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2 [2+2] Cycloadditions of Carbonyl Compounds

2 [2+2] Cycloadditions of Carbonyl Compounds

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30



S. Inagaki



1.2.1  Delocalization Band

Four-membered ring formation between unsaturated carbon bonds and carbonyl

compounds is a photochemical reaction [25]. This is an excited-state reaction in the

delocalization band (Scheme 6).

1.2.2  Pseudoexcitation Band

The [2+2] cycloaddition could occur thermally in the pseudoexcitation band. In fact,

an alkyne with electron-donating group, ethoxyacetylene, and electron accepting

carbonyl compound, perfluoroacetone, form the oxetene at low temperature (−78

°C) without light irradiation (pseudoexcitation band in Scheme 6) [26, 27].

Thermal [2+2] cycloaddition reactions of carbonyl compounds were catalyzed

by a Lewis acid. The catalyst forms complexes with the carbonyl compounds and

enhances the electron-accepting power. The reaction shifts from the delocalization

band to the pseudoexcitation band. Catalyzed [2+2] cycloaddition reactions were

observed with acetylenic compounds [28] and ketenes [29–31].

Olefins (enamines) unsymmetrically substituted with strong electron-donating

(amino) group and CS2 generate zwitterions (1,4-dipoles) [32, 33]. Polar additions

are proposed here to be reactions in the pseudoexcitation band.



1.3  [4+2] Cycloadditions

The mechanistic spectrum suggests that [4+2] cycloadditions should shift from

concerted reactions in the delocalization band to stepwise reactions through intermediates in the pseudoexcitation band. The HOMO–LUMO interactions, important

in the delocalization band, are allowed by the orbital symmetry (Sect 3.5 in Chapter

“Elements of a Chemical Orbital Theory” by Inagaki in this volume). The reactions

occur in a concerted manner. In the pseudoexcitation band the HOMO–HOMO–

LUMO–LUMO interaction is important. The HOMO–HOMO and LUMO–LUMO

interactions are, however, forbidden by the orbital symmetry at the six-membered

ring transition states for the Diels–Alder reactions. The pseudoexcitation band does

not prefer concerted [4+2] cycloaddition reactions. The HOMO–HOMO and

LUMO–LUMO interactions can be free from the symmetry restriction, when one

pair of the reaction sites is closer to each other than another pair of sites (Scheme

7). This geometry leads to a zwitterion intermediate. Even the symmetry-allowed

[4+2] cycloaddition reactions can be changed to such polar addition reactions as

electrophilic additions when the donors and acceptors are strong. This suggests that

polar additions could be reactions in the pseudoexcitation band.

1.3.1  Delocalization Band

An electron donating butadiene with the methoxy substituents at the 1 and 4 positions

was calculated to undergo a concerted [4+2] cycloaddition reaction with TCNE as



A Mechanistic Spectrum of Chemical Reactions



31

OCH3

CN

CN



OCH3

NC



CN



NC



CN



+

CN

CN



OCH3



OCH3



+



HN



HN



+

CN



CN



Delocalization band

NH2



NH2

NC



CN



NC



CN



+

NC



NH2



HN



NH2



CN

CN

CN



NH2



NH2



OCH3



CN

CN



HN



HN



+



CN

CN



OCH3



OCH3



Pseudoexcitation band

Scheme 7  Mechanistic spectrum of [4+2] cycloaddition reactions



usual (delocalization band in Scheme 7) [34]. Some calculations [35] showed the

[4+2] cycloaddition reaction of the N-protonated 2-aza-1,3-butadiene cation with

acrylonitrile takes place in a concerted manner (delocalization band in Scheme 7).



1.3.2  Pseudoexcitation Band

A stronger donor, the butadiene with the amino groups in place of the methoxy group

in the 1,4-positions, was calculated to react with TCNE via a zwitterion (pseudoexcitation band in Scheme 7) [34]. The loss of the stereochemical integrity was observed

in the [4+2] cycloaddition reactions between some strong donors, 1,4-bis(dimethylamino)

butadienes, and acceptors, fumaric and maleic dinitriles [36].

In hetero [4+2] cycloaddition (delocalization band in Scheme 7) the cationic

diene is a strong acceptor and the dienophile is substituted by an electron accepting

group. Replacement with a strong donating substituent in the dienophile shifts the

reaction from the delocalization band to the pseudoexcitation band. The methoxysubstituted dienophile gives a zwitterionic intermediate (pseudoexcitation band in

Scheme 7) [35]. This is a polar reaction in the pseudoexcitation band.



32



S. Inagaki



According to the calculations at high levels of theory, the [4+2] cycloaddition

reactions of dienes with the singlet (1Dg) oxygen follow stepwise pathways [37, 38].

These results, which were unexpected from the Woodward–Hoffmann rule and the

frontier orbital theory, suggest that the [4+2] cycloadditions of the singlet (1Dg) oxygen

could be the reactions in the pseudoexcitation band.



1.4  Cycloisomerization of Conjugate Polyenes

According to the Woodward–Hoffmann rule [6, 7], conjugate polyenes with 4n and

4n+2 p electrons undergo cyclizations in conrotatory and disrotatory fashions under the

thermal conditions, respectively. Recently, novel cycloisomerizations were found to be

catalyzed by Lewis acid and to afford bicyclic products [39] as photochemical reactions

do [40]. The new finding supports the mechanistic spectrum of chemical reactions.

1.4.1  Delocalization Band

Hexatrienes undergo disrotatory ring closure by thermal activation to afford cyclohexadienes in agreement with the Woodward–Hoffmann rule (delocalization band in

Scheme 8) [41–43]. Photo-irradiation of hexatrienes is known to give bicylic products

in a stereospecific [4pa+2pa] manner (delocalization band in Scheme 8) [40] in contrast

to this rule.

O2N



CO2Et



O2N



Ph







CO2Et



H



Ph



Ph







Ph

Me



H

H



H



Delocalization band



O2N



O2N



CO2Et



Me



Lewis acid



H

H

EtO2C



Pseudoexcitation band

Scheme 8  Mechanistic spectrum of cycloisomerizations of hexatrienes



Me



A Mechanistic Spectrum of Chemical Reactions



33



1.4.2  Pseudoexcitation Band

Trauner and colleagues [39] recently found a striking contrast in the thermal and

catalyzed reactions of a triene. Thermal reaction of a trienolate readily underwent

disrotatory electrocyclization to afford cyclohexadiene (delocalization band in

Scheme 8) in accordance with the Woodward–Hoffmann rule. Surprisingly, treatment of the trienolate with Lewis acid did not result in the formation of the

cyclohexadiene but rather gave bicyclo[3.1.0]hexene in a [4pa+2pa] manner (pseudoexcitation band in Scheme 8). The catalyzed reaction is similar to the photochemical reaction in the delocalization band.

The hexatriene is polarized by unsymmetrical substitution with the C=O group,

and further activated by coordination with Lewis acid. The catalyzed reaction is

polar. The similarity between the catalyzed and the photochemical reactions can be

understood if polar reactions belong to the pseudoexcitation band as has been

proposed in Sect 1.



1.5  Electrophilic Aromatic Substitutions

The mechanistic spectrum shed new light on a familiar textbook example of organic

reactions, i.e., electrophilic aromatic substitution (Scheme 9).

1.5.1  Delocalization Band

No electrophilic aromatic substitution reactions of toluene, ethylbenzene, and cumene

occur with BBr3 in the dark: the electrophile is too weak for these reactions. The photochemical reactions followed by hydrolysis give the p-isomers of the corresponding boronic acids as the major products (delocalization band in Scheme 9) [44].

1.5.2  Pseudoexcitation Band

Electrophilic aromatic substitution reactions take place between aromatic compounds

and strong acceptors (pseudoexcitation band in Scheme 9). The substitutions are



CH3



+



BBr3



1) hν

2) hydrolysis



CH3



B(OH)2



Delocalization band



+



E+



Pseudoexcitation band



Scheme 9  Mechanistic spectrum of electrophilic aromatic substitutions



E+



34



S. Inagaki



regarded as reactions in the pseudoexcitation band. Addition of AlCl3 causes the

haloboration with BBr3 [45]. Complex formation of BBr3 with ACl3 generates a

more electrophilic species [BBr2]+[AlCl3Br]− and shifts the reaction from the delocalization band to the pseudoexcitation band.



1.6  Reactions of Indoles with Unsaturated Acceptors

The theory of the mechanistic spectrum generally suggests that photochemical

reactions between donors and acceptors in the delocalization band could be similar

to thermal reactions between strong donors and acceptors in the pseudoexcitation

band. This is further supported by the reactions of indoles with electron-accepting

alkenes.

A photochemical reaction of indole with acrylonitrile gave an a-cyanoethylated

indole (delocalization band in Scheme 10) [46]. This is a photochemical reaction in

the delocalization band.

A stronger acceptor, TCNE, undergoes a similar reaction without irradiation to give

tricyanovinylindole after the elimination of HCN by pyridine (pseudoexcitation band

in Scheme 10) [47].



2  Delocalization Band

The reactions in this band are controlled by the frontier orbital interactions (Sect

3 in chapter “Elements of a Chemical Orbital Theory”), which were described in

detail earlier [48–51]. A few recent interesting advances are reviewed in this

section.



CH3

C



+



h�



CH2 CHCN



H



CN



N

H



N

H



Delocalization band



NC



NC

NC



CN



H

CN



+

N

H



NC



CN



N

H



NC



CN



CN

CN



−HCN

N

H



Pseudoexcitation band



Scheme 10  Mechanistic spectrum of the reactions of indoles with unsaturated acceptors



A Mechanistic Spectrum of Chemical Reactions



35



2.1  [4+2] Cycloadditions of a Ketene

Staudinger observed that the cycloaddition of ketenes with 1,3-dienes afforded

cyclobutanones from a formal [2+2] cycloaddition [52] prior to the discovery of the

Diels–Alder reaction. The 2+2 cycloadditions were classified into the symmetryallowed p2s+p2a cycloaddition reactions [6, 7]. It was quite momentous when

Machiguchi and Yamabe reported that [4+2] cycloadducts are initial products in the

reactions of diphenylketene with cyclic dienes such as cyclopentadiene (Scheme

11) [53, 54]. The cyclobutanones arise by a [3, 3]-sigmatropic (Claisen) rearrangement of the initial products.



O

O



O



+

Ph



Ph



Ph



Ph



Ph



Ph



Scheme 11  [4+2] Cycloaddition reaction of diphenylketene



2.2  Exo-addition in Diels–Alder Reactions

Endo-selectivity of the Diels–Alder reactions of olefinic dienophiles are well understood in terms of the secondary frontier orbital interaction [55]. However, exo–endo

selectivity of the reactions of acetylenic dienophiles was difficult to investigate,

since exo and endo transition states produce diastereomerically identical adducts.

Ishihara and Yamamoto [56, 57] reported the first example of an enantioselective

Diels–Alder reaction of acetylenic dienophiles with dienes, which have prochiral

reactive centers, in the presence of chiral boron Cu(II) catalysts. The secondary

orbital interaction is antibonding between the lobes on the 2-position of the dienes

and carbonyl oxygen of the dienophiles (Scheme 12). The Diels–Alder reactions of

acetylenic aldehydes is resistant to the endo-transition structure, in contrast to that

of olefinic aldehydes. The predominance of the exo-transition structure, confirmed

by ab inito calculations, is in agreement with the observed enantioselectivity.



HOMO



+

O



LUMO

O



Scheme 12  Orbital phase environment in the Diels–

Alder reactions of acetylenic aldehydes: exo-selectivity



endo-TS



exo-TS



36



S. Inagaki



Similar enantioselective Diels–Alder reactions between cyclopentadiene and

a,b-acetylenic aldehydes catalyzed by a chiral super Lewis acid were reported by

Corey and Lee [58].



2.3  [4+2] Cycloadditions on Surface

Reactions on the surface are interesting. The adsorptions of unsaturated organic

molecules on the surface provide a means for fabricating well-ordered monolayer

films. Thin film organic layers can be used for diverse applications such as chemical

and biological sensors, computer displays, and molecular electronics.

Diels–Alder reactions are allowed by orbital symmetry in the delocalization

band and so expected to occur on the surface. In fact, [4+2] cycloaddition reaction

occurs on the clean diamond (100)-2 × 1 surface, where the surface dimer acts as a

dienophile. The surface product was found to be stable up to approximately 1,000

K [59, 60]. 1,3-Butadiene attains high coverage as well as forms a thermally stable

adlayer on reconstructed diamond (100)-2 × 1 surface due to its ability to undergo

[4+2] cycloaddition [61].

Diels–Alder reactions also take place on the Si(100)-2 × 1 [62] and Ge(100)-2 ×

1 [63, 64] surface. The experiments by Hammers and his colleagues [65] indicate that

the [4+2] cycloaddition reactions of 1,3-cyclohexadiene and 1,3-dimethylbutadiene

on the Si(001) surface compete with the [2+2] cycloaddition reactions.



3  Pseudoexcitation Band

Some typical reactions in the pseudoexcitation band are reviewed in this section.

The importance of pseudoexcitation [1] in chemical reactions was supported by the

detailed numerical analysis of the electronic structures of the transion states [66].

The concept of pseudoexcitation appeared in physics [67–69].



3.1  Reactions of Singlet Molecular Oxygen O2 (1Dg)

Singlet molecular oxygen O2 (1Dg) is an electron acceptor powerful enough to react

with olefins in the pseudoexcitation band. The [2+2] cycloaddition and ene reactions

and the stereoselectivities are reviewed in this subsection.

3.1.1  Quasi-Intermediate, Perepoxide

The interaction between the HOMO of alkenes and the LUMO of singlet oxygen 1O2

(Scheme 4) is the most favored in the perepoxide structure (Scheme 13). This suggests



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