Tải bản đầy đủ - 0 (trang)
3 [4+2] Cycloadditions on Surface

3 [4+2] Cycloadditions on Surface

Tải bản đầy đủ - 0trang

A Mechanistic Spectrum of Chemical Reactions



O

O



37



O

O



Scheme 13  Perepoxide quasi-intermediate



a path via the perepoxide intermediate or a perepoxide-like transition state [13]. We

earlier predicted the following property and role of perepoxide from the calculated

potential surface [70]. If an energy minimum exists (for perepoxide), it is very shallow – we may say, perepoxide itself cannot be isolated; in such a sense, perepoxide

cannot be a genuine intermediate. However, if the flat region on the surface (for a

perepoxide-like structure) is high, some kinetic and dynamic effects could possibly be

observed as if the perepoxide intermediate actually intervened. This property is an

attribute of the true intermediate. Such partial but not complete fulfillment of the conditions for reaction intermediate deserves the designation, quasi-intermediate.

After 28 years the perepoxide quasi-intermediate was supported by a two-step

no intermediate mechanism [71, 72]. The minimum energy path on the potential

energy surface of the reaction between singlet molecular oxygen O2 (1∆g) and

d6-teramethylethylene reaches a valley-ridge inflection point and then bifurcates

leading to the two final products [73].

Katsumura, Kitaura and their coworkers [74] found and discussed the high reactivity of vinylic vs allylic hydrogen in the photosensitized reactions of twisted 1,3-dienes

in terms of the interaction in the perepoxide structure. Yoshioka and coworkers [75]

investigated the effects of solvent polarity on the product distribution in the reaction

of singlet oxygen with enolic tautomers of 1,3-diketones and discussed the role of the

perepoxide intermediate or the perepoxide-like transition state to explain their results.

A recent review of the ene reactions of 1O2 was based on the significant intervention

of the perepoxide structure [76], which can be taken as a quasi-intermediate.

A perepoxide intermediate [77] or a peroxy diradical intermediate [78–81] have

been proposed.

3.1.2  [2+2] Cycloaddition Reactions

[2+2] Cycloaddition reactions can occur with retention of configuration in the

pseudoexcitation band (Sect 1.1) whereas [2ps+2ps] reactions are symmetry-forbidden

in the delocalization band. Experimental evidence is available for the stereospecific

[2+2] cycloaddition reactions between 1Dg O2 and olefins with retention of configuration (Scheme 14) [82]. A perepoxide intermediate was reported to be trapped in the

epoxide form [83] in the reaction of adamantylideneadamantane with singlet oxygen

affording dioxetane derivatives [84].



38

Scheme 14  Stereospecific [2+2] cycloaddition reactions of

O2 (1Dg)



S. Inagaki

EtO



EtO



+

EtO



O



1O2



O

EtO



3.1.3  Concerted/Stepwise Boundary

The concept, quasi-intermediate [70], was introduced in 1975 to symbolize a

boundary between concerted and stepwise mechanisms. Recent advances in computer

chemistry are allowing us to investigate subtle problems more clearly in the years

since 2000. Concerted/stepwise boundary mechanisms were proposed for other

diverse reactions than those of singlet molecular oxygen O2 (1Dg).

Reactions that would be concerted based on the potential energy surface can

nonetheless end up as a stepwise process [85]. The potential energy surface for the

rearrangement of (CH3)3C–CHCH3–OH2+ indicates loss of the water leaving group

and migration of the methyl group take place in a concerted manner. However, most

trajectories involve a stepwise route. The carbocation prior to the methyl migration

can be termed a quasi-intermediate.

A mechanism at the SN2Ar/SN1 boundary was proposed for the nucleophilic

substitution reaction of aryldiazonium ions in water [86].

A single transition structure was located on the potential energy surface of the

intramolecular bicyclization of protonated and Lewis acid activated (2E,4Z)hepta-2,4,6-trienal and the corresponding methyl ester to provide the bicylo[3.1.0]

hexene derivatives [87]. These are models for the reactions in the pseudoexcitation

band (Scheme 8). The five-membered ring is formed through the transition

structure. The subsequent formation of the three-membered ring is barrierless.

The reaction cannot be considered a stepwise process because no intermediate is

found along the reaction path. It is not a concerted mechanism because of the timing

of the bond-formation process. The five-membered ring structure can be termed a

quasi-intermediate.

It is noteworthy that these are the reactions in the pseudoexcitation band if the

polar reactions are taken as proposed in Sect 1.

3.1.4  Ene Reactions

Following the discovery of the ene reaction of singlet molecular oxygen O2 (1Dg)

(Scheme 15) in 1953 by Schenck [88], this fascinating reaction continues to receive

considerable mechanistic attention today. The importance of a path via the perepoxide

intermediate or a perepoxide-like transition state [13] or the perepoxide quasi-intermediate [70] was proposed for the ene reactions of singlet oxygen 1O2 with alkenes

affording allylic hydroperoxides.

The HOMO of alkenes is an out-of-phase combination of the p and sCH orbitals.

The amplitude is larger on p. The LUMO of singlet oxygen is p*. The frontier

orbital interaction occurs most effectively when the alkenes and the singlet oxygen



A Mechanistic Spectrum of Chemical Reactions



+



1



39



O



O2

O



CH3



HOMO (p−s) LUMO (p ∗)



O



O



O



O

H



s*CH LUMO (p ∗)



Scheme 15  An ene reaction of O2 (1Dg)



assume a three-membered ring geometry (Schemes 15 and 4). This is a structure of

the perepoxide quasi-intermediate. The interaction reduces the sCH bonding electron

density and elongates the sCH bond. The positively charged and weakened sCH bond

can readily accept electron density from p * of the oxygen having accepted partial

electron density from p . The sCH* orbital is lowered enough to interact with p *.

As a result, the partial electron density is promoted (pseudoexcited) from the

HOMO (p) to an unoccupied orbital (sCH * ) of alkenes. The ene reaction is a reaction in the pseudoexcitation band.

The significant role of the quasi-intermediates is in agreement with the small

deuterium isotope effects in the ene reactions (kH/kD = 1.1−2.4 for 1-methylcyclohexene

relative to the value 12.2 for 1,5-hydrogen shift of cis-1,3-pentadiene) [89].

Orfanopoulos and Stephenson [90] interpreted the results of their extensive investigation of the reaction of singlet oxygen with isotopically-labelled 2,3-dimethyl-2-butene

to support a reactive intermediate with “structural requirements not dissimilar to those

of the perepoxide”. Shuster and coworkers [91] proposed reversible formation of an

exiplex or encounter complex in the first identifiable step, followed by irreversible

conversion to a perepoxide in the rate-determining step of the ene reaction.

3.1.5 HOMO Amplitudes, Quasi-Intermediate Structures,

and Mode Selectivities

The geometrical structure of the perepoxide quasi-intermediate was suggested to

play critical roles in determining diverse selectivities of the reactions of 1O2 with

substituted olefins [92].

The HOMO amplitude of olefins determines (Sect 3.4 in Chapter “Elements of

a Chemical Orbital Theory” by Inagaki in this volume) which carbon atom attracts



40



S. Inagaki



the incoming oxygen more strongly. For example, an electron donating substituent

X enlarges the HOMO amplitude on the b carbon. This implies unsymmetrical

structure of the quasi-intermediate (Scheme 16). The b-attack is preferable. In this

case, the exocyclic tailing oxygen in the three-membered ring quasi-intermediate

cannot react with the substituents (R2, R3) on the b carbon but with R1 and/or X.

Otherwise, a [2+2] cycloaddition reaction occurs to form a dioxetane.

R2 R3



R3



O



R2

O+



Scheme 16  HOMO polarized by X deforms perepoxide quasi-intermediates



R



O



O



1



R X

HOMO

1



X



LUMO



The lone pairs on the nitrogen and oxygen atoms make a significant difference

in the chemical reactions (Scheme 17). b-Arylenamines undergo [2+2] cycloaddition reactions [93] whereas b-arylenol ethers undergo [2+2+2] cycloaddition reactions [94]. The mode selectivity was attributed [95] to the HOMO amplitude or the

p bond polarity.

Ph

N



+



1O

2



Ph



N



O



Ph



O



+

Scheme 17  HOMO amplitude controls

the selectivities of reaction modes



O



1O

2



O



OR

OR



The nitrogen lone pair enlarges the HOMO amplitude on the b carbon more than

the oxygen lone pairs or the aromatic rings since the lone pair orbital of the nitrogen

lies higher in energy. In the case of the amino substituent, the transient threemembered ring of the perepoxide quasi-intermediate may collapse at an early stage

and the incoming oxygen attacks the b carbon of the enamines. The [2+2] cycloaddition reaction results.

The alkoxy oxygen lone pairs and the phenyl group polarize the HOMO to a

similar extent in opposite directions. The HOMO polarization is not significant.

The symmetrical perepoxide structure cannot collapse at an early stage. The tailing

oxygen atom can attack the phynyl ring on the a carbon to undergo the [2+2+2]

cycloaddition reactions.

In the photooxygenation of electron-rich olefins with allylic hydrogen atoms,

ene reactivity usually dominates [96]. Nevertheless, other reactions become the

preferred reaction mode. Inagaki et al. [92] attributed the exclusive [2+2] cycloaddition



A Mechanistic Spectrum of Chemical Reactions



41



reaction of indene [96], the [2+2+2] cycloaddition reaction of diphenylmethylenecyclobutane (no ene reactions) [97], to the HOMO amplitude or to the polarized p bond.

3.1.6  Attraction by Substituents and Selectivities

Attraction of the exocyclic tailing oxygen atom with X steers the oxygen atom to

the same side of the double bond [92]. Lone pairs (Scheme 18a) on X and aromatic

rings (Scheme 18b) can attract the tailing oxygen. The reactions can take place with

X or the substituent R3 on the same side of the double bond rather than with those

(R1, R2) on the opposite side.

R2



O



1



R



R3



Scheme 18 a,b  Attraction between

1

O2 and the substituents of alkenes:

a a lone pair and b a phenyl p bond



O



O

O



X

HOMO



LUMO

LUMO



HOMO



a



b



In fact, the hydrogen abstraction in the ene reactions was experimentally

substantiated to occur from the group on the same side of the methoxy group

(Scheme 19a) [98]. The E-isomer of enol ether yielded the hydroperoxide by a

process which involves a cyclopropyl H-abstraction, whereas the Z-isomer led, via

a [2+2+2] cycloadduct, to the epoxide (Scheme 19b) in agreement with the findings

by Foote [99] cited in [92]. Recently, a similar effect of an alkenyl nitrogen functionality on the mode selectivity (and the diastereoselectivity) was found for the

reactions of singlet oxygen with enecarbamates [100], but in that case the competition occurred between the ene reaction and [2+2] cycloaddition. Such a steering

effect is exercized by allylic nitrogen [101] or oxygen [102].

3.1.7  Cis-Effect

The argument of the directing effect of lone pairs on the substituent [92] easily

extends to the alkyl cases. The orbital interaction (Scheme 20) [103] in the perepoxide quasi-intermediate suggests the stabilization occurs by the simultaneous

interaction of 1O2 with two allylic hydrogens on the same side of the alkene.

Photooxygenation of trisubstituted olefins revealed a strong preference for

H-abstraction from disubstituted side of the double bond [104, 105].

3.1.8  Hydrogen Bonding Effects

Hydrogen bonding to the pendant (tailing) oxygen (Scheme 21) in the perepoxide

quasi-intermediates controls the facial/diastereoselectivty of the ene reactions of



42



S. Inagaki

OOH

1O

2



OOH



+



OCH3



OCH3



72%



OCH3



28%



a the regioselectivity



CH3O



CH3O



1O

2



H

OOH



CH3O

1



O2

O



CH3O



O



b the mode selectivity

Scheme 19a,b  Nonbonded attraction controls the regioselectivity (a) and the mode selectivity (b)



LUMO



O



O



HOMO



the interaction in the perepoxide

quasi-intermediate



preferred hydrogen abstractions



Scheme 20  HOMO–LUMO interaction in the perepoxide quasi-intermediate for the cis-effect

and the regioselectivity (percent) of the hydrogen abstractions



singlet oxygen with allylic alcohols [106, 107] and amines [108, 109]. The allylic

alcohol exhibits a striking diastereoselectivity for the threo (S*S*) b-hydroxy

allylic hydroperoxide while its acylated derivative exhibits a modest erythro (S*R*)

diastereoselectivity.

The steering effect of the hydrogen bonding was applied to a highly diastereoselective dioxetane formation from a chiral allylic alcohol (Scheme 21) [110].



A Mechanistic Spectrum of Chemical Reactions



43



O

1



OH



H



H



O



O2



Me



O



O



H



O

OH



Me



H



Me



Scheme 21  Hydrogen bonding effects



3.1.9  Photooxygenation in Zeolites

In 1996, Ramamurthy reported that photooxygenation of 2-methyl-2-pentene was

regioselective and afforded a single allylic hydroperoxide product (Scheme 22) [111].

The result can be explained in terms of the complexation of the cation in the zeolite

with the tailing oxygen in the perepoxide quasi-intermediate (Scheme 22) [112–114].

The steric interaction keeps the large substituent (ethyl group) away from the zeolite

framework. Hydrogen abstraction occurs on the side of the double bond opposite to

the large substituent or from the methyl group, favoring formation of the less hindered

hydroperoxide. There is no substituent geminal to the ethyl group. Perepoxide quasiintermediate plays an important role in the photooxygenation in zeolites.



1

O2

in Zeolite

NaY



O

OOH



O

H



+



Na

Zeolite framework



100%



Scheme 22  Regioselectivity in zeolites



3.2  [2+2] Cycloadditions of Bent Unsaturated Bonds

Bending of unsaturated bonds reduces the overlap between the p-orbitals and weakens

the interaction. The p orbital lies high in energy and the p * orbital lies low. Bent

unsaturated bonds are electron acceptors as well as donors. The energy gap between

p and p * is small. Bent unsaturated bonds are readily pseudoexcited to undergo

[2+2] cycloaddition reactions.

3.2.1  Reactions of Benzyne

Benzyne shares a feature with 1D▵g O2 in the [2+2] cycloaddition reactions. The

HOMO–LUMO interaction prefers the three-centered interaction (Scheme 4) [115].

This is in agreement with the calculated reaction path [116].



44



S. Inagaki



The 2+2 cycloadditions of benzyne to cis- and trans-propenyl ether gave cis- and

trans-benzocyclobutanes as the main products, respectively [117, 118]. Stereospecific

[2+2] cycloaddition reactions were observed between the benzyne species generated

by the halogen–lithium exchange reaction of ortho-haloaryl triflates and the ketene

silyl acetals (Scheme 23) [119].



X



Y



Y



X



+

R13SiO



OR2



R13SiO



R2O



OR



OR



Scheme 23  Stereospecific [2+2] cycloaddition reaction of a benzyne



3.2.2  Reactions of Cycloalkynes

Reactions of cyclopentyne with alkenes gives [2+2] cycloadduct with complete

retention of stereochemistry (Scheme 24) [120]. Laird and Gilbert observed the

expected [2+2] cycloadduct along with the polycyclic adduct in the reaction of

norbornyne with 2,3-dihydropyran (Scheme 24) [121], and located a cyclopropylcarbene intermediate [122].



R



R



+

R



O



R



+



O



O



O



Scheme 24  [2+2]Cycloaddition reactions of cycloalkynes



3.3  [2+2] Cycloadditions of Ketenes

Ketenes have cumulative bonds and can undergo [2+2] cycloaddition reactions

across C=C and C=O bonds. Interestingly, most of the products obtained are

cyclobutanones rather than oxetanes. Thermal [2+2] cycloaddition reactions in the

pseudoexcitation band occur between electron donors and acceptors. Alkenes are

donors while ketenes are acceptors. In contrast to the experimental observations,



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

3 [4+2] Cycloadditions on Surface

Tải bản đầy đủ ngay(0 tr)

×