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A domino fashioned allylic rearrangement, intramolecular decarboxylative coupling, propargyl-allene rearrangement and diradical 1,3-dien-5-yne cycloaromatization of aryl propiolates of the Baylis-Hillman alcohols in the presence of catalytic Pd(PPh 3 ) 4 and DBU leading to the formation 4-benzylnaphthoates has been described. Interestingly, the meta -substituted propiolates ( 1i – t ) provided sterically hindered cyclization products ( 12 – 23 ) via the unusual ortho -selective cycloaromatization; while the meta -alkoxy ( 1i – m , 1r – t ) group provided predominantly the ortho -selective 1,3-dien-5-yne cycloaromatization products, meta -Me group ( 1 n – q ), products formed both via ortho -selective cyclization and para -selective cyclization products in comparable ratio. A novel mechanism involving diradical species has been proposed to explain the unusual ortho -selectivity in 1,3-dien-5-yne cycloaromatization. Preference for sterically hindered ortho -selective cyclization may be attributed to the formation of a transition state radical possessing more stable ortho -methoxy cyclohexadiene radical moiety (driving force). [ABSTRACT FROM AUTHOR]

An Aza-Wittig reaction of 1,1′-bis(triphenylphosphoranylidenamino)-ferrocene with 3-isocyanato-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-1-oxyl and subsequent intramolecular [2+2]-cycloaddition of the bis(carbodiimide) intermediate was used for the synthesis of a 1,3-diazetidine-2,4-diimine moiety bearing two radical groups and bridging the cyclopentadienyl (Cp) rings. According to single-crystal X-ray data, the planar 2,4-diimino-1,3-diazetidine moiety adopts a perpendicular orientation with respect to the eclipsed Cp rings. One of the radical groups lies almost within the plane of the four-membered 1,3-diazetidine ring, while the other has a conformationally preferred orientation with dihedral angles ±62.3°. The two Cp rings are constricted by the 1,3-diazetidine-2,4-diimine moiety so that the angle between their planes is 11.0°. The synthesized diradical contains ferrocene and nitroxide redox signaling units that can be oxidized step-by-step at E 1/2 = 0.26 and 0.48 V ( vs Fc/Fc + ), respectively. Electron spin resonance spectroscopy revealed a moderate exchange interaction (| J | ∼ a N ) between the two nitroxide radical moieties and the following values of zero-field splitting parameters: ∣D∣ = 3.5 mT and E/D = 0 were obtained. These data were in agreement with density functional theory calculations. The newly developed approach to multispin systems may be interesting for the construction of weakly coupled rigid polyradicals for quantum technologies, the molecular design of magnets, and the creation of ferrocene-based electron-paramagnetic-resonance–active chemical sensors. [ABSTRACT FROM AUTHOR]

Bis-2,5-dimethylene-2,5-dihydrothiophenes have been generated in the gas-phase by flash vacuum pyrolysis (FVP) of diester precursors. These thiophene-based bis-( p -quinodimethanes) are shown to undergo reactions leading to macrocycles. The conversions are consistent with a mechanism involving cyclic diradical intermediates followed by disproportionation of the radical centers. [ABSTRACT FROM AUTHOR]

Abstract: There is evidence that the crystalline-state thermochromic behavior of dispiropyracene acr-1 with an ultralong C–C bond [1.791(3)Å at 413K, 1.771(3)Å at 93K] might be due to thermal generation of a bond-dissociated triplet diradical. The C1–C2 bond lengths (d) in the newly prepared 1,1,2,2-tetraarylpyracenes 1a–e [1.717(4)–1.761(4)Å at 113/123/153K] are also much larger than the standard value for C(sp3)–C(sp3) (1.54Å). The fact that there is no correlation between d and the radical-stabilizing parameter (σ•) provides evidence that there is no contribution from a crystallographic artifact caused by contamination of the diradical in the crystal of 1 at room temperature or below. Further indication for the covalent nature of the ultralong C–C bond in tetraphenylpyracene 1c was attained by Raman spectroscopy, showing the far red-shifted C1–C2 stretching vibration (638cm−1) and by the theoretical prediction of very large vertical singlet–triplet energy gap (58.3kcalmol−1 at the UB3LYP/6-31G∗ level). However, at elevated temperatures bond dissociation might occur forming a triplet diradical. [Copyright &y& Elsevier]