Cyclopentane-1,3-diyl

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... 

Calculations and Experiments on the Ground States of Cyclopentane-1,3-diyls. Our calculations on 2,2-difluoropropane-l,3-diyl (10) led to another prediction that was subsequently tested by us experimentally and confirmed. Both (2,2)CASSCF and SD-CI calculations predict the triplet to be the ground state of propane-1,3-diyl (8) ° and cyclopentane-1,3-diyl (16a), " a cyclic derivative of 8. Prior experiments by Gloss had shown that the ground state of 16a is, in fact, a triplet and Dougherty, Adam, Wirz, and their co-workers found the same to be true for the 1,3-diphenyl derivative (16b) (Fig. 22.12). ... 

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.

The photochemically generated cyclopentane-1,3-diyl diradials (87) were part of a study of spin delocalization through the EPR Z)-paramctcr. These biradicals were a model system for cumyl and benzyl radicals and experimental data were combined with MO calculations to map the electronic effects on D by varying the aromatic substituent (Ar = heterocycle).218 This parameter was also measured for a related series of... 

The phenomenon of such bimodal lifetime distribution proposed for reaction 1 on the basis of direct quasiclassical trajectory calculations were tested experimentally with the reaction of diaza-[2.2.1]bicycloheptane to [2.1. Ojbicyclopentane [Equation (2)].6 8 Experimental study on reaction 2 showed that the exo isomer 5x is formed favorably over the endo isomer 5n by about 3 1 in the gas phase. One explanation for the preferential formation of 5x invokes a competitive concerted and stepwise mechanism the concerted pathway directly from 4 to 5 gives 5x with the inversion of configuration at the carbon from which N2 is departing, whereas the stepwise pathway goes through the radical intermediate and leads to both 5x and 5n in equal amount. Alternatively, the product stereochemistry can be rationalized by dynamic matching between the entrance channel to the cyclopentane-1,3-diyl radical intermediate and the exit channel to bicyclo[2.1. Ojpentane as was assumed for reaction 2. 

Fragmentation of 2,3-diazabicyclo[2.2.1]hept-2-enes via photoinduced electron transfer offers a convenient route to cyclopentane-1,3-diyl radical cations, in turn undergoing interesting and selective rearrangements [42]. 

Case Study 5.2 Mechanistic photochemistry - cyclopentane-1,3-diyl biradicals... 

Figure5.17 MO interaction diagram for cyclopentane 1,3 diyl (CP). The symmetry labels a and s refer to reflection through the plane defined by the CH2 group at position 2...

IV. The Influence of Heteroaryl Substituents on the D Parameter of Cyclopentane-1,3-diyl Triplet Diradicals, 238... 

The most prominent representatives of sufficiently persistent diradical intermediates are the cross-conjugated (non-Kekule type) trimethylene-methanediyl (1) [10] diradical (e.g., the Berson diradical 2 as its cyclic derivative [11]), or the diradicals 3 and 4 [12] and the localized triplet cyclobutane-1,3-diyl (5) [13], cyclopentane-1,3-diyl (6) [14 16], or 4,5-diazacyclopentane-l,3-diyl (7) [17] diradicals (Fig. 1). We have focused our attention on the localized type, namely, the structurally related and cyclo-pentane-annellated cyclopentane-1,3-diyl triplets of 6 [Eq. (1)], specifically... 

A typical first-order derivative spectrum of randomly oriented triplet diradicals is given schematically in Figure 4, which shows how readily the D and E values can be read out. The six high-field signals belong to the Ams = +1 transitions, whereas the forbidden Ams = 2 transition appears at half-field. The latter is characteristic for triplet diradicals and serves as a criterion for their unambiguous assignment. Illustrative examples are the triplet cyclobutane-1,3-diyls (5) [13], the cyclopentane-1,3-diyls (6) [14 16], and the 4,5-diazacyclopentane-1,3-diyls (7) [17], whose zfs parameters are summarized in Table 1. 

Of the known triplet states, the cross-conjugated non-Kekule diradicals are classified as delocalized triplet diradicals (e.g., 14). In contrast, the radical centers in localized triplet diradicals (e.g., 5-7) are not joined through a n system. For the localized 1,3-diphenyl-substitutcd cyclopentane-1,3-diyl triplet diradicals 6b and c, a planar five-membered ring with coplanar arrangement of the aryl groups is expected to allow the maximum contribution of the benzylic resonance stabilization, which is worth about 50-54 kJ mol 1 [28], For such aryl-substituted triplet diradicals (e.g., 9-12), the magnetic dipole interaction of the two uncoupled spins is given by Eq. (6). 

The aforementioned analysis makes it evident that no special electronic effects (captodative stabilization, spin polarization, etc.) play a significant role in the localized triplet diradicals 9-11. Nevertheless, the question arises whether the electronic substituent effects in such triplet diradicals are additive, that is, whether the substituents on the two aryl groups act independently of one another. This feature was tested by comparison of the AD values of the unsymmetrical monosubstituted triplet diradicals 9 with those of the corresponding symmetrical disubstituted derivatives 10. Indeed, a linear correlation (Fig. 8) with a slope of nearly one-half (m = 0.558) unequivocally establishes that the electronic effects on the D parameter of localized triplet diradicals are additive [7], Thus, the localized triplet diradicals 9-11 may be considered to be electronically a composite of two independent (except for dipolar interactions) cumyl-type monoradicals, embedded in the molecular framework of the planar cyclopentane- 1,3-diyl ring system. This fact allows us to assess electronic substituent effects on the cumyl monoradicals 14 from the experimentally determined D parameter of the cyclopentane-1,3-diyl triplet diradicals 9-11 through the changes of the spin densities. 

Spin densities (p) are theoretical quantities, defined as the sum of the squared atomic orbital coefficients in the nonbonding semi-occupied molecular orbital (SOMO) of the radical species (Hiickel theory). For monoradical species, the spin density is connected to the experimental EPR hyperfine coupling constant a through the McConnell equation [38]. This relation provides the opportunity to test the spin density dependence of the D parameter [Eq. (8)] for the cyclopentane-1,3-diyl triplet diradicals 10 by comparing them with the known experimental hyperfine coupling constants (ap) of the corresponding substituted cumyl radicals 14 [39]. The good semiquadratic correlation (Fig. 9) between these two EPR spectral quantities demonstrates unequivocally that the localized triplet 1,3-diradicals 9-11 constitute an excellent model system to assess electronic substituent effects on the spin density in cumyl-type monoradicals. 

IV. THE INFLUENCE OF HETEROARYL SUBSTITUENTS ON THE D PARAMETER OF CYCLOPENTANE-1,3-DIYL TRIPLET DIRADICALS... 

Besides spin densities, the D parameter also depends on the interspin distance dAB [Eq. (8)]. Thus, structural effects on the D parameter may be probed through variations of the interspin distance dAB at constant spin densities pA and pB. The influence of such structural features on the D parameter is most prominently displayed by the two localized diradicals 5d [13] and 6b [15] (Table 5) for which the 1,3-diphenyl substitution provides essentially constant spin densities at the radical sites. Thus, any changes in the D parameter of these triplet diradicals should derive from geometrical effects on the interspin distance dAB. Optimized geometries from MO calculations for the cyclobutane-1,3-diyl (5d) gave an interspin distance of 208.5 pm and for the cyclopentane-1,3-diyl (6b) a value of 237.1 pm [64], which accounts nicely for the observed trends in the experimental D values that is, a lower D value (Table 5) for the latter due to the inverse d3 dependence [Eq. (8)]. 

Cyclopentane-1,3-diyl Formation from 2,3-Diazabicyclo[2.2.1 ]hept-2-ene... 

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