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Supplement A3: The Chemistry of Double-Bonded Functional Groups. Edited by Saul Patai Copyright 1997 John Wiley & Sons, Ltd.

ISBN: 0-471-95956-1

CHAPTER 8

Complex formation involving compounds with double-bonded functional groups

LUCIANO FORLANI

 

Dipartimento di Chimica Organica ‘A. Mangini’, viale Risorgimento 4,

 

40136-Bologna, Italy

 

Fax: 051-64-43-654; e-mail: forlani@ms.fci.unibo.it

 

I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

367

II. OLEFINS. NON-COVALENT INTERACTIONS . . . . . . . . . . . . . . . . .

369

A. Olefins: Electron Donor Acceptor Complexes . . . . . . . . . . . . . . . . .

369

B. Complexes Between Olefins and Ozone . . . . . . . . . . . . . . . . . . . . .

373

C. Complexes Between Olefins and SO2 or Ketenes . . . . . . . . . . . . . . .

375

D. Complexes Between Olefins and Carbenes . . . . . . . . . . . . . . . . . . .

376

E. Complexes Between Olefins and Halogens: Introduction . . . . . . . . . .

377

F. Complexes Between Olefins and Cl2, Br2, I2 . . . . . . . . . . . . . . . . . .

378

G. Interactions of Fluorine with CDC Double Bond . . . . . . . . . . . . . . .

386

H. Oxymercuration of Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

387

I. Electrophilic Addition to Olefins: A Tentative Parallel

 

with Other Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

388

J. Hydrogen Bonding Involving the System of CDC Double Bond . . .

394

III. OLEFINS. COVALENT COMPLEXES WITH NUCLEOPHILES . . . . . .

395

IV. CARBONYL GROUP. ELECTRON DONOR ACCEPTOR

 

COMPLEXES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

399

V. IMINO GROUP. ELECTRON DONOR ACCEPTOR COMPLEXES . . .

403

VI. COVALENT COMPLEXES INVOLVING CDO AND CDN GROUPS

 

AND NUCLEOPHILES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

409

VII. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

416

I. INTRODUCTION

Weak interactions in non-covalent complexes are not only an interesting field for speculative investigations, and a useful tool for explaining many properties or phenomena, but

367

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Luciano Forlani

are also important for projecting new practical synthetic routes, in the fields of scientific and technical research.

Interactions between molecules and between parts of the same molecule are of fundamental importance for investigating and explaining many different behaviours in different fields.

There are very important factors that affect the interactions between molecules of solutes and solvents and explain solvent effects1. Simple solubilization of a compound in a solvent is possible if certain solute/solvent interactions take place. The usual general solvent2 parameters may be used to explain the physical and chemical behaviours of solutes, but often better explanations are obtained when specific interactions depending on the peculiarities of interacting molecules are considered3 5.

When two or more reactants first approach one another, non-covalent recognition and attraction of the molecules or of parts of the molecules takes place and permits system aggregation, which may be of importance in obtaining the reaction products. For instance, the approach of a carbocation to a system of olefins starts by forming a charge-transfer (CT) complex which precedes the formation of the C C bond6.

Usually, simple chemical reactivities (regarding covalent bond forming and breaking) can be estimated by changes in energy (or enthalpy). In more complicated processes, such as biological processes, the entropy changes may be more important than energy changes and some modifications are considered to be entropy-controlled processes. The formation of weak interactions may be related to a small enthalpy variation, but to a significant entropy change.

Generally speaking the interest in the formation of weak interactions, generally noncovalent interactions, such as electron donor acceptor interactions7 or hydrogen bonding interactions8, may be summarized by the following main points:

(i)Physical properties, with particular emphasis on spectroscopic properties, regarding not only solute/solvent interactions, but also self-association of solutes, population of conformers and positions of tautomeric equilibria.

(ii)Chemical properties: the immediate neighbourhood of a solute (a reagent in a solvent) may be very important for defining its reactivity.

(iii)Investigation of reaction pathways: many reactions, both organic and inorganic, may be unified in a common general mechanism if the electrophilic reagent and nucleophilic reagents are considered electron acceptors and electron donors, respectively forming a molecular complex by means of weak interactions, involving or competing with the

solvents7.

The free energy relationship for electron transfer (FERET) starts with the presence of weak interactions (similar to donor acceptor complexes) yielding the ion pair radical7.

(iv)Molecular recognition is an important step in self-aggregation of molecules, including Diels Alder reactions and some types of olefin polimerizations9.

(v)In biological processes10 the non-covalent interactions are of fundamental importance: the catalysis of chemical reactions, enzyme catalysis in particular, neutralization of toxins, hormone action in stimulating cellular activities and understanding the action of

pharmaceutical chemicals are examples11 of processes starting with non-covalent interactions between receptors and ligands, with solvent intervention.

While a complete classification of the complexes (and of some intermediates)12 is beyond the scope of this chapter, the term ‘complexes’ is often applied to several different kinds of interactions (involving also hydrogen bonding interactions). A formal distinction is made between non-covalent and covalent complexes.

The main interactions may be divided into three main groups:

(i) Intermolecular and intramolecular hydrogen bonding interactions, which are both well known13,14.

8. Complex formation involving double-bonded functional groups

369

(ii)Donor acceptor interaction15 is an attractive electrostatic interaction16. Chargetransfer complexes start with this kind of interaction.

(iii)Van der Waals interactions are important when interactions (i) and (ii) are absent

or very weak17,18. Basically, van der Waals systems involve attractive forces other than chemical bonds, including both points (i) and (ii). With true van der Waals molecules the main attraction comes from dispersion energy19.

II.OLEFINS. NON-COVALENT INTERACTIONS

A.Olefins: Electron Donor Acceptor Complexes

The CDC olefin double bond is usually considered to be an electron-rich centre which is prone to reactions with electrophiles. When strong electron-releasing groups such as amino, alkoxy or phenyl groups are bonded to the carbon atoms forming the double bond, the bond’s capacity to release electrons is enhanced and the olefin is defined as an electron-donor olefin. On the other hand, when the CDC double bond bears substituents that attract electrons, the olefin is an electron-acceptor olefin.

The ability of molecules to form donor acceptor complexes depends not only on their ionization potential, electron affinity and polarizability, but also on the requirements and properties of partners.

In principle, the same olefin can act as a donor molecule or an acceptor molecule depending on the properties of the partner20. In the same way, the CDC double bond is a donor or a acceptor21, depending on the nature of the groups bonded to it.

Donor acceptor interactions are the first step in some Diels-Alder reactions7,22 between electron-poor olefins and electron-rich olefins, and also in spontaneous polymerization which can occur during cycloaddition reactions of olefins23.

True CT complexes are formed between unsaturated electron acceptors, among which the derivatives with cyano and nitro groups predominate. The most common strong electron acceptors are 1,2,4,5-tetracyanobenzene, 7,7,8,8-tetracyanoquinodimethane (1), tetracyano-p-benzoquinone (2), tetracyanoethylene (TCNE) (3) and many electron-donor

NC

CN

 

 

C

 

O

 

 

 

NC

CN

 

 

 

 

 

NC

CN

 

 

 

C

 

O

 

NC

CN

 

 

(1)

 

(2)

 

NC

CN

Me2 N

NMe2

C

C

C

C

NC

CN

Me2 N

NMe2

(3)

 

(4)

 

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hydrocarbons including alkenes, cycloalkanes, alkynes and aromatic hydrocarbons. In the field of the unsaturated electron donors, tetrakis(dimetylamino)ethylene (4)15 is a strong electron donor.

Generally, equilibrium 1 is quickly established, and it is a standard common model in donor acceptor interaction investigations.

 

 

K

 

DONOR

C

ACCEPTOR CHARGE-TRANSFER COMPLEX

1

 

 

 

CT complexes exhibit spectral properties different from those of the separate compounds, and spectroscopic methods are widely used in their investigations and in quantitative evaluations of K (equation 1) values15.

Scheme 1 illustrates the Diels Alder reaction through a complex.

NC

CN

NC

CN

OR

 

CN

OR

 

CN

 

OR

 

 

 

 

 

 

 

 

+

 

 

 

 

 

 

CN

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NC

CN

NC

CN

 

 

 

 

 

 

 

charge-transfer complex

 

CN

 

 

 

 

 

 

 

 

 

 

SCHEME 1

 

 

 

Tetrakis(phenylethynyl)ethene

(5) forms

complexes

with 2,4,7-trinitrofluoren-9-

one (6) and (2,4,7-trinitrofluoren-9-ylidene)malonitrile (7)24. In the solid state the CT complexes show a 1:2 stoichiometry; in solution (CHCl3) the purple-coloured complex between 5 and tetracyanoethylene shows a 1:1 stoichiometry. Such complexes may be of interest as potential new materials e.g. in the field of non-linear optics.

The most common donor/acceptor ratio for a CT complex is 1:1, but there are examples of different ratios, such as 2:1. In theory a 1:2 ratio donor/acceptor is possible, depending on the experimental conditions and on the relative amounts of partners used.

7,7,8,8-Tetracyanoquinodimethane (1) and tetracyanoethylene (3) are able to form CT complexes with crown ethers which are electron-donor molecules25. A recent study has recorded the spectral properties and stability constants of 89 tetracyanoethylene CT complexes with donors26. The main interaction in these complexes is an electron transfer ( ! Ł ) between the HOMO of the donor and the LUMO of the acceptor.

The formation of CT complexes between alkenes is considered to be the first step of the cycloaddition reactions, and it may also be the first step of some types of olefin polymerization23. The CT complex obtained from strong electron donors and strong electron acceptors may produce a complete charge separation with formation of an ion-radical pair (cation radical and anion radical pair), as illustrated by Scheme 2.

9-Cyanoantracene (and indene) form CT complexes with TCNE; these complexes were studied27 using time-resolved picosecond spectroscopy. Irradiation of the CT complex produced an ion radical pair as shown in Scheme 2.

The electron transfer (if there is an ion radical pair) intermediate is generally thought to participate in the polar mechanism for cycloaddition reactions28. Recently, the electron transfer mechanism for the zwitterionic cycloaddition of tetracyanoethylene and bis(4- methoxycinnamyl) ether has been discounted and there is now strong support for the theory that a polar mechanism is also operative for other systems29.

The ground states of electron donor acceptor complexes between trans-stilbene and electron-deficient alkenes (fumaronitrile, dimethyl fumarate and maleic anhydride) are formed and the isomerization of trans-stilbene (and of fumaronitrile to maleic nitrile) has

 

 

8. Complex formation involving double-bonded functional groups

371

R

R

R1

R1

R

R R1

R1

 

R

R

R1

R1

 

 

+

 

 

 

 

 

 

 

 

 

 

 

R1

 

 

 

 

 

 

 

 

 

 

 

 

 

R

R

R1

R1

R

R R1

R1

 

R

R

R1

R =electron-withdrawing group

 

CT complex

 

 

 

ion radicalpair

 

R1 =electron-releasing group

 

 

 

 

 

 

 

 

 

 

SCHEME 2

 

 

(5)

 

 

O

NC

CN

 

 

 

O2 N

NO2

O2 N

NO2

 

 

 

O2 N

 

O2 N

 

(6)

 

(7)

been investigated to check the importance of the presence of oxygen, which favours the isomerization of trans-stilbene.

‘Exciplexes’ are defined as molecular complexes which are stable under electronic excitation30. The picosecond (or femtosecond) laser photolysis methods are suitable for investigating the very rapid photo-induced processes related to CT complexes.

trans-Stilbenes (8)/fumaronitrile complexes are used to investigate some aspects of the microdynamics of these complexes (by picosecond absorption spectroscopy)31,32, e.g. for the formation of contact ion pairs and solvent-separated ion pairs, as shown in Scheme 333.

The excited trans-stilbene/fumaronitrile complex produces a locally excited triplet state, which is considered to be responsible for isomerization of substituted stilbenes. Similarly, the CT complexes between aromatic hydrocarbons and fumaronitrile produce the isomerization of fumaronitrile to malonitrile34.

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Luciano Forlani

 

 

Ph

 

CN

 

 

Ph

CN

 

 

 

 

 

 

 

 

 

 

 

 

Ph

+

CN

 

 

Ph

CN

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CT complex

 

 

 

 

 

 

hν

 

 

 

Ph

 

 

 

Ph

CN

 

 

 

CN

 

 

 

 

 

 

 

 

 

 

 

 

 

 

+

 

 

 

+

 

 

Ph

 

CN

Ph

CN

 

 

 

 

 

 

 

 

 

 

 

 

 

Contact ion pair

Solvent-seperated

 

 

 

 

 

 

ion pair

 

 

 

 

SCHEME 3

 

 

R1

 

 

 

 

R1

= H; R2 = OCH

 

 

 

 

 

 

3

 

 

 

 

 

R1 = R2 = CH3

 

 

 

 

 

R1 = H; R2 = CH3

 

 

 

 

 

R1 = Cl; R2 = CH3

 

 

 

 

 

R1 = R2 = H

 

 

 

 

 

R1 = H; R2 = Cl

 

 

 

 

 

R1

= H; R2

= F

 

 

 

 

 

R1 = R2 = Cl.

 

 

 

 

 

R2

 

 

 

(8)

 

 

 

 

 

 

trans-Stilbene/amine exciplexes have been investigated by using trialkylamines and the bicyclic diamine, 1,4-diazabicyclo[2,2,2]octane, in benzene35 and in acetonitrile36: in the more polar solvent, a solvent-separated radical ion pair is formed either directly from the complex or from the exciplex.

Recently37, the importance of CT complexes in the chemistry of heteroaromatic N- oxides has been investigated in nucleophilic aromatic substitutions. Electron acceptors (tetracyanoethylene and p-benzoquinones) enhance the electrophilic ability of pyridine- N-oxide (and of quinoline-N-oxide) derivatives by forming donor acceptor complexes which facilitate the reactions of nucleophiles on heteroaromatic substrates.

Complexes between tetranitromethane (which is a powerful electron acceptor) and different electron donors (aromatic substrates, alkenes, amines, sulphides, ethers) may be observed and isolated as moderately stable coloured complexes (if stored in the dark). These complexes are usually classified as CT complexes. Irradiation of complexes between alkenes and trinitromethane forms interesting products, which are derived from the nucleophilic attack of the trinitromethide.

Scheme 4 shows the reaction between styrenes and tetranitromethane38 to produce the isoxazolidine derivatives 9 with 85% yields, by irradiation of the styrene/tetranitromethane

8. Complex formation involving double-bonded functional groups

373

complex. The structure of 9 was investigated by X-ray diffraction. Scheme 4 suggests that in this case the trinitromethide anion (which is an ambident nucleophile) acts as an oxygen nucleophile39. In other cases, such as photonitration of the CT complex between naphthalene40 (and 1-methoxy-naphthalene41) and tetranitromethane, the trinitromethide anion is a carbon nucleophile, as shown in Scheme 5.

+C(NO2 )4 CT complex

X

hν

 

(MeCN)

 

 

 

 

O2 N NO2

 

 

NO2

 

 

N

 

O

O

 

X

X

 

X = H, Me

(9)

SCHEME 4

X

X C(NO2 )3

hν, CT

+ C(NO2 )4 CT complex

H NO2

X = H, OMe

SCHEME 5

The main products of the photolysis42 of the complex between 1-methoxynaphthalene and tetranitromethane are 1-methoxy-4-nitronaphthalene and 1-methoxy-4-trinitromethyl- naphthalene.

The ability of compounds with a quinonic structure to form donor acceptor interactions and CT complexes is useful in regioselective halogenation of phenol (or naphthols and their derivatives).

Chlorination of phenol43 is obtained mainly in positions 2 or 4, depending on the hexachlorocyclohexadienone used, as shown in Scheme 6. A hydrogen bond between the OH and CDO groups favours the formation of CT complexes (10, Scheme 6). In the same way, 2-chloro-1-naphthol and 4-chloro-1-naphthol44 are obtained from ˛-naphthol.

B. Complexes Between Olefins and Ozone

A weakly bonded complex between ozone and ethylene was observed by using a pulsed beam Fourier transform microwave spectrometer45. The C2H4 O3 complex shows the two partners on two parallel planes at a distance of 3.30 A˚ (R in 11). This complex (which is probably a van der Waals complex) enables the primary ozonide 12 to be

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Luciano Forlani

 

 

 

OH

O

H

 

O

OH

O

Cl

Cl

Cl

 

Cl

Cl

 

 

 

+ Cl

 

Cl

 

 

 

 

Cl

 

 

Cl

 

OH

O

H

 

O

OH

O

 

Cl

Cl

Cl

 

Cl

 

 

 

 

 

+

 

 

 

 

 

Cl

Cl

 

Cl

Cl

Cl

 

 

 

 

 

 

 

(10)

 

 

 

 

SCHEME 6

 

 

 

formed. Theoretical results confirm microwave results suggesting the internal rotation (in a van der Waals complex, 11) of ethylene: this rotation exchanges the equivalent pair of hydrogen (or deuterium) atoms for ethylene46.

O

C

O

R

C

 

O

O

 

O

C

C

 

O

(11)

 

(12)

The weakly bonded complex (11) is close to the transition state (TS) of the cycloaddition reaction to obtain the ozonide (12) and it must occur prior to this TS, which shows a C O distance of 2.3 2.0 A,˚ shorter than that calculated for the complex (11).

Despite the complexities of alkene ozonolysis47, the reaction between alkenes and ozone may be summarized by Scheme 7. The reaction involves several steps48 with the formation of a variety of intermediates, such as a primary ozonide (1,2,3-trioxolane) (12), its isomer of rearrangement 13 and a carbonyl oxide (14).

Kinetic data for ozonization reactions of a given number of alkenes in carbon tetrachloride at different temperatures are consistent with the presence of a pre-association equilibrium between the alkene and the ozone when electron-rich alkenes are used49.

The reaction of trans-di-tert-butylethylene (15) with ozone is shown in Scheme 8. The ozonide (16) is an unusually stable (at 60 °C) trioxolane50. The decomposition of (16) forms the aldehyde (17) and the carbonyl oxide (18) (as illustrated in Scheme 8, for non-radical ozonization reactions). The carbonyl oxide is an oxygen carrier.

The decomposition of (18) in the presence of electron-deficient oxygen acceptors such as tetracyanoethylene forms the tetracyanoethylene oxide (19)51, with 60% yield. The oxygen atom transfer may be considered a general reaction of carbonyl oxides in ozonolysis of CDC double bonds when oxygen-accepting substrates are present.

 

8. Complex formation involving double-bonded functional groups

375

R

R

 

 

 

R

R

 

 

 

 

R

 

 

 

 

 

 

 

 

R

 

 

 

 

 

 

 

 

O+ O2

 

 

O

 

+ O3

 

CT

 

 

 

 

 

R

O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

O

R

R

 

 

 

R

R

 

 

 

 

R

 

 

 

 

 

 

 

 

(12)

 

 

 

 

 

 

R

 

 

 

 

 

R

R

 

 

 

 

 

 

C

 

 

 

O

 

 

 

 

 

 

 

 

R

+

 

 

 

O

 

 

 

 

 

R

 

 

O

O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

O+

 

 

 

 

 

 

 

 

 

C

 

 

 

 

R

R

 

 

 

 

 

R

 

 

 

O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(14)

 

 

(13)

 

 

 

 

SCHEME 7

 

 

 

 

 

 

 

 

 

 

 

O

 

 

 

 

 

 

 

 

O3

 

O

O

 

 

 

 

 

O

+

 

+O O

C C

 

 

 

C

C

 

 

C

C

 

 

 

 

 

 

 

 

 

 

H

 

 

H

 

 

 

 

 

 

 

 

(15)

 

 

 

(16)

 

 

 

(17)

 

 

(18)

 

 

 

 

SCHEME 8

 

 

 

 

 

 

 

 

CN

CN

 

C

 

 

 

+

 

 

 

 

 

 

 

 

 

 

 

 

 

NC C

C

CN

 

 

O

O

C

 

 

 

 

 

 

 

 

O

 

 

C

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(19)

 

 

 

 

(20)

 

 

 

A reasonable explanation for the epoxide 19 formation is the presence of a complex such as 20. The mechanism involving 20 parallels the reaction of ozonolysis of fluoro olefins52.

In the ozonization reactions of olefins, the radical production may be a significant side reaction53. In this case, again, the reaction may be deemed to proceed through a CT complex between ozone and olefins54.

C. Complexes Between Olefins and SO2 or Ketenes

The microwave spectrum of mixtures of ethylene and sulphur dioxide indicates the presence of an interaction involving the electrons of ethylene55. The SO2 molecule

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Luciano Forlani

may be arranged in two main structures, depending on the fact that sulphur may be closer to the carbon atoms (with structures similar to that illustrated by 11, for the ozone/olefin complex) than the oxygen atom, as illustrated in 21. In complex 21, the distance between the centre of the S O bond and a carbon atom of ethylene is 3.51 A˚ .

The properties of weakly bonded van der Waals complexes in reactive systems are studied by pulsed Fourier Transform microwave spectroscopy, which is a powerful tool for investigating many complexes.

The microwave spectrum of the complex between ethylene and ketene (and of deuterated derivatives) reveals56 a crossed structure (22), while the ketene/acetylene complex shows a planar geometry57. This difference in geometry is explained by the different quadrupole moments of two unsaturated hydrocarbons.

C

O C C O

R

S C

O C

C

(21)(22)

From the qualitative point of view, the structure of the ethylene/ketene complex is similar to the geometry of the TS of the same system in cycloaddition reaction58. In 22, R (D 3.46 A)˚ is the distance between the centre of mass of the ethylene and the carbon of the carbonyl group of ketene; this carbon is the most electrophilic centre of the ketene. In the case of the complex between acetylene and ketene, the same distance between the centre of mass of acetylene and the carbon of ketene was evaluated (by the same method) at 3.60 A˚ .

D. Complexes Between Olefins and Carbenes

In thermolysis and photolysis of 3-chloro-3-benzyldiazirines 23, carbenes 24 are formed as indicated by reaction 2. In both electron-rich (tetramethylethylene) and electronpoor (diethyl fumarate) alkenes (as solvents), cyclopropanes (and chlorostyrenes) are obtained59. The probable reaction pathway is the multi-step model shown in Scheme 9.

ArCH2

N , (hν)

ArCH2

C

 

 

C

 

Cl

(2)

 

 

Cl

N

 

 

 

(23)

 

 

(24)

 

 

Negative activation energies for cycloaddition reactions of some carbenes are reported60 and they confirm the presence of a pre-association equilibrium on the reaction pathway. In addition, the entropy control of the cycloaddition of halocarbenes to the CDC double bond was extensively reported61 and explained by the presence of a weakly bound intermediate complex (25), which is reversibly formed and is probably a CT complex62. Its presence is also supported by direct kinetic data59,63.

The nature of the complex63 probably involves the electrons of the alkene and the vacant p orbital of carbene by CT interaction. The possibility that the intermediate 25 is

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