Isomerization of Cr

The trans cis isomerization of [Cr(C204)(H20)2] is also catalyzed by metal ions and a detailed study of catalysis by Mg11 has been published.540 Dissociation of oxalato complexes of Crin of the type [Cr(ox)n(OH2)6 2n](3 2n)+ (n = l, 3) is also promoted by a series of transition metal ions.541... 

The trans-cis isomerization of [Cr(mal)2(H20)2] ion (mal = malonate ) has been investigated in mixed aqueous-organic solvents (MeOH, EtOH, Me2CO, and dioxane). The rates increase with a decrease in the water concentration, but are independent of the organic diluent used. This contrasts with the behavior of the corresponding oxalato complexes, and the rate-determining step is traced to the loss of... 

In this case, as in the case of isomerization of [Cr(SCN)(OH2)6] +, there is a close similarity between the kinetic parameters for isomerization and for aquation. This suggests a common activated complex as intermediate. 

Activation parameters have been determined for cis- trans and for trans- cis isomerization of [Cr(tfac)s], both in toluene and in 1,2,4-trimethyl-benzene solution. The kinetic parameters are considerably more sensitive to the nature of the solvent than are the equilibrium constants. ... 

Up to about 10 percent of crs-stilbene was obtained when trimethyl-dioxetane 129 was decomposed in the presence of trans-stilbene 182) the electronic excitation energy of the excited carbonyl compounds formed in the cleavage of 129 (see Section V.) was transferred to trans-stilbene, so effecting the photochemical trans-cis isomerization. When bis (2.4-dinitrophenyl) oxalate reacted with hydrogen peroxide (see Section V. C. in the presence of o-tolyl-propane-1.2-dione 130, 2-methyl-2-... 

Isomerization of 1,3-dienes (12, 36).3 The 1,5-hydrogen shift in isomerization of 1,3-dienes catalyzed by (naphthalene)Cr(CO)3 (1) can be used for synthesis of aryl-substituted exocyclic alkenes, which are not readily available by coupling of aryl halides with exocyclic vinyl halides. 

Based on data from competition experiments, trapping of vinyl radicals occurs via a cr-type intermediate, which is lower in energy than the alternative jt-radical structure [55, 56], Stabilization of cr-radicals via hyperconjugation is small, which causes vinyl radicals to be more reactive than e.g. the methyl radical. /Z-Isomerization of a strained cr-vinyl radical proceeds with a rate constant k 3 x 108-1010 s-1 to provide the thermodynamically most favorable geometry [56],... 

A similar isomerization of an allenyl ketone, catalyzed by a Cr(CO)sL complex, is most probably the mechanistic key step of the palladium-catalyzed conversion of chromium carbene complexes and propargyl bromide to furans. In control experiments different aryl and alkyl allenyl ketones 96 isomerized to the furans 99 in the presence of 10 mol% of Cr(CO)5(NEt3) in good yields (Scheme 15.31) [70],... 

The conversion of cr -> tt by external removal of a hydride (with < 3C+) and the subsequent conversion of tt -> a by external supply of hydride (with NaBHg) has been suggested 43) as a model system for the doublebond isomerization of chemisorbed olefins on metal surfaces in which the metal can act as the internal banking site for hydride ... 

Fig. 2.15 Plot of AS (J K- mol ) vs AK (cm moK ) for racemization and geometrical isomerization of a variety of octahedral metal complexes. Only a few entries are selected from the 27 reactions tabulated in Ref. 180. The deviation of (four) Cr(IlI) complexes represented by Cr(phen) + (3) from the linear plot (best fit for 23 complexes) may indicate that these recemize by twist, and not dissociative, mechanisms. Racemization of Cr(C204)3 -(l), Co(Ph2dtc)3(2), Cr(phen)3 (3), Ni(phen)f+(4). Geometrical isomerization of trans-Cr(C204)2(H20)2 (5), trans-Co(en)2(H20) +(6), (3-Co(edda)en+(7).

Example IV The thermal and oxidative isomerization of tetraalkyl-substituted tetrahedrane clusters (CR)4 to cyclobu-tadienederivatives. 

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