Related effects in other types of compounds

The key requirement for the anomeric effect is that there is a heteroatom with a lone pair (O, N, S usually) adjacent to (that is, in a position to interact with) a low-lying antibonding orbital—usually a C–X σ* (where X=halogen or O). The C–X bond doesn’t have to be within the ring—for example, the nitrogen heterocycle on the left prefers to have the R group axial so that the nitrogen gets an equatorial lone pair. Equatorial lone pairs are parallel with bonds within the ring, one of which is C–O, and this conformation is therefore stabilized by an N lone pair/C–O σ* interaction.

It would be a bit much for the 1,3,5-triazine on the right to have all three t-butyl groups axial (too much steric hindrance), but it can get away with having one of them axial, benefitting from the resulting equatorial lone pair, which can overlap with two C–N σ*s in the ring. It’s not only in six-membered rings that stereoelectronic interactions between fi lled and unfilled orbitals stabilize some conformations more than others. Stereoelectronic effects control the conformations of many types of molecules.

●Any conformation in which a lone pair is anti-periplanar to a low-energy antibonding orbital will be stabilized by a stereoelectronic interaction.

We shall look at three common compounds that are stabilized by stereoelectronic effects: in two cases, the stabilization is specifi c to one conformation, and we can use stereoelectronics to explain what would otherwise be an unexpected result. We start with a compound that is so simple that it has only one conformation because it has no rotatable bonds: dichloromethane. You may have wondered why it is that, while methyl chloride (chloromethane) is a reactive electrophile that takes part readily in substitu tion reactions, dichloromethane is so unreactive that it can be used as a solvent in which substitution reactions of other alkyl halides take place. You may think that this is a steric effect: indeed, Cl is bigger than H. But CH2Cl2 is much less reactive as an electrophile than ethyl chloride or propyl chloride: there must be more to its unreactivity. And there is: dichloro methane benefi ts from a sort of ‘permanent anomeric effect’. One lone pair of each chlorine is always anti-periplanar to the other C–Cl bond so that there is always stabilization from this effect. Among the most widespread classes of acyclic compounds to exhibit stereoelectronic control over conformation are acetals. Take the simple acetal of formaldehyde and methanol, for example: what is its conformation? An obvious suggestion is to draw it fully extended so that every group is fully antiperiplanar to every other—this would be the lowest energy conformation of pentane, which you get if you just replace the Os with CH₂s. The trouble is, in this conformation none of the oxygen lone pairs get the chance to donate into the C–O σ* orbitals. Although putting the bonds anti-periplanar to one another makes steric sense, electronically, the molecule much prefers to put the lone pairs anti-periplanar to the C–O bonds, so the bonds themselves end up gauche (synclinal) to one another. This is known as the gauche effect, but is really just another way in which the stereoelectronic effects that give rise to the anomeric effect turn up in acyclic systems. Finally, an even more familiar example that you may never have thought about. You are well aware now that amides are planar, with partially double C–N bonds, and that tertiary amides have one alkyl group cis to oxygen and one trans. But what about esters? Esters are less reactive than acyl chlorides because of donation from the oxygen p orbital into the carbonyl π*, so we expect them to be planar too, and they are. But there are two possible planar conformations for an ester: one with R cis to oxygen and one with R trans. Which is preferred?

Here are the two conformations drawn out for ethyl acetate. When the ethyl group (= R) and O are cis, not only can one oxygen lone pair interact with the C=O π*, but the other lone pair can also donate into the σ* of the C=O bond. This is not possible when Et and O are trans: they are no longer anti-periplanar. The cis conformation of esters is generally the preferred one, even in formate esters, where the alkyl group ends up in what is clearly a more sterically hindered orientation.

Cyclic esters—lactones—cannot lie cis because of the ring, and this is one of the reasons why lactones are distinctly more reactive than esters and, in many reactions, behave more like ketones: lactones are quite easy to reduce with NaBH₄, for example.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم الشؤون الفكرية ينظم ندوة عن الأبعاد الفكرية والعقدية لثورة الإمام الحسين (عليه السلام)

المجمع العلمي يواصل برامجه ودوراته القرآنية في عدد من المحافظات العراقية

لليوم الرابع.. قسم العلاقات العامة يواصل إقامة فعاليات مخيّم البذرة الواعدة

العتبة العباسية المقدسة تطلق التشغيل التجريبي لمعمل غسيل السجاد والأغطية

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

Baldwin’s rules and ring opening

Combatting ΔS‡—the Thorpe–Ingold effect

Thermodynamic control over ring size

Making heterocycles: ring-closing reactions

Some substituents of saturated heterocycles prefer to be axial: the anomeric effect

Determining stereochemistry when coupling constants are no help: the nuclear Overhauser effect

Oxygen heterocycles

Aziridine: ring strain promotes ring opening

Reactions of saturated heterocycles

More heteroatoms in fused rings mean more choice in synthesis

Quinolones also come from anilines by cyclization to an ortho position

Quinolines and isoquinolines