Norcaradienes complexes

There is limited experimental magnetic information on norcaradienes and most of the currently available information comes from theory. Pikulik and Childs examined the H NMR spectra of the type 110 and pointed out that the chemical shifts of the C(7) and C(l)/C(6) proton resonances were anomalous as compared to complexes of related cyclopropyl compounds such as 111 and 112246. They argued that the chemical shifts of 110 could be accounted for on the basis of an induced ring current. 

A related >/4-norcaradiene tricarbonyliron complex is obtained upon reaction of tricy-clo[4.3.1.0l6]deca-2,4-diene with Fe3(CO)12 in boiling benzene (equation 143). However, the [4.3.1]propellane ring system is not retained in the analogous tricarbonylchromium complex. Instead, as suggested from solution NMR and solid state X-ray analyses, the complex assumes a homoaromatic structure, which is intermediate between a norcaradi-ene and a cycloheptatriene system (equation 144)193,194. It is noteworthy that the Cr(CO)3 group prefers the same conformation as the Fe(CO)3 group in the analogous norcaradiene iron complex. 

Further experimental and theoretical studies on the rotational barriers of the metal fragment in (cycloheptatriene)Cr(CO)3 complexes195 suggest that (cycloheptatriene)-Cr(CO)3 complexes in general are in equilibrium with their norcaradiene valence isomers and their ground state conformation is controlled by the same electronic factors which effect the cycloheptatriene-norcaradiene equilibrium195. 

In principle, this is a simple process we determine the causes of instability in the target molecule and eliminate them by appropriate transformations (substitution, complex-ation, etc.). Indeed many abnormal molecules with varied structures have now been made.32 For each problem, several solutions are possible. There is not enough space to illustrate every conceivable approach here, so we will merely consider four representative molecules norcaradiene (p. 226), cyclobutadiene, trimethylenemethane and carbene. 

However, it has been shown that substituents can change the position of the cycloheptatriene/ norcaradiene equilibrium. Thus n-electron-withdrawing substituents in 1 (X = Y = CN X = COjH, Y = H ) are able to stabilize the norcaradiene structure by shortening the distal (C2-C3) bond and lengthening the vicinal (C1-C2, C1-C3) bonds. With 7r-electron-do-nating substituents the situation is more complex. Only strong n-donors (e.g. 0 , CHj) effect stabilization by lengthening the cyclopropane bonds."... 

Bis(phenylethynyl)-l,6-methanocycIodeca-l,3,5,7,9-pentaene (45) reacted with cyclopen-tadienylbis(triphenylphosphane)cobalt to form an intermediate metallacyclopentadiene complex 46 (with the methanocyclodecapentaene 45 converted to its norcaradiene valence tautomer) which gave the corresponding iminocyclopentadiene complex of 47 upon treatment with an... 

The energy of activation for isomerization of norbornadiene itself is considerably largeThe reaction is mechanistically complex, and yields toluene and decomposition products as well as cycloheptatriene. Under the conditions used, part of the toluene is formed by isomerization of the cyclo-heptatriene , possibly via norcaradiene. In the gas phase the energy of activation for formation of cycloheptatriene from norbornadiene is about 51 kcal.mole and log is about 14.8 (refs. 24, 190). Activation parameters for the formation of toluene directly from norbornadiene in the gas phase are Ecf = 53 kcal.mole and log A — 14.2. These reactions probably involve initial cleavage of the C-1, C-7 bond in norbornadiene to yield an allylic diradical which can cyclize to norcaradiene (a precursor for both cycloheptatriene and toluene) as well as undergo other reactions. 

Studies of the kinetic parameters indicate that it takes place by a one-step concerted mechanism and that the norcaradiene is in fact a transition complex having a partial C(2) - C(7) bond and elongated HC-CO bonds [152]. 

Arenes suffer dearomatization via cyclopropanation upon reaction with a-diazocarbonyl compounds (Btlchner reaction) [76]. Initially formed norcaradiene products are usually present in equilibrium with cycloheptatrienes formed via electrocyclic cyclopropane ring opening. The reaction is dramatically promoted by transition metal catalysts (usually Cu(I) or Rh(II) complexes) that give metal-stabilized carbenoids upon reaction with diazo compounds. Inter- and intramolecular manifolds are known, and asymmetric variants employing substrate control and chiral transition metal catalysts have been developed [77]. Effective chiral catalysts for intramolecular Buchner reactions include Rh Cmandelate), rhodium carboxamidates, and Cu(I)-bis(oxazolines). While enantioselectivities as high as 95% have been reported, more modest levels of asymmetric induction are typically observed. 

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