Electrophilic addition may be preferred to substitution with furan

 So far, thiophenes and furans look much the same as pyrrole but there are other reactions in which they behave quite differently and we shall now concentrate on those. Furan is less aro matic than pyrrole, and if there is the prospect of forming stable bonds such as C–O single bonds by addition, this may be preferred to substitution. A famous example is the reaction of furan with bromine in methanol. In non-hydroxylic solvents, polybromination occurs as expected, but in MeOH no bromine is added at all!

Bromination must start in the usual way, but a molecule of methanol captures the fi rst formed cation in a 1,4-addition to furan.

The bromine atom that was originally added is now pushed out by the furan oxygen atom to make a relatively stable conjugated oxonium ion, which adds a second molecule of methanol.

This product conceals an interesting molecule. At each side of the ring we have an acetal, and if we were to hydrolyse the acetals, we would have ‘maleic dialdehyde’ (cis-butenedial)—a molecule that is too unstable to be isolated. The furan derivative may be used in its place.

The same 1,4-dialdehyde can be made by oxidizing furan with the mild oxidizing agent dimethyldioxirane, which you met on p. 432. In this sequence, it is trapped in a Wittig reaction to give an E,Z-diene, which is easily isomerized to E,E.

We can extend this idea of furan being the origin of 1,4-dicarbonyl compounds if we consider that furan is, in fact, an enol ether on both sides of the ring. If these enol ethers were hydrolysed we would get a 1,4-diketone.

This time the arrow is solid, not dotted, because this reaction really happens. You will dis cover in the next chapter that furans can also be made from 1,4-diketones so this whole process is reversible. The example we are choosing has other features worth noting. The cheapest starting material containing a furan is furan-2-aldehyde or ‘furfural’, a by-product of breakfast cereal manufacture. Here it reacts in a typical Wittig process with a stabilized ylid.

Now comes the interesting step: treatment of this furan with acidic methanol gives a white crystalline compound having two 1,4-dicarbonyl relationships. You might like to try and draw a mechanism for this reaction.

The thiophene ring can also be opened up, but in a very different way. Reductive removal of the sulfur atom with Raney nickel reduces not only the C–S bonds but also the double bonds in the ring and the four carbons in the ring form a saturated alkyl chain. If the reduction follows two Friedel–Crafts reactions on thiophene the product is a 1,6-diketone instead of the 1,4-diketones from furan. Thiophene is well behaved in Friedel–Crafts acylations, and reaction occurs at the 2- and 5-positions unless these are blocked.

اخر الاخبار

اخبار العتبة العباسية المقدسة

أهالي كربلاء يحيون اليوم السابع لذكرى شهادة الإمام الحسين (عليه السلام)

مشروع أمير القراء الوطني يشهد مشاركة أكثر من 100 طالبٍ في مرحلته الثالثة

قسم الشؤون الفكرية يصدر دليلًا يوثّق القصص الفائزة في مسابقة فتوى الدفاع المقدسة

قسم الشؤون الفكرية يعلن عن توفير عددٍ من المكتبات الإلكترونية المتخصّصة للمؤسّسات الدينية والمكتبات العامّة

المزيد

الآخبار الصحية

نصائح عملية للحد من التعرق المفرط في الصيف

أطباء يحذرون: مكمل غذائي شائع قد يدمر الجهاز العصبي

تحذير طبي من تأثير جانبي "غير متوقع" لحقن التخسيس

حمامات الثلج قد تكون خطيرة وأحيانا مميتة.. ما السبب؟

المزيد

الآخبار التكنلوجية

أكبر جدارية شمسية في العالم تدخل موسوعة غينيس.. مشروع ضخم

تسارع دوران الأرض يثير القلق.. وتحذيرات من "كوارث محتملة" ؟

"سوذبيز" تطرح أكبر قطعة من المريخ على الأرض في مزاد علني

أوبن أيه آي تعتزم إطلاق متصفح إنترنت يعمل بالذكاء الاصطناعي

المزيد

مواضيع ذات صلة

There are thousands more heterocycles out there

Fusing rings to pyridines: quinolines and isoquinolines

Putting more nitrogen atoms in a six-membered ring

Benzo-fused heterocycles-Indoles are benzo-fused pyrroles

Five-membered rings with two or more nitrogen atoms Imidazole

Nitrogen anions can be easily made from pyrrole

Five-membered heterocycles act as dienes in Diels–Alder reactions

Furan and thiophene are oxygen and sulfur analogues of pyrrole

Five-membered aromatic heterocycles are good at electrophilic substitution

Pyridine as a catalyst and reagent

Pyridine N-oxides are reactive towards both electrophilic and nucleophilic substitution

Pyridones are good substrates for nucleophilic substitution