Stabilized ylids, that is ylids whose anion is stabilized by further conjugation, usually with a carbonyl group, give E alkenes on reaction with aldehydes.

These stabilized ylids really are stable—this one, for example, can be recrystallized from water and the ylid is more stable than the phosphonium salt from which it might be made. This stability means though that they are not very reactive, and often it is better not to use the phosphonium salt but a phosphonate instead.

Phosphonate esters can be deprotonated with sodium hydride or alkoxide anions to give enolate-type anions that react well with aldehydes or ketones to give E alkenes. Alkene forming reactions with phosphonates are called Horner–Wadsworth–Emmons (or Horner Emmons, Wadsworth– Emmons, or even Horner–Wittig) reactions. This example is a reaction that was used by some Japanese chemists in the synthesis of polyzonimine, a natural insect repellent produced by millipedes.

The synthesis below offers a nice illustration of the contrasting selectivity of the two classes of Wittig reagent. The female silkworm moth attracts mates by producing a pheromone known as bombykol. Bombykol is an E,Z-diene, and in this synthesis two successive Wittig reactions use fi rst a stabilized and second an unstabilized ylid to control the stereochemistry of the product.

So why is there a change to E stereoselectivity when the ylid is stabilized? Again, the details are still unclear, and there are several possible explanations. Here we give one which is gaining ground, supported by recent experimental and computational evidence. It seems that, as with unstabilized ylids, the stereochemistry of the alkene product is determined by the stereo chemistry of the intermediate oxaphosphetane, which with stabilized ylids must be anti.

It used to be thought that the formation of the anti oxaphosphetane was under thermo dynamic control, but it now seems likely that it too is formed stereoselectively under kinetic control. The difference from the alkyl-substituted unstabilized ylids lies in the repulsion experienced between the polarized C=O bond of the aldehyde and the electronegative stabilizing group shown here as an ester CO₂R. As the four-membered ring flattens out, the CO₂R and Ph groups end up on opposite sides of the four-membered ring.

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 Z-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