Cycloadditions strained hydrocarbons

Reactions which formally can be classified as cycloadditions or retrocycloadditions have been observed in homogeneous solution with single electron oxidative initiation, so the observation of parallel activity on irradiated semiconductor suspensions is certainly reasonable. The first example of such a reaction involved the photocatalysis by ZnO or CdS of the ring-opening of a strained hydrocarbon which could also be opened in the dark by a single electron oxidant, ceric ammonium nitrate, Eq. (38)... 

It appears likely that transient metallacyclobutanes are involved in a variety of organic reactions which are catalyzed by transition metal complexes. Thus, cycloadditions of activated alkenes to strained hydrocarbons such as quadricyclane and bicyclo[2.1.0]pentane are catalyzed by complexes such as Ni(CH2=CHCN)2 and probably involve initial formation of a nickelacyclobutane (Scheme 2) (79MI12200). The nature of the organometallic intermediates in related metal-catalyzed rearrangements (72JA7757) and retro-cyclo-addition reactions (76JA6057) of cyclopropanoid hydrocarbons, e.g. bicyclo[n.l.O]alkanes, has been discussed. 

It is now well established that the cation radicals of unsaturated and strained hydrocarbons undergo a variety of isomerization (e.g., Scheme 18) and cycloaddition reactions with much faster rates than those of the corresponding neutral molecules [162-165]. A cation radical chain mechanism analogous to Scheme 17 was reported for one-way photoisomerization of cis-stilbene (c-S) to truws-stilbene (f-S) via photoinduced electron transfer, as shown in Scheme 18 [166], Once c-S + is formed, it is known to isomerize to t-S + [167,168]. The free energy change of electron transfer... 

Two transition metal catalyzed [27t + 2strained hydrocarbons, reacts with exo-tricyclo[3.2.1.0 " ]oct-6-ene (5) to give tetracyclo[3.3.0.0 .0 ]octane (6) in quantitative yield. The endo-isomer is unreactive. ... 

As shown in Section 2.2.2.3.1., bicyclopropylidene (1) is capable of undergoing [2-1-2] cycloadditions with electron-deficient alkenes such as diethyl fumarate under nickel(O) catalysis. The [3 -I- 2] cyclodimer and a cyclotrimer are obtained only as minor products from this reaction. In contrast, exclusive [3 -I- 2] cycloaddition can be achieved with many other substrates when palladium(O) catalysts are employed. These cycloaddition products are also produced with phosphite-modified nickel(O) catalysts, but both yields and selectivities are markedly lowered. The reactions of 1 with norbornadiene and norbornene serve as examples for the reaction with strained hydrocarbons, providing the cyclodimers 2 and 3 in 61% and 66% yield, respectively. ... 

In the presence of transition metal complexes, certain strained hydrocarbon systems are activated under mild thermal conditions and undergo characteristic transformations. Methylenecyclopropane (XXVI) (Noyori et al., 1970, 1972b), bicyclo[2.1.0]pentane (XXVII) (Noyori et al., 1971c, 1974a), and quadricyclane (XXVIII) (Noyori et al., 1975a) add to electron-deficient olefins with the aid of a nickel(0) catalyst such as bis(l,5-cyclooctadiene)-nickel(O) or bis(acrylonitrile)nickel(0). These cycloaddition reactions proceed... 

Intramolecular [2 + 2] cycloaddition also occupies a pivotal position among the methods available for the synthesis of highly strained compounds. Owing to the proximity effect, this reaction occurs easily even in sterically encumbered cases. The synthesis of one of the first representatives of exotic hydrocarbons, Dewar benzene 388 by van Tamelen, was achieved by a surprisingly short route (Scheme 2.130). The readily available Diels-Alder adduct 389 was first converted into diene 389a. The latter underwent intramolecular [2 -i- 2] cycloaddition which led to the formation of the [2.2.0] bicyclohexene framework of the key intermediate 390. 

Bicyclo[1.1.0]butanes (1 equation 1), the smallest bicyclic hydrocarbons, are highly strained but stable at ambient temperature. Under the influence of a catalytic amount of a nickel(0) complex, however, they undergo cleavage of the central bond and one of the four peripheral bonds and enter into cycloaddition reactions with electron deficient alkenes to afford allylcyclopropane derivatives. Reaction of... 

Cydoadditifms to strained bicyetie hydrocarbons. CSI reacts with simple alkenes by [2 + 2] cycloadditions. However, as the complexity of the system is increased [1+4 , [1+5], and [1+6] cycloadditions can be realized. Thus CSI reacts with tricyclo[4.1.0.0 ]heptane (1) to give, after hydrolysis, the lactam (2) as the major product. Under the same conditions the lactam (4) is obtained as the major product... 

The ubiquitous and reversible formation of radical cations in photoelectrochemical transformations allows pericyclic reactions to take place upon photocatalytic activation since the barriers for pericyclic reactions are often lower in the singly oxidized product than in the neutral precursor. For example, ring openings on irradiated CdS suspensions are known in strained saturated hydrocarbons [176], and formal [2 -I- 2] cycloadditions have been described for phenyl vinyl ether [ 177] and A-vinyl carbazole [178]. The cyclization of nonconjugated dienes, such as norbomadiene, have also been reported [179]. A recent example involves a 1,3-sigmatropic shift [180]. 

In addition to strained and unstrained hydrocarbons, as well as unsaturated esters, a variety of cyclic substrate types undergo [3-1-2] cycloaddition with methylenecyclopropanes. Some representative examples of such cyclopentaannulation reactions are discussed in this section. 

Bicyclo[3.2.0]hept-l-ene (151) can be prepared from the tosylhydrazone of norbornan-7-one via norbornan-7-ylidene 150 in 74% as a stable olefin (144). Similarly, 152 has been obtained from norborn-2-en-7-one in a yield of 67% (144a). Introduction of a further double bond leads to the strained triene 153, which has been prepared from 105b (Scheme 3). Compound 153 dimerizes readily at room temperature to a 1 1 mixture of hydrocarbons 154 (145). The most likely structures of these dimers arise from a formal [2 + 2]cycloaddition of the bridgehead double bonds of 153 with each other. Support for the intermediacy of 153 comes from trapping experiments with 1,3-diphenyl-isobenzofuran, which yield a mixture of stereoisomers via reaction with the bridgehead double bond of 153. 

Diels-Alder reaction of the highly strained alkene (48) with benzene is one of the strikingly unusual reactions. The driving force of the unusual cycloaddition would be a favorable electron transfer from benzene to the low-lying LUMO of the highly electron-deficient double bond of 48 [15]. In contrast, the corresponding hydrocarbon (50) dimerizes and polymerizes rapidly at ambient temperature (Scheme 1.43) [16]. 

The majority of cycloaddition reactions involve interactions between two (occasionally three) n systems as in the Diels-Alder reaction [equation (5.284)] or the analogous additions of allyl cations to dienes (148)-(150). Such n cycloadditions usually take place by cis addition to each conjugated system because the corresponding transition states are less strained. The reactions are then of Hiickel type (see, e.g.. Figs. 5.35a and 5.36a) and follow the same rules for aromaticity as do ordinary conjugated hydrocarbons. Some examples follow ... 

We have found that azodicarboxylates possess a unique level of reactivity in their reactions with various imsaturated hydrocarbons under on water conditions. We first noticed this phenomenon in the context of our study on strained olefins. In particular, 1,2-diazetidines such as 40 were accessed via the 2cr - - 2cr -F 2tt cycloaddition of quadricyclane (38) with dimethyl azodicarboxylate (39 Fig. 11.10). This reaction typically requires prolonged reaction times and/or elevated temperatures when carried out in an organic solvent or in the absence of solvent. However, it proceeds rapidly at room temperature or below when performed on water . In this case, the on water reaction appears to be 2—3 orders... 

A variety of photoreactions of olefins such as rearrangements of diolefins and of strained systems, 2+2 cycloadditions as well as 2+4 cycloadditions are effectively catalyzed by Cu(I) salts. Cu(I) halides (CuCl, CuBr), which are highly insoluble in water in the presence of an olefin show limited solubility in organic solvents, hydrocarbons and in particular ether. Subsequently to the use of halides the trifluoromethanesulfonate (triflate, OTf) of Cu(I) was introduced by Kochi et al. [5], which was found to be a superior reagent, due to its higher solubility and to the higher ionic character of the Cu-OTf-bond that facilitates olefin bonding. The solubility is due to complex formation, either of 1 1 or 1 2 Cu olefin-stoichiometry. In many cases complexes cannot be isolated but are formed in pre-equilibria in solution. The 1,5-cyclooctadiene (COD) complex 1, isolable as a compound and reasonably stable in solution, has been studied with respect to photoconversion of the complex itself as well as its role as a catalyst. 

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