“…More specifically, they found that the β-position in β-hydroxytetraline is more susceptible to electrophilic substitution than the α-site. This intriguing selectivity induced by the fused small ring has been investigated and confirmed subsequently by a number of researchers [5][6][7][8][9][10][11]. These experimental findings were supported recently by our theoretical studies [12][13][14].…”
“…More specifically, they found that the β-position in β-hydroxytetraline is more susceptible to electrophilic substitution than the α-site. This intriguing selectivity induced by the fused small ring has been investigated and confirmed subsequently by a number of researchers [5][6][7][8][9][10][11]. These experimental findings were supported recently by our theoretical studies [12][13][14].…”
“…The annelation of benzene by small rings induces a significant bond alternation of the type originally proposed by Mills and Nixon to explain the enhanced positional selectivities in electrophilic aromatic substitutions. , Recently, such a bond fixation has become a subject of considerable theoretical as well as experimental scrutiny; , particularly noteworthy is the use of benzenoid hydrocarbons which are tris-annelated by small bicyclic rings, as represented in the generic structure I ( GS I ) below …”
A versatile method for the high-yield synthesis of various tris-, bis-, and mono-annelated benzenes (as well as cyclooctatetraene) is based on the Pd-catalyzed coupling of three (or four) ethylenic units comprised of α,β-dibromoalkenes and α'-alkenyl Grignard reagents all carried out in a single pot. The particular application to tris(bicyclopentyl)-annelated benzene yields the syn isomer 1s in high purity; X-ray diffraction analysis confirms the aromatic bond alternation relevant to the Mills−Nixon effect. Most importantly, the efficient synthesis of 1s crystals of extraordinary quality allows us (for the first time) to make precise electron-density measurements of the "banana-type" distortion and the ellipticity (π-character) of the various aromatic C−C bonds sufficient to identify the electronic origin of the classical Mills−Nixon effect. The unique electron-donor properties of tris-annelated benzenes also relate to their highly reversible one-electron oxidation potentials even in nonpolar solvents.
“…It appears that protonation and formation of the a -s p 3 center induces severe structural perturbations. The geminal C(l)-C(2) and C(l)-C(6) bonds are substantially lengthened and the C(2)-C(l)-C (6) angle is smaller than 120°, in harmony with rehybridization and the lack of r-bonding. A rather small HC(1)H angle of 103.4" shows that the p content in the CH bonds is higher than in cannonical sp3 hybridization.…”
Section: Structural Propertiesmentioning
confidence: 97%
“…Lloyd and Ongley examined nitration, Friedel-Crafts acetylation, and hydrobromination of benzocyclobutene, which yielded substitution predominantly at the P-position [ 5 ] . Further, it was found that products of bromination [6] and the reactivity toward protodesilylation and protodetritiation [7] strongly favor the P-site in indan, whereas there was practically no discrimination in tetralin. Reactivity of biphenylene was explored by Taylor and results were again in harmony with the original Mills-Nixon (MN) hypothesis [8].…”
Structure and properties of Wheland's intermediates are studied in some model systems. The effect of the annelated small ring is simulated by bending two vicinal CH bonds in benzene toward each other. The extent of bending describes the ring-size effect of a small fused fragment molecule. The electrophilic substituent is represented by a proton. Calculations at the HF/6-31G* and MP2//HF/6-31G* levels of sophistication confirm the Mills-Nixon (MN) hypothesis. It is conclusively shown that P-complexes are more stable than are a-protonated species. The results are interpreted in terms of the ground-state density distribution dictated by rehybridization and of the transition structure ?r-electron redistribution triggered by protonation. Their effects are additive to a large extent as far as CC bond distances are concerned. Energetic properties are determined by the interplay of two characteristic ?r-electron localization patterns, one caused by the initial angular strain and the accompanying rehybridization related to the ground state and the other one occurring during formation of the proton a-complex. This interplay destabilizes a-intermediates relative to P-intermediates.
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