Cyclobutanes formation

As final examples, the intramolecular cyclopropane formation from cycloolefins with diazo groups (S.D. Burke, 1979), intramolecular cyclobutane formation by photochemical cycloaddition (p. 78, 297f., section 4.9), and intramolecular Diels-Alder reactions (p. 153f, 335ff.) are mentioned. The application of these three cycloaddition reactions has led to an enormous variety of exotic polycycles (E.J. Corey, 1967A). 

Fluorinaied dienophiles. Although ethylene reacts with butadiene to give a 99 98% yield of a Diels-Alder adduct [63], tetrattuoroethylene and 1,1-dichloro-2,2-difluoroethylene prefer to react with 1,3-butadiene via a [2+2] pathway to form almost exclusively cyclobutane adducts [61, 64] (equation 61). This obvious difference in the behavior of hydrocarbon ethylenes and fluorocarbon ethylenes is believed to result not from a lack of reactivity of the latter species toward [2+4] cycloadditions but rather from the fact that the rate of nonconcerted cyclobutane formation is greatly enhanced [65]... 

For the 2-1-2 pathway the FMO sum becomes (ab — ac) = a b — c) while for the 4 -I- 2 reaction it is (ab-I-ab) — a (2b). As (2b) > (b — c), it is clear that the 4 + 2 reaction has the largest stabilization, and therefore increases least in energy in the initial stages of the reaction (eq. (15.1), remembering that the steric repulsion will cause a net increase in energy). Consequently the 4 - - 2 reaction should have the lowest activation energy, and therefore occur easier than the 2-1-2. This is indeed what is observed, the Diels-Alder reaction occurs readily, but cyclobutane formation is not observed between non-polar dienes and dieneophiles. 

Figure 15.12 Orbital correlation diagram for cyclobutane formation...

After the discovery of the photopolymerization of 2,5-DSP crystals, several types of photoproducts were found, not only the linear polymers, but some other derivatives, e.g. the V-shaped dimer or cyclophane (Hasegawa and Hashimoto, 1992). The photopolymerization occurs in a step-growth mechanism by cyclobutane formation between the excited olefin and the olefin in the ground state. 

The modification of molecular conformation from the highly strained non-isolable dimer molecule to the V-shaped dimer molecule (6 OPr-dimer) is explained in terms of relaxation of the strain energy due to the bond angle in the non-isolable dimer, which accumulated during the cyclobutane formation. Therefore, strictly speaking, the process going from the non-isolable dimer into the V-shaped dimer (6 OPr-dimer) is not a... 

The photochemical behaviour of 7 OEt is the first example in which the reaction of achiral molecules in an achiral crystal packing does not occur at random but stereospecifically, resulting in a syndiotactic structure. As no external chiral catalyst exists in the reaction, the above result is a unique type of topochemical induction , which is initiated by chance in the formation of the first cyclobutane ring, but followed by syndiotactic cyclobutane formation due to steric repulsions in the crystal cavity. That is, the syndiotactic structure is evolved under moderate control of the reacting crystal lattice. 

The mechanism of oxetane formation is similar to the one discussed for cyclobutane formation in chapter 4.3.3. The 1,4-diradicals can be efficiently trapped with molecular oxygen. The resulting 1,2,4-trioxanes are interesting synthetic intermediates (4.81) 495>. 

III. CYCLOBUTANE FORMATION A. Copper(l) Triflate Controlled Reactions... 

The irradiation of 1,4-quinones in the presence of olefins can also give oxetanes and/or cyclobutanes. For example, with cyclooctene, 1,4-benzoquinone gives good yields of the oxetane,75 while chloranil can give both the oxetane and/or the cyclobutane products, depending upon the olefin concentration.75,76 A plausible explanation for this concentration effect is that the oxetane (formed when the olefin is dilute) may be a reaction of the triplet, while in excess olefin, cyclobutane formation can compete with intersystem crossing for the singlet.71... 

Cyclobutane formation via light-induced [2 + 2] cycloaddition is probably one of the best studied photochemical reactions and has been reviewed thoroughly up to 1972 (Houben-Weyl, Vols. 4/5 a and 4/5 b). The most important types of C —C double-bond chromophores undergoing such reactions arc alkenes, 1,3-dienes, styrenes, stilbenes, arenes, hetarenes, cycloalk-2-enones, cyclohexa-2,4(and 2,5)-dienones, 1,4-benzoquinones, and heteroanalogs of these cyclic unsaturated carbonyl compounds. For p notocyciodimerizations see Houben-Weyl, Vol. 4/5 a, p 278 and for mixed [2 + 2] photocycloadditions of these same chromophores to alkenes see Section 1.3.2.3. 

Siloxanes 1 (R = Me, Ph synthesized from 3-phenylallyl alcohol and dimethyl- and diphenyldichlorosilane, respectively) on photolysis afforded one cyclobutane product, the all-p-am-isomer 2, in almost quantitative yield.2 Photolysis of 3-phenylallyl alcohol under the same conditions resulted in very little cyclobutane formation. 

Virtually all reactions involving cyclobutane formation via cycloaddition of a cumulene to another C —C double-bond system involves excitation of this latter moiety, e.g. an enone or a quinone, and not of the allene or ketene itself.1 Earlier examples of such reactions have been discussed in Houben-Weyl, Vol. 4/5 b, pp 926 931. 

Photochemical reactions of quinones with allenes have also been studied and in some cases cyclobutane formation occurs, although in competition with products derived from attack of the allene on the carbonyl oxygen. Thus, photocycloaddition of tetramethyl-l,4-benzoquinone with 1,1-dimethylallene affords the four-membered carbocycle 6 in good yield.12... 

MINDO/3 calculations of cyclobutane formation reveal a large singlet-triplet splitting of 10 kcal - mol-1 in the cisoid butyl 1,4-diradical, which might be due to a radical-radical interaction.79 The results of a more recent SINDOl calculation are also in agreement with a nonconcerted cleavage of cyclobutane via a diradical pathway.80 Additional quantitative evidence for the 1,4-diradicals has been obtained by thermochemical studies.81... 

Photocycloadditions of higher order than 2 + 2) are sometimes encountered, but they are not so general as the (2 +2) reactions. Often they arise in reactions that occur by way of radical cations 2.83), when electrophilic attack on an aromatic ring may divert the reaction from cyclobutane formation, or in those that are promoted... 

Oxetane formation is presumed to occur via the singlet exciplex however, excitation of the ground state charge-transfer complex may be necessary in order for the formation of 39 to compete with the rapid isomerization of c-1. The factors which favor oxetane versus cyclobutane formation in this reaction are not understood. 

Analogously, the thermal formation of fused strained tricycles 77 can be rationalized by a mechanism which includes an exocyclic diradical intermediate 80 through an initial carbon-carbon bond formation, involving the proximal allene carbon and the internal alkene carbon atom (path C, Scheme 28). The alternative pathway leading to tricyclic 2-azetidinones 77 is proposed in path D (Scheme 28). This proposal involves an endocyclic diradical intermediate 81 arising from the initial attack of the terminal olefinic carbon onto the central allene carbon. The final ring-closure step of the diradical intermediates account for the cyclobutane formation. 

Scheme 9.13 Loss of stereochemical integrity in cyclobutane formation from TCNE and c/s-propenyl methyl ether.

Photoreaction of cyanonaphthalenes with tetramethylethylene leads to cyclobutane formation in nonpolar solvents via exciplex intermediates, whereas in polar media electron transfer occurs [115-117], 1-Azetine is formed in the photoreaction of highly electron-donating alkene with 1-cyanonaphthalene [118]. 

Most of the bimolecular absolute asymmetric syntheses are limited to 2+2 cyclobutane formation or polymerization of olefins. Koshima et al. reported a unique example of bimolecular reaction whereby acridine 20 and diphenylacetic acid are assembled in a 1 1 molar ratio by hydrogen bonding, and crystallized in a chiral space group, P2i2i2i.[18] Irradiation of the crystals caused stereospecific decarboxylating condensation to give chiral 21 in 33-39% ee. 

Cyclobutanes are usually more difficult than cyclopropanes to prepare by cyclization. Although their ring strain is as high as that of cyclopropanes, more bonds must assume a suitable conformation for cyclobutane formation, resulting in a higher entropic barrier (Table 9.1). 

Chesick, J. P. Cyclobutane formation from mercury-photosensitized reactions of ethylene. J. Amer. chem. Soc. 85, 3718 (1963). 

In nonpolar solvents, the cyclobutane 44 is the only product upon irradiation of trans-stilbene/dimethyl fumarate mixtures. Exciplex emission, the detection of a weak ground state complex, as well as the stereospedfity of the cyclobutane formation support the proposed mechanism outlined in Eq. (26). 

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