of the elimination step means that the ratio of the final alkene geometrical isomers will reflect the stereoselectivity of this addition step. When R is not conjugating or anion-stabilizing, the syn diastereoisomer of the oxaphosphetane is formed preferentially, and the predominantly Z alkene that results reflects this. The Z-selective Wittig reaction therefore consists of a stereoselective fi rst step, to form the syn oxaphosphetane, followed by a stereospecific elimination from this intermediate to give a Z alkene.

Why the syn oxaphosphetane is favoured with unstabilized ylids is the subject of much debate because the mechanism by which the oxaphosphetane is formed is not entirely under stood. One possible explanation relies on rules of orbital symmetry, which you will meet, if the ylid and carbonyl compound react together to give the oxaphosphetane in one step, they will do so by approaching one another at right angles. Here we have drawn the ylid in its phosphorane form, adding to benzaldehyde. Keeping the large substituents apart produces a transition state like that shown below, which (correctly) predicts that the oxaphosphetane will have syn stereochemistry.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم العلاقات العامة يختتم فعاليات محطات فتية الكفيل الثقافية في محافظة نينوى

ما الخدمات التي قدّمها موكب أشبال الطفّ لزائري عاشوراء؟

العتبة العباسية المقدسة تنظّم مسيرات عزائية لإحياء ذكرى عاشوراء في نينوى

قسم الشؤون الفكرية يحيي ذكرى استشهاد الإمام الحسين (عليه السلام) في القارة الإفريقية

المزيد

الآخبار الصحية

تناول الجبن.. مفيد للجسم أم خطر يهدد صحتك؟

طبيبة أسنان تحذر.. لا تتجاهل هذه العلامة بعد التنظيف

يجب أن تأكل المزيد من الشمام هذا الصيف.. إليك السبب ؟

تأثير صحي كبير.. كم بيضة يجب أن تتناول يوميا؟

المزيد

الآخبار التكنلوجية

أداء خلايا البيروفسكايت الشمسية تحت الماء قد يزيد كفاءة تحويل الكهرباء

مسافة الأمان: أساس القيادة الآمنة وتجنب الحوادث

هل القيادة السريعة تعني استهلاك اكثر للوقود؟

لماذا يعتبر كوكب المريخ غير صالح للحياة؟

المزيد

مواضيع ذات صلة

E-selective reduction of alkynes using sodium in liquid ammonia

Amine synthesis using functional group interconversions

Functional group interconversion

Multiple step syntheses: avoid chemoselectivity problems

Synthons are idealized reagents

The E-selective Wittig reaction

Stereoselectivity in the Wittig reaction depends on the ylid

The Peterson reaction is stereospecific

The one-step Julia olefination

The Julia olefi nation is stereoselective

The Julia olefination is regiospecific and connective

Sulfoxide elimination—oxidation to enones