Four-membered ring compounds containing bis(methylene)cyclobutene or tetrakis(methylene)cyclobutane moieties. Benzocyclobutadiene, benzodicyclobutadiene, biphenylene, and related compounds

Computed NMR Shielding Effects over Fused Aromatic / Antiaromatic Hydrocarbons

“…Thus the shielding effects are dominated by the moderate degree of aromaticity of the two benzene rings, with little effect of the deshielding four-membered ring, as reported by Kleinpeter using a somewhat different method [12]. X-ray data generally agree with the computed bond lengths: 1.37 to 1.43 Å range (∆d = 0.06 Å) for the six-membered ring; 1.51 Å between the benzene rings and 1.43 Å for the bonds shared with the benzene rings [24,25]. Thus, with a four-membered ring between benzene rings, the latter retain much of their aromaticity.…”

Studies in the cycloproparene series: cycloaddition reactions of diarylmethylidenecycloproparenes

“…13 In like manner, reaction of the dimethoxy 4c and bis(trifluoromethyl) 4d lead to the corresponding cyclobutarenes 11c (32%) and 11d (42%).…”

“…When heated with DPIBF in dry degassed toluene for several days 4b provides a pale yellow solid 1:1 addition product the 13 and the exocyclic olefinic bond are recorded at 1665 and 1738 cm -1 , respectively. 13 In like manner, reaction of the dimethoxy 4c and bis(trifluoromethyl) 4d lead to the corresponding cyclobutarenes 11c (32%) and 11d (42%).…”

Synthesis of Tetrahydrobiphenylene <i>via</i> Pd(0)-Catalyzed C(<i>sp</i>²)–H Functionalization

“…1) This class of compounds possesses an interesting aromaticity 2) and the aryl-aryl C-C bond can be cleaved to give metallocyclic complexes. [3][4][5] In contrast, the synthesis of partially hydrogenated biphenylenes, including tetrahydro-and hexahydrobiphenylenes has received little attention, despite these compounds showing interesting bioactivity.…”

“…[21][22][23][24][25][26][27] Recently, we have investigated Pd(0)-catalyzed benzylic C(sp 3 )-H functionalization for the synthesis of various heterocycles and tetrahydro-2H-fluorenes. [28][29][30][31][32][33][34][35][36][37][38][39][40][41] In the synthesis of oxindoles, C(sp 3 )-H activation was favorable over competitive C(sp 2 )-H activation because of the presence of the sp 2 nitrogen linkage 29) (Chart 2a). In contrast, two methyl groups were required for the synthesis of tetrahydro-2H-fluorene through C(sp 3 )-H activation in the presence of the sp 3 carbon linkage (Chart 2b).…”