Cycloheptatriene, reaction with metal

A transmetalation of the styrylcarbene chromium complex 62 in the presence of stoichiometric amounts of [Ni(cod)2] to give the nickel carbene intermediate 63 was applied to the synthesis of Cr(CO)3-coordinated cycloheptatriene 64 upon reaction with terminal alkynes [57] (Scheme 37). The formation of pen-tacarbonyl(acetonitrile)chromium is expected to facilitate the metal exchange. 

The reactions of cycloheptatriene with metal vapors may be altered by the addition of other ligands (133, 140) ... 

Complexes of Cr, W, Mo, Fe, Ru, V, Mn and Rh form stable, isolable arene if -complexes. Among them, arene complexes of Cr(CO)3 have high synthetic uses. When benzene is refluxed with Cr(CO)6 in a mixture of dibutyl ether and THF, three coordinated CO molecules are displaced with six-7r-electrons of benzene to form the stable i/fi-benzene chromium tricarbonyl complex (170) which satisfies the 18-electron rule (6 from benzene + 6 from Cr(0) + 6 from 3 CO = 18). Complex formation is facilitated by electron-donating groups on benzene, and no complex of nitrobenzene is formed. Complex formation has a profound effect on reactivity of arenes, and the resulting complexes are used in synthetic reactions. The metal-free reaction products can be isolated easily after decomplexation by mild oxidation using low-valent Cr. Cycloheptatriene also forms a stable complex with Cr(CO)3 and its synthetic applications are discussed below. 

Cycloheptatriene (23.62) can act as a 67r-electron donor, and in its reaction with Mo(CO)6, it forms (q -CvH8)Mo(CO)3. The solid state structure of this complex (Figure 23.24a) confirms that the ligand coordinates as a triene, the ring being folded with the CH2 group bent away from the metal... 

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. 

Several metal carbonyls are known to react with l,3,S-cycloheptatriene to give metal complexes in which carbonyl groups have been displaced by three olehnic bonds and it seems possible that an analogous reaction might... 

The metal-mediated and metal-catalyzed [6 + 2]- and [6 + 4]-cycloaddition reactions, pioneered by Pettit and co-workers105 106 and Kreiter and co-workers,107 respectively, involve the cycloaddition of metal-complexed cyclic trienes with 7r-systems such as alkenes, alkynes, and dienes. The [6 + 2]-reactions produce bicyclo[4.2.1]nonadiene derivatives and the [6 + 4]-reactions produce bicyclo[4.4.1]undecatrienes (Scheme 32). Trienes complexed to chromium, which can be prepared on large scale (40 g) as reported by Rigby and co-workers,108 react with 7r-systems upon thermolysis or irradiation.109-111 Chromium and iron-catalyzed [6 + 2]-reactions of cycloheptatrienes and disubstituted alkynes... 

Rigby and colleagues also demonstrated that the regioselectivities in the reactions of 1- and 2-substituted dienes with 1-substituted cycloheptatrienes, which do not proceed under metal-free conditions, were generally high. In the case of 1-substituted dienes, this may be completely attributed to steric hindrance. 2- And 3-substituted cycloheptatrienes hardly showed any regioselectivity305. 

When the reactions of 494 with some dienes were carried out under thermal conditions, the adducts were obtained metal-free. This suggested the possibility of effecting these transformations using a catalytic amount of an appropriate Cr(0) source. Rigby and colleagues showed that the reaction between cycloheptatriene 511 and 1-acetoxy-1,3-butadiene (341) can be catalyzed by employing a catalytic amount of 513 (equation 149). The yield of 512 was 36% in this instance, whereas a yield of 20% was obtained when a catalytic amount (10 mol%) of 494 was used as the catalyst305,308. 

The carbonyl undergoes a variety of complex formation reactions, involving partial or total replacement of CO groups with other donors. Many reactions have synthetic applications. Such donors include pyridine (py), diglyme, toluene, aniline, cycloheptatriene, alkyl disulfide and metal cyctopentadiene. A few examples are given below ... 

The reactions of cycloheptatriene with transition metal atoms are similar to those of cyclopentadiene. In both cases, the reactions can involve extensive migration of hydrogen in the formation of the final product (9, 10, 133, 136) ... 

Some interesting reactions have recently been found to occur on heating cycloheptatriene with some organosiliconruthenium carbonyls. An unexpected feature of these reactions is the migration of trimethyl-silyl groups from the metal to the 7r-ligand (R = Me) (17, 74) ... 

In the course of a study of the formation of fluorophosphoranes from chloro-phosphines (22) we observed one exception in the compound chloromethyldi-chlorophosphine, which reacted smoothly with antimony trifluoride to give the flammable fluorophosphine, C1CH2PF2, under conditions where many other chloro-phosphines were invariably converted into fluorophosphoranes. As this fluorophosphine is readily available, its interaction with a metal carbonyl derivative was studied, and cycloheptatriene molybdenum tricarbonyl, obtained from the reaction of molybdenum hexacarbonyl with cycloheptatriene (I, 2), was chosen as a starting compound. 

Ultraviolet activation of the complexes hexacarbonylbis( / 5-cyclopen-tadienyl)dimolybdenum and -ditungsten (135) has been studied in detail (136-138). In addition to homolytic cleavage of the metal-metal bonds, loss of carbon monoxide has also been observed. The products of the photochemical reactions of [(t/5-CH3C5H4)M(CO)3]2 (M = Mo, W) with the dienes la- 1c, lg and It and 1,3,5-cycloheptatriene (68a) differ markedly from those obtained from the thermal reaction of [(f/s-C5Hs)Mo(CO)B]2 (n = 2,3) with dienes (139,140). 

Mechanistic studies of the metal catalyzed cycloaddition reactions establish the intermediacy of a species bearing cyclopolyenyl or cyclopolyene ligand and a n -bound alkene or alkyne. Low-temperature solution studies established that coupling takes place in two steps, often with rearrangement of the alkene or alkyne fragments. This process is illustrated for alkyne addition to a cycloheptatriene ligand in Scheme 17. 

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