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PAPER No. 05: TITLE: ORGANIC CHEMISTRY-‐II MODULE No. 20: TITLE: Norbornyl system, Common Carbocation Rearrangements

Subject Chemistry

Paper No and Title 05, ORGANIC CHEMISTRY-II

Module No and Title 20, Common Carbocation Rearrangements and non classical Norbornyl cation

Module Tag CHE_P5_M20

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TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Carbocation Rearrangement 3.1 1,2-Hydride Shift 3.2 1,2-Alkyl Shift 4.Norbornyl System 5. Norbornyl Cation 6. Examples 7. Summary

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1. Learning Outcomes

After studying this module, you shall be able to

• Know about carbocation rearrangements • Rearrangements involving 1,2- hydride shift and 1,2 alkyl shift • Know about Norbornyl system • Learn about Norbornyl cation • Identify how Norbornyl cation works and rearrangement happen • Understand the concept of nonclassical carbocation.

2. Introduction

The chemistry of the carbocation or carbonium ion has been extensively studied and many outstanding organic chemists have largely enriched and developed this field. The term carbenium ion is more appropriate as suggested by G. A. Olah. This module is all about carbocation intermediates and their rearrangements. 3. Carbocation Rearrangements In a rearrangement a group moves from one atom to another in the same molecule. Most are migrations between adjacent atoms and are called 1,2-shifts. Carbocation rearrangements occur more frequently on secondary carbocations to form tertiary which are more stable and energetically more favourable. Simple alkyl primary carbocations are too high in energy to form so you don’t tend to see a primary carbocation. There are some exceptions to this general rule for primary carbocations. Certain structural conditions such as being benzylic or allylic, due to resonance, allows for the formation of primary carbocations. Tertiary carbocations do not need to rearrange as they are already the most stable. In general, the bonding electrons of a carbocation may shift between adjacent atoms to form a more stable carbocation. If a secondary carbocation is vicinal to a tertiary carbon bearing a hydrogen, a 1,2- hydride shift should occur. If a secondary carbocation is vicinal to a quaternary carbon, a 1,2-alkyl shift should occur. The general rule in alkyl shifts is: the smaller alkyl substituent tends to be the substituent that shifts.

2o carbocation vicinal to a 3o carbon 2o carbocation vicinal to a quaternary carbon

There are two types of carbocation rearrangements namely, 1,2-Hydride shift and 1,2-Alkyl shift as you can see from these reactions:

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The following “rules” hold for carbocation rearrangements:

• Carbocation rearrangements are equilibrium processes. • Usually lead to more stable carbocations. • Sometimes lead to carbocations of equal stability (not so common). • Sometimes lead to less stable carbocations (very unusual but does happen). • Hydride shift is more common, favorable, than alkyl shift. • The least bulky alkyl substituent shifts (usually CH3). • Only groups adjacent to C+ can migrate. • Only carbon groups and H atoms can shift (1, 2-OH shift is forbidden).

3.1 1, 2-Hydride Shift If a carbocation is vicinal to a tertiary carbon bearing a H atom, a 1,2-Hydride shift should occur. Hydride shift leads to a 3o carbocation which is more stable than a 2o.

The hydride shift would occur more readily than the alkyl shift. For nucleophilic substitution, the pattern of bonds that form and break is pretty straightforward. You break C-(leaving group) and you form C-(nucleophile), a straight swap. But you might see a “weird” substitution reaction. If you look closely at the pattern of bonds formed and bonds broken in the second reaction below, there’s an extra set. A normal Substitution Reaction

Substitution with Rearrangement

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In other words it’s a substitution reaction where the hydrogen has moved or rearranged. As it turns out, reactions that go through carbocations can sometimes undergo rearrangements. And looking back at substitution reactions, recall that the SN1 reaction goes through a carbocation intermediate. Carbocations contains six electrons bearing a positive charge. In other words, they are electron deficient (–2 electrons short of a full octet). So, it would make sense that carbocations become more stable as you increase the number of electron donating groups attached to them. Alkyl groups are a perfect example. That’s why carbocation stability increases as you go from primary to secondary to tertiary. Carbocation with the same substitution pattern can rearrange if it result in a resonance stabilized carbocation.

Resonance stabilized carbocation

Carbocations are also stabilized by resonance, which allows the positive charge to be delocalized or “spread out” over a greater area on the molecule. One rearrangement pathway where an unstable carbocation can be transformed into a more stable carbocation is called a hydride shift.

Hydride shift pathway

In this rearrangement reaction, the pair of electrons in the C-H bond is transferred to the empty p orbital on the carbocation. In the transition state (or called non classical carbocation) of this reaction, there’s a partial C-H bond on C3 and a partial C-H bond on C2. The transition state here is kind of like that split second in a relay race where one sprinter is passing the baton to another sprinter and they both have their hands on it. Then, as the C2-H bond shortens and the C3-H bond weakens, we end up with a carbocation on C3 (3o carbocation) in the product which is more stable. Here are some examples of “allowed” rearrangement reactions. Notice how we are always going from a less substituted carbocation to a more substituted carbocation. One exception is at the very bottom; the rearrangement is favorable because the new carbocation has increased stabilization. Some example of “allowed” H rearrangements to make more stable carbocation:

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Example:

SN1 reaction- Hydride shift

3.2 1, 2-Alkyl Shifts If a carbocation is vicinal to a 3o carbon, a 1,2-alkyl shift should occur. You might note something with this example, however: There is only possibility to form more stable 3o carbocation if an alkyl group migrates. The most common situation where alkyl shifts can occur is when a quaternary carbon is adjacent to a 2o carbocation.

The pair of electrons from the C-C bond can be donated into the empty p-orbital on the carbocation (this means they have to be aligned in the same plane). In the transition state, there

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are partial bonds between the carbon being transferred and each of the two adjacent C-atoms. Then, as one bond shortens and the other lengthens, we end up with a (more stable) 3o carbocation.

Alkyl shift pathway

Once a carbocation is formed , rearrangements can potentially occur at any time. That includes SN1 reactions. Here’s an example of an SN1 with an alkyl shift.

SN1 reaction - Alkyl shift

It doesn’t always have to be a methyl group that moves. One interesting example is when a carbocation is formed adjacent to a strained ring, such as a cyclobutane. Even though the CH3 could potentially migrate in this case, it’s favorable to shift one of the alkyl groups in the ring, which leads to ring expansion and the formation of a less strained, 5-membered ring.

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SN1 reaction - alkyl shift leads to ring expansion

4. Norbornyl System

Norbornane is an organic compound and its chemical formula C7H12. Its IUPAC name is [2.2.1] heptane. It is a 1,4 bridged bicyclohydrocarbon . The norbornane molecule possesses a rigid structure with uncommon steric characteristics. Carbon atoms 1-6 constitute a cyclohexane structure in the higher-energy boat conformation. Moreover, the 7-methylene group not only locks this ring system into a rigid boat conformation, but the constraint so produced accentuates the steric crowding within the boat structure. Much of the chemistry of norbornane is dominated by the far greater steric accessibility of the exo as compared to the endo position. For example, in many free radical substitution reactions the exo product is produced preferentially.

Norbornane 5. Norbornyl Cation Many naturally occurring terpenes are derivatives of Norbornane. Molecular rearrangements are common among these bicyclic terpenes. Winstein and roberts (1950) actively pursued the mechanism of the rearrangemts and the associated stereochemical problems using the basic norbornyl system.

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Acetolysis of 2-norbornyl brosylates. Each of the isomeric brosylates (1 and 2) is prepared and subjected to acetolysis. (solvolysis using acetic acid as solvent and nucleophile)

Observations regarding these reactions:

(i) Rate of solvolysis of 1 to 2 is about 400 (ii) Both 1 and 2 give 3, the exo-acetate (iii) Optically active exo-brosylate (1) gives 3 which is racemic (±) (iv) The recovered brosylate (1) is also racemic (v) The rate of racemisation (measured from specific Rotation) is greater than

solvolysis in the case of the exo-isomer (vi) The rate of racemization is equal to the rate of solvolysis in the case of endo-

isomer. Following are for comparison:

• Solvolysis rate of endo-brosylate is nearly equal to cyclohexyl brosylate • Relative rate of 1 to cyclopentyl brosylate is 13.

Interpretation and notes A satisfactory explanation for these observations is needed and also identify the nature of the intermediate norbornyl carbocation and its reactivity. Beginning 1950 and till 1980, the topic became one of the very important area in theoretical organic chemistry. A new concept has been evolved, namely non-classical carbocation, non-classical resonance, the sigma (σ) bridged norbornyl cation. The researches have raised healthy debates and also great controversies.

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Acetolysis of 2-exo and 2-endo-norbornyl brosylates

Acetolysis of 2-exo and 2-endo-norbornyl brosylates follow the following changes: • A → B → C → D is acetolysis with retention of configuration. The norbornyl cation C is attached by actetate ion from the exo face (preferred) • B→E is ion pair rearranging to another (C1 – C6 bond migration to C2) • A → B →E →G (H) is acetolysis of A with rearrangement. D and I are (+),(-) forms of the acetate (mirror images) • J → B is a slow ionization of endo–brosylate (the C1- H and C1 – C6 bonds hinder ionisation) No such hindrance exist with A. B→A is ion pair return and E→F is the rearranged ion pair (E) return. A and F are enantiomers and hence racemic. There are two proposals to represent the intermediate carbocation (1) Two rapidly equilibrating carbocations (C) and(G). (2) Sigma bridged non –classical norbornyl cation

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The carbocation (b) is unconventional and unusual therefore called non – classical as against (a) which is localized carbocation. The cation (b) is also spoken as split – sigma bond (C1 – C6), bridged cation. Some call these delocalized structure as non – classical resonance. We will not enter into any discussion at this stage but sum-up the main points. Summing up – There appears to be small anchimeric assistance to solvolyis of A – from C1 – C6 bond at the T.S. This bond is just located at the back of the 2 – exo position. The intermediate derived from this Transition State is stabilized in comparison with localized carbonium ion by about 3kcal / mole (calculated from rates of acetolysis of A and J) this energy difference is too small to think of major reorganization of bonds. The non – classical norbornyl carbocation has a plane of symmetry (passing through C4, C5 and C6 and the midpoint of C1 - C2 and therefore achiral. The achiral carbocation is expected to give a racemic acetate (product). 6. Some Examples of carbocation rearrangements. 1. Examples of SN1 reactions with rearrangements.

1.

2.

3. 2. Hydride shift followed by alkyl shift

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3. Carbocations are intermediates in SN1 reaction and quite often yield rearranged products. The intermediate 2o carbocation formed rearranges to a 3o carbocation. A 1,2 hydride shift occurs which is more stable. The carbocation reacts with CH3OH and an SN1 reaction occurs.

4. Wagner –Meerwin Rearrangement a)

b)

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5.An interesting example involving rearrangement is as given below:

Notice here the rearrangement is taking place towards the formation of more stable cyclic systems

7. Summary

• Carbocation rearrangement is an important tool for organic reaction mechanism • The more substituents on the carbocation, the more stable the carbocation, and the faster it

can form (Hammond Postulate). • A reaction will take the path of the more stable carbocation intermediate go to the

regioselective product. • If a carbocation can become more stable by rearrangement, rearrangement occurs involving

hydride shifts, methyl shifts, and ring expansions. • Carbocation rearrangement is an important tool for organic reaction mechanism. • Many of the features of 2-norbornyl carbocation can be explained on the basis of two

equilibrating rearranging classical carbocations or sigma bridged non- classical carbocation.

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