Cyclopentane-1,3-singlet

Steady-state kinetics. The cycloaddition reaction between the singlet ground state of 2-isopropylidene cyclopentane-1,3-diyl ( = S ) with acrylonitrile (A) is believed to occur by way of a biradical intermediate (BR),17... 

In alicyclic hydrocarbon solvents with aromatic solutes, energy transfer (vide infra) is unimportant and probably all excited solute states are formed on neutralization of solute cations with solute anions, which are formed in the first place by charge migration and scavenging in competition with electron solvent-cation recombination. The yields of naphthalene singlet and triplet excited states at 10 mM concentration solution are comparable and increase in the order cyclopentane, cyclohexane, cyclooctane, and decalin as solvents. Further, the yields of these... 

Figure 22.12. Cyclopentane-1,3-diyl (16a) has been both calculated " and found to have a triplet ground state, and the 1,3-diphenyl derivative (16b) has also been found to have a triplet ground state. In contrast, calculations on 16c and 16g have predicted both diradicals should have singlet ground states, and experiments on derivatives of 16d, 16e, and 16f found these three diradicals do, in fact, have singlet ground states.

Treatment of cyclopentane with FSOsH-SbFj in SO2CIF below —10°C yields the cyclopentyl cation [49] (Olah and Lukas, 1968). The H-nmr spectrum showed only one singlet at 8 4.75 which was unchanged down to —130°C. Rapid 1,2-hydride shifts around the ring (42) were suggested to be... 

Reaction with norbornene. Norbornene (1) has been reported to be inert toward singlet oxygen. However, Jefford and Boschung find that it does react, although slowly, to give (2) and (3) as major products. One is cij-cyclopentane-... 

Unstable trimethylene cyclopentane 190, generated in situ from 189 by a Wittig reaction, was trapped with PTAD to give a 25% yield of the ( ) isomer 191 (88CB2127). cw-Methoxystyrene easily reacts with singlet... 

The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone. Calculations have shown that decarbonylation of cyclobutanone occurs from the nji triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state.Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed. Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5). ... 

Insertion into C-H bonds is more probable than insertion into C-C bonds. Insertion into C-C bonds does not appear to occur at all. For example, photolysis of diazomethane in cyclopentane at —75 °C produced only methylcyclopentane, with cyclohexane not being observed. Singlet carbenes are thought to add to C- H bonds by a concerted process, while triplet carbenes can produce net addition through hydrogen abstraction and then recombination of the alkyl radicals. It was found that o-(2-endo-norbornyl)phenylcarbene inserts into the 3-position in such a way as to give a trans-junction (Scheme 5.47). 

Paquette s synthesis of dodecahedrane, which is briefly summarized in the Scheme (overleaf), features the homo-Norrish photochemical bond-formation of a ketone carbonyl carbon to a neighbouring carbon atom, e.g. (l)->(2), (3) -> (4), and (5) -> (6), to stitch together many of the cyclopentane rings in the molecule, with the final ring being closed by dehydrogenation of (7). Dodecahedrane is a crystalline solid which does not melt below 450 °C. As anticipated, the molecule shows sharp singlets in both its and n.m.r. spectra, at S 3.38 and S 66.93 p.p.m. respectively. 

There have been reports of a number of reactions of CPNA 73 that result in cleavage of the strained C—C o-bond under thermal conditions. The formed reactive intermediate 74 undergoes insertion and cheletropic [1+2]-, [3+2]-, and [3-1-4] cycloaddition reactions under thermal conditions (Scheme 6.13a). The reactivity profiles reported to date are consistent with such a a-delocalized singlet species 74 that can react either as a 1,1- or as a 1,3-dipole. Moreover, the 2-alkylidenecyclopropanone acetal 75 derived from a CPNA 76 is a useful precursor of dialkoxy trimethylenemethane (TMM) 77. MUd thermolysis of 75 in the presence of an electrophile generates 77, which undergoes a [3+2] cycloaddition to form cyclopentane derivative 78 (Scheme 6.13b). These results were reviewed by Nakamura and coworkers [32]. 

Powell, H.K. and Borden, W.T, CASSCF and CASPT2N calculations predict singlet cyclopentan-2-one-l,3-diyl is stable toward closure to bicyclo[2.1.0]pentan-5-one,/. Org. Chem., 60,2654-2655, 1995. 

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