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Concerted Reactions.
Introduction: scope and some definitions Much of the discussion in this course focuses on reactive intermediates. To the extent that transition states (the key species between the various energy minima) are considered it is largely to point to the extent that these resemble the reactive intermediates, and hence that reactivity patterns may be predicted with considerable confidence by consideration of the factors that affect the stability of the various intermediates. In the discussion which follows in this section we shall examine an important feature of single step reactions. The discussion applies to each step of any multistep process, but is perhaps seen most clearly in transformations involving only stable species (without formation of an intermediate). Such processes have been given a number of names but that which we will use is "concerted reactions". Concerted reactions are those in which the breaking of one bond is accompanied by formation of another (i.e. the breakage and formation processes are "in concert"). The energy diagram for such a process has a single maximum.
Reaction progress
Energy
The particular category of concerted reactions that we are most concerned with here are pericyclic reactions, concerted reactions with a cyclic transition state. Of the categories of pericyclic reactions, we shall discuss in particular, (a) cycloaddition reactions (b) electrocyclic reactions (c) sigmatropic rearrangements
a) Cycloaddition two components come together to form a ring or at least react via a cyclic TS e.g. Diels-Alder
∆
the reverse reaction may be called retro-Diels-Alder, cycloreversion, cycloelimination or retrocycloaddition The Diels-Alder reaction involves 4 electrons on one component and 2 electrons on the other and may be referred to as a [4+2] cycloaddition or even as a [4π+2π] cycloaddition. (b) Electrocyclic reaction The formation of a single bond between the ends of a linear system of n π-electrons, and the reverse process.
≠
also
∆
(I) Some reactions represented by cyclic arrow shifts go easily and some do not. (for reasons not immediately apparent)
"Dewar benzene" ∆H ≈ −60 kcal mol-^1 (-250 kJ mol-^1 ) but reaction is not fast at room
temperature (t (^) ½ ≈ 2 days at 25°C, ∆H‡^ = 23 kcal mol-^1 (96 kJ mol-^1 )
by contrast
R R
R
R
rapid at room temperature (e.g. ∆H‡^ ≈7 kcal mol-^1 (29 kJ mol-^1 ) when R = COOCH 3 ); an example of "valence tautomerism". (ii) Many reactions for which a cycloaddition is readily imagined do not take place by a concerted mechanism.
Cl 2 + CH 2 =CH 2 → CH 2 Cl-CH 2 Cl
Not by H^2 C CH^2
Cl Cl
Chlorination of alkenes does not take place by a cycloaddition Instead it occurs by either an ionic pathway:
Cl (^) Cl H 2 C CH 2
Cl H 2 C CH 2 Cl
or perhaps by a radical pathway: Cl• + CH 2 =CH 2 → Cl-CH 2 -CH 2 • + Cl 2 → ClCH 2 CH 2 Cl + Cl• but not by a concerted cycloaddition (iii) Thermal and photochemical reactions differ readily goes photochemically (not thermally)
readily goes thermally, not photochemicaly (Diels-Alder) Questions: (i) Why the stereospecificity? (ii) Why the differences in ease of reaction? (iii) Why the differences between photochemical and thermal processes? Since about 1965 a number of ways of looking at concerted reactions have been proposed. In somewhat different ways, these look at the symmetries of the reacting molecular
hν ≠
each higher energy has 1 extra nodal plane try n = 3, 5, 6, 7, 8 (nodes & orbitals) ODD ones have nodes where orbitals should be (occasionally!) For cyclic polyenes there is a convenient mnemonic for the pattern of MO energies. A convenient mnemonic for the orbitals (not really a practical calculation) is the circle method. Draw a circle with radius 2β. (β is the “bond integral” from a quantum mechanical treatment of π systems; it is a negative quantity.) For monocyclic hydrocarbons, inscribe in the circle the polygon corresponding to the hydrocarbon, with one corner touching the lowest point in the circle. The molecular orbital energy levels correspond to the heights of the corners of the polygon.
-1^ -2ββ (^12) ββ
To construct a drawing accurately enough to read off the energies is generally more work than to solve the determinant, but it is a good way to remember the qualitative order. The same method can be used for linear polyenes: for a polyene of m carbons, inscribe a polygon with 2m+2 corners, and take the end of the polyene as one up from the bottom. Thus for ethene we use the same diagram as for benzene, but reinterpret it.
-1β 1 β
Cycloaddition reactions, for which the most natural seeming approach is that of: FMO Theory Basic assumption: in a concerted bimolecular process there is a flow of electrons from the highest-energy occupied MO (HOMO) of one molecule to the lowest-energy unoccupied MO (LUMO) of the other. Rule: If the HOMO-LUMO interaction involves overlap of orbitals of the same symmetry (.i.e. net bonding overlap) the reaction is symmetry- allowed and may proceed by a concerted. pathway. If the orbitals are of different symmetry the reaction is symmetry- forbidden, i.e. there is a high activation barrier, making the concerted pathway unfavorable. Some simple cycloadditions provide illustration.
(a) H 2 + D 2 2HD
In FMO terms HOMO (σ) LUMO (σ*)
(There is a similar picture if one considers the HOMO of D 2 and the LUMO of H 2 .) The HOMO and LUMO have different symmetries (symmetric and anti-symmetric) and the net overlap is zero. Reaction is forbidden. According to ab initio calculations the activation energy for the
"forbidden" reaction is 123 kcal mol-^1 (514 kJ mol-^1 ). other mechanisms have lower energies, e.g.
D D
H H
(Diels-Alder)
butadiene ethene 3 nodes 1 node 2 nodes
1 node 0 nodes 0 nodes
LUMO (ψ 2 ') Bonding overlap ∴ allowed. HOMO (ψ 2 )
or alternatively
ψ 4
ψ (^3)
ψ 2
ψ 1
ψ 2 '
ψ 1 '
A
B
HOMO (ψ 1 ') also bonding overlap ∴ allowed LUMO (ψ 3 )
contrast this with the photochemical reaction of two ethylenes:
2 CH 2 =CH 2 hν
HOMO for excited ethylene Positive overlap ∴ allowed
LUMO (for ground state ethylene, as before) (this analysis applies to a singlet excited state: the triplet excited state can not react by a concerted mechanism: it must react by another stepwise mechanism:
(^3) * (^) + isc
FMO theory: for a reaction to be allowed requires:
- The above examples emphasize the need for HOMO and LUMO to have the appropriate symmetry for positive overlap. This a necessary but not sufficient condition for a reaction to be allowed by FMO theory.
- A further point: the energies of the interacting MO's must be reasonably close.
the new bonds on opposite sides of one ethylene. Such a mode is called antarafacial. The reaction may therefore be described as a [π2s + π2a] cycloaddition. Geometrical considerations: Suprafacial vs. Antarafacial addition Suprafacial: for a π-system suprafacial bonding occurs if overlap of two lobes occurs on the same side of a nodal plane. Antarafacial: overlap of two lobes on the opposite side of a nodal plane these terms also apply to σ-bonds and single π-orbitals
Suprafacial Antarafacial Previously showed LUMO zero overlap, forbidden HOMO
Each ethylene has 2π electrons. The suprafacial addition for each molecule can be symbolized: 2 πs
One way of describing the above situation is to say the [2πs 2πs] cycloaddition is forbidden. There is an alternative mode of addition: suprafacial for one ethylene and antarafacial for the other. This actually occurs for some systems and can be imagined for ethylene dimerization.
HOMO
LUMO
or LUMO
HOMO Note: addition is antarafacial to the LUMO ethylene, and the substituents initially cis in the LUMO become trans.
b b
a a
c d
a a c d b b c c d d
a (^) a
c d
b b
d c
(along with the enantiomer)
This can happen if the crowding can be lessened. CH 2 =CH 2 plus CH 2 =CH 2 doesn’t work, but CH 2 =CH 2 plus CH 2 =C=O does.
HOMO
+ve overlap ∴allowed
LUMO
e.g.
N N
COOMe
COOMe (^) N
N
COOMe COOMe
[π14a + π2s] HOMO +ve, overlap ∴ allowed
CN
CN
NC
NC
H H
NC CNCNCN
Orbital symmetry restrictions apply to ionic species as well.
Ph C
CH 2
CH 2
Ph Ph
Ph
Ph
Ph
(π4s + π2s] HOMO
- ve overlap ∴ allowed LUMO (Note: Use HOMO of the anion and LUMO of the alkene. Why?)
Note: There is more than one applicable way of designating a reaction, but this does not affect the application of the rule. E.g. the Diels-Alder reaction can also be analyzed as a (π2s + π2s + π2s] or a [π2s + π2a + π2a] cycloaddition
- or even a (ω0s + ω2s + ω0a + ω2a + ω0s + ω2s] process* (where ω refers to single orbitals)! The rule predicts that each of these processes is allowed.
- Woodward and Hoffmann cite this particular possibility "partly in a sportive spirit" (Angew. Chem. Int. Ed. Engl., 8, 781-853, esp. p. 850).
Pericyclic rule protocol: (1) Draw the orbitals (without + or - signs). (2) Take each "largest unit” (for simplicity) and determine (a) whether the attachment being made to this unit is a or s, (b) if a then whether (4r)a or (4r+2)a, and if s whether (4r+2)s or (4r)s. (3) Sum the number of (4r)a and (4r+2)s sets (ignore the (4r)s and (4r+2)a sets). (4) If the sum is odd the reaction is allowed, if the sum is even the reaction is forbidden. For cycloaddition reactions the FMO picture is particularly convenient; now we turn to electrocyclic reactions where another picture is more convenient.
- Orbital Correlation Diagrams and Electrocyclic Reactions As noted in the examples of p. 96, electrocyclic reactions are stereospecific. In discussing this topic it is helpful to define two forms of ring-closure (or opening):
Conrotatory:
Disrotatory:
Note that in the conrotatory mode
a two-fold axis of symmetry (C 2 -axis) is conserved.
