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B. Photocycloaddition of Copper-Tethered Alkenes

B. Photocycloaddition of Copper-Tethered Alkenes

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Scheme 22 Octamethylcubane formation.

Scheme 23 Synthesis of homosecohexaprismane structure using 2ỵ2 photocycloaddition.

Scheme 24 Pyridine based cubane-like synthesis.


Cubane-like Structures

Other approaches to the novel multicyclic strained systems have been

reported [36]. Chou et al. have demonstrated that novel cage structures can

be obtained using the alkene ỵ alkene methodology (see Sch. 23). More

recently an aza analogue has been reported. This example is shown in

Sch. 24.


High Energy Compounds

The imidazolinone shown in Sch. 25 has been dimerized by irradiating in

acetone [37]. The bis-urea substituted cyclobutane that was obtained has

been further modified to produce several high energy explosives. The most

energetic product is the nitrourea shown. It is hydrolytically and thermally

stable, but sensitive to impact.

Copyright © 2005 by Marcel Dekker

Photocycloaddition of Alkenes

Scheme 25

Intermolecular photocycloaddition of imidazolinone.

Scheme 26

Tethered norbornene photocycloaddition.

Scheme 27

Ipsdienol photocycloaddition.



Cage Structures

Norbornene structures can be linked by long chain diesters. The copper

catalyzed photocycloaddition of these alkenes gives rise to the cyclobutane

shown in Sch. 26 [38]. The intramolecular reaction is stereoselective.



A recent publication described the carbon tethered 2ỵ2 photocycloaddition

of the terpene ipsdienol [39]. The pseudoterpene photoproduct (see Sch. 27) is

formed as a single stereoisomer. The authors suggest that the hydroxyl group

avoids the developing gem-dimethyl group due to steric interactions.

They propose the transition state shown although the reaction is triplet

sensitized and presumably proceeds in a step-wise fashion. The monoterpene,

Copyright © 2005 by Marcel Dekker



Scheme 28 Grandisol synthesis.

b-necrodol, has also been synthesized using an alkeneỵalkene photocycloaddition [40]. In this case the authors use a copper catalyzed approach.


Insect Pheromone

Grandisol has previously been synthesized using a number of photochemical

procedures. Two recent reports demonstrate the utility of the copper

catalyzed photocycloaddition (see Sch. 28) in the synthesis of the four

member ring observed in the boll weevil pheromone. The first example [41] is

a chiral synthesis and the second pathway [42] provides the racemic product.





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Copyright © 2005 by Marcel Dekker

Photocycloaddition of Alkenes























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Copyright © 2005 by Marcel Dekker


Di-p-Methane Rearrangement

Diego Armesto, Maria J. Ortiz, and Antonia R. Agarrabeitia

Universidad Complutense, Madrid, Spain



The chemistry of alkenes is one of the pillars of organic chemistry. These

substrates undergo many synthetically useful transformations such as

electrophilic and nucleophilic additions, oxidations, reductions, [1ỵ2] and

[2ỵ2]-cycloadditions, metathesis, polymerizations [1]. This chemistry also

applies to dienes and higher polyenes. Conjugated dienes undergo specific

reactions, such as conjugated additions and [4ỵ2]-cycloadditions (DielsAlder reactions) that have found important synthetic applications [1].

However, the double bonds of nonconjugated dienes behave as two separate

alkenes in most reactions, an exception being 1,5-dienes that undergo

Cope and Claisen rearrangements [1]. In particular, C–C double bonds in

1,4-dienes tend to react as two separated entities in most of the reactions

mentioned above, although a few specific reactions of compounds containing the 1,4-diene have been observed in the chemistry of some natural

products such as polyunsaturated fatty acids and prostaglandins.

The photochemical reactivity of alkenes is also of great interest [1,2].

Studies in this area have led to an expansion of the synthetic utility of these

substances. Typical photochemical reactions include cis-trans isomerizations, inter- and intramolecular cycloadditions, photooxidations, and

electrocyclic ring opening and closing of conjugated dienes and polyenes.

Many of these photoreactions have thermal counterparts. In contrast,

1,4-dienes can undergo a unique photochemical transformation affording

vinylcyclopropanes, as shown schematically in Sch. 1.

This process is known as the di-p-methane (DPM) rearrangement [3].

One of the first examples of this reaction was reported in 1966 by


Copyright © 2005 by Marcel Dekker


Armesto et al.

Scheme 1

Scheme 2

Zimmerman et al. in the triplet-sensitized irradiation of barrelene 1 that

yields semibullvalene 2 (Sch. 2) [4]. Zimmerman also recognized the

generality of the process and proposed a mechanistic interpretation for

the reaction that it is still commonly accepted [5]. Since then, a large number

of studies by Zimmerman and his coworkers, and by other research groups

worldwide, have demonstrated that the rearrangement is very general and

that it usually occurs with a high degree of regioselectivity and stereochemical

control [3].

In many instances, high chemical and quantum yields have been

observed for this process. The reaction also takes place when an alkene unit

is replaced by an aryl ring [3]. A representative example of the aryl-di-pmethane version of the rearrangement is found in the conversion of 3 to the

cyclopropane 4, in 93% yield (Sch. 3) [6]. The large number of different

cyclopropane derivatives that can be obtained by using the DPM

rearrangement reaction is remarkable, making the reaction particularly

useful from a synthetic point of view [3].

The di-p-methane rearrangement is not restricted to 1,4-dienes. Other

1,4-unsaturated systems also undergo similar photoreactions. The extension

of the rearrangement to b,g-unsaturated ketones occurred almost simultaneously with the discovery of the DPM rearrangement [7]. These compounds

react from their triplet exited state to give the corresponding cyclopropylketones regioselectively, in what is known as the oxa-di-p-methane (ODPM)

version of the rearrangement. The large number of studies which have been

carried out on the photochemistry of b,g-unsaturated ketones show that the

ODPM reaction usually occurs with a high degree of diastereoselectivity and,

under special conditions, even enantioselectivity [3a,3e,8]. Therefore, it is not

surprising that the ODPM rearrangement has been applied as the key step in

the synthesis of natural products and other highly complex molecules that

Copyright © 2005 by Marcel Dekker

Di-n-Methane Rearrangement


Scheme 3

Scheme 4

are difficult to obtain by alternative preparative routes [8e]. The reaction has

been extended recently to b,g-unsaturated aldehydes that have been

considered for many years unreactive in this mode [9]. Because of its

important synthetic applications, the ODPM rearrangement is discussed in a

separate Chapter.

In spite of the large number of studies carried out on the DPM and the

ODPM rearrangements since 1966, ten years elapsed before the reaction was

extended to other 1,4-unsaturated systems, particularly to C–N double bond

derivatives. The first example of a 1-aza-di-p-methane (1-ADPM) rearrangement was reported by Nitta et al. in a study on the photoreactivity of

the tricyclic oximes 5 [10]. Direct irradiation of compound 5a brought about

the formation of the DPM product 6 and the 1-ADPM derivative 7a, in the

first example of competition between these two processes. However, the

methyl substituted derivative 5b yielded the 1-ADPM photoproduct 7b,

exclusively (Sch. 4) [10a].

The first 1-ADPM rearrangement of an acyclic derivative, reported

five years later, was observed in studies of the sensitized irradiation of the

b,g-unsaturated imine 8 that yielded the corresponding cyclopropylimine 9

exclusively (Sch. 5) [11]. This photoproduct undergoes hydrolysis during

isolation to generate the corresponding cyclopropane carbaldehyde 10.

A series of studies in this area since then have demonstrated that the

1-ADPM rearrangement is as general as the DPM and ODPM counterparts

Copyright © 2005 by Marcel Dekker


Armesto et al.

Scheme 5

Scheme 6

and that it can be used to regioselectively form hydrolytically stable C–N

double bond containing cyclopropylimine derivatives, such as oxime esters,

acylhydrazones, and semicarbazones [3e,12].

For more than 30 years, di-p-methane processes were limited to the

three different types of b,g-unsaturated systems mentioned above. However,

recent studies have shown that the rearrangement is applicable to 2-aza-1,4dienes [13]. Thus, triplet-sensitized irradiation of compound 11 affords the

cyclopropylimine 12 and the N-vinylaziridine 13 (Sch. 6). The photoreaction

of 11 represents the first example of a 2-aza-di-p-methane rearrangement

(2-ADPM) that brings about the formation of a heterocyclic product. The

reaction has been extended to other 2-azadienes that yield the corresponding

cyclopropylimines regioselectively [13b].

A few examples of DPM rearrangements in b,g-unsaturated compounds in which the methane carbon is replaced by a boron or silicon atom

have been described. However, the di-p-borate [14] and di-p-silane [15]

alternatives of the rearrangement do not have the scope and synthetic

applications of the other alternatives mentioned before.

Those interested in seeking detailed information about covering

different versions of the rearrangement reaction should consult the many

reviews which have been published over the years [3,8,12].



The classical di-p-methane rearrangement can be considered as a 1,2 shift of

one of the p units to the other followed or accompanied by ring closure to

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Di-n-Methane Rearrangement


Scheme 7

form a cyclopropane ring. Concerted and biradical mechanisms have been

proposed to account for the rearrangement. Studies on the reaction

stereochemistry have shown that the configuration at C-1 and C-5 of the

1,4-diene is retained [16] while the configuration at C-3 (i.e., the

sp3-hybridized carbon) is inverted during rearrangement to the vinylcyclopropane [17,18]. These stereochemical features can be explained considering

that the rearrangement takes place via a [p2aỵp2aỵs2a] concerted transition

state. However, a biradical mechanism as shown in Sch. 7 for compound

14 has been normally used to justify the rearrangement because it provides

a convenient way to understand the regioselectivity and the influence of

substitution observed for the reaction [19]. Thus, compound 14 affords

the vinylcyclopropane 17 exclusively.

The alternative regioisomer 19 is not formed in this reaction. This

regioselectivity can be easily explained considering that irradiation of 14

generates the cyclopropyldicarbinyl biradical 15. Ring opening in 15 by

breaking of bond a affords the more stable 1,3-biradical 16, the precursor of

17. The alternative ring opening in 15 by breaking of bond b does not occur

because biradical 18 is less stable than 16. This interpretation permits a

prediction of the regioselectivity of the reaction. However, it should be

pointed out that the biradical species shown in Sch. 7 should not be

considered necessarily as true intermediates but they might merely represent

points along a reaction surface. The stereoselectivity observed for the

reaction can also be explained by using the biradical mechanism if the rates

of bond rotation in these species are lower that those for bond formation

and cleavage. However, the issue of whether the DPM reaction occurs

via a concerted or a biradical mechanism is still under discussion. Thus,

while theoretical calculations support the concerted 1,2-vinyl migration

[20], kinetic studies on the aryl-di-p-methane rearrangement favor the

Copyright © 2005 by Marcel Dekker


Armesto et al.

Scheme 8

conventional biradical mechanism [21]. Nevertheless, the latter mechanistic

approach is the most widely used since it depicts the reaction course in a

very simple way.

Biradical mechanisms, similar to that shown in Sch. 7, have been used

to account for the regioselectivity and the influence of substituents in the

ODPM and 1-ADPM versions of the rearrangement [8,12]. In these

instances the ring opening of the 1,4-cyclopropyl biradical intermediates

always restores the C–O and C–N double bonds, affording the corresponding cyclopropylketones and cyclopropylimines, exclusively. This

regioselectivity can be explained based on the stability of the intermediates

as shown in Sch. 8 for the 1-ADPM rearrangement of oxime acetate 20 [22].

Thus, triplet-sensitized irradiation of 20 affords the cyclopropyl

biradical 21. Ring opening of bond a in 21 gives the 1,3-biradical 22, the

precursor of the observed photoproduct 23. The alternative ring opening in

21, by rupture of bond b, does not occur because the 1,3-biradical 24 is less

stable than 22. In this instance the reaction is also diastereoselective

affording the trans-cyclopropane 23, exclusively. Similar stereoselectivity

affording predominantly or exclusively the most stable diastereoisomer has

been observed in other instances [23].

However, the regioselectivity observed in the triplet-sensitized

2-ADPM reaction (Sch. 6) is more difficult to justify based on the differences in stability of the 1,3-biradical intermediates and further studies

are necessary to understand the factors that control this novel rearrangement [13].

For more than 30 years di-p-methane rearrangements have stood as the

paradigm reactions that occur in the excited state manifold exclusively.

However, recent studies have shown that rearrangements of the di-p-methane

type can also occur in ground state of radical-cation and radical-anion

Copyright © 2005 by Marcel Dekker

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