Arene complexes from diene

Co arene chemistry has been expanded by the preparation of a number of (arene)Co(j7" -diene)+ and (arene)Co( , -enyl) complexes starting from (10). Hydrogenation of (10) in the presence of an arene and a base (piperidine or quinuclidine for obvious stabilization of coordinatively unsaturated cobalt intermediates) gives cyclooctenyl complexes (37) (equation 53). This reaction occurs with a large niunber of arenes even CeFg gives an j -arene complex. The resulting complexes (37) are moderately active catalysts in the pyridine synthesis from alkynes and nitriles (Section 5.1.4). 

In the following reaction hydride ions are removed from a diene complex to give an arene complex (Eq. 2-79)... 

Arene complexes display structures with distortions of the ring from planarity and carbon-carbon bond lengths that differ from typical distances. " Tlie aromaticity is largely lost in these structures, and the arene is bound, at least formally, with the diene portion of a hypothetical "Kekule structure." Structurally well-characterized examples... 

Although synthetic routes using Ru(II) arene dimers can give high yields of relatively pure half-sandwich products, they are limited by the availability of suitable precursor dienes. Alternative routes involve, for example, substitution of weakly bound naphthalene in the Ru(0) complex [(ri -naphthalene)Ru(Ti -COD)] (where COD is cyclo-octa-1,5-diene) by an added arene [38] followed by conversion to the Ru(II) arene dimer on addition of HCl [39]. This pathway can provide access to Ru(I I) arene complexes with functional side-chains on the arene [40]. Other routes involve thermal displacement of coordinated p-cymene by sterically-demanding arenes such as hexamefhylbenzene [41], photoinduced substitution ofe.g. benzene in [(ri -benzene)Ru(amidinate)(Cl)] [42], derivatization of coordinated arenes in [(ri -arene)Ru(Cp)] complexes [43], and stoichiometric cyclotrimerization of alkynes using [(ri -naphthalene)Ru(r -COD)] [44, 45]. It is also possible to synthesize mono- and bis-arene Ru(II) complexes starting from [Ru(II)(H20)g] which can aromatize cyclic olefins when heated in water or other suitable protic solvents [46, 47]. 

Rare earth arene complexes are good precursors for difficult-to-synthesize rare earth compounds (Bochkarev, 2002) and the same was true for the scandium arene complexes supported by NN . Hessen previously showed that a scandium complex of the 2,2 -bipyridyl radical anion could be readily accessed from a reduced 1,3-diene scandium complex (Beetstra et al., 2003). Similarly, the addition of 2,2 -bipyridine to a CeDs solution of Sc2-naph (Scheme 3A) led to the formation of the previously reported radical anionic bipyridyl complex (NN )Sc(2,2 -bipyridine) (Williams et al., 2010). 

The RCM of 1,6-dienes into cyclopentenes is sometimes accompanied by a cycloisomerisation reaction to give cyclic products with an exo-methylene substituent, especially when ruthenium-arene complexes are used as catalyst precursors (see for instance compounds 32 in Section 7.3.1.6). In many cases, this side reaction was undesired and could be effectively suppressed by the addition of various co-catalysts. It is nevertheless possible to alter the reaction path to obtain the cycloisomerisation products with very high selectivities. Early examples of NHC-Ru complexes suitable for this task included the unstable cationic alle-nylidene complexes 36 prepared in situ from more robust chelated precursors 35 (Equation (7.6)). Alternatively, a combination of IMes-HCl/Cs2C03/[RuCl2(p-cymene)]2 also provided an efficient catalytic system. A mechanism involving oxidative coupling of the 1,6-diene to a ruthenium(II) centre followed by p-elimination to generate a hydrido ruthenium(IV) intermediate and reductive elimination was proposed for the transformation (Scheme 1.9)2 ... 

Co-condensation of Hf and Zr atoms from an electron-gun evaporation device, with P(Me)3 and arenes at 77K gave good yields of the species [M(arene)2P(Me3)]. Metal vapor synthesis led to Fe(i7 -arene)L2 and Fe(i7 -arene)-(i7 -diene), where L is a phosphorus ligand. In addition, complexes of stoichiometry Fe(T) -diene)L3 (where L is again a... 

The processes depend on the formation of the cyclohexadienyl anion intermediates in a favorable equilibrium (carbon nucleophiles from carbon acids with pKt > 22 or so), protonation (which can occur at low temperature with even weak acids, such as acetic acid) and hydrogen shifts in the proposed diene-chromium intermediates (25) and (26). Hydrogen shifts lead to an isomer (26), which allows elimination of HX and regeneration of an arene-chromium complex (27), now with the carbanion unit indirectly substituted for X (Scheme 9). 

The formation of Crf, FeCr+, FeMn+, and MnJ has been reported in the spectra of it complexes containing two or more metal atoms (176). The loss of carbonyl groups from such compounds is not straightforward. The diene-Fe(CO)3 complexes do not lose CO stepwise, and in some arene-Cr(CO)3 complexes only arene-Cr(CO)+ (w = 0,1 and 3) were seen. Some derivatives of CpMn(CO)3 lose all three CO groups at once, and if a second of these groups is present on a ferrocene moiety, the next three CO groups are also lost simultaneously. 

RuCl2(arene)]2 complexes (1) react with 1,5-cyclooctadiene and 1,3- or 1,4-cyclohexadiene in the presence of ethanol and Na2C03 or zinc dust to give Ru°(776-arene)(V diene) compounds of type 196-198 in 60% yield [Eq. (20)] thus, this reaction appears to be the reverse of the 198-> 1 reaction [Eq. (19)]. The same reaction with ethylene leads to the bis-ethylene ruthenium(O) complex 205 (37%) (131,10). The norbornadiene complex 207 is prepared similarly from derivative 206 (125). Combination of transformations 206 - 207 [Eq. (21)] or 1 - 198 [Eq. (20)] with trans-... 

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