Cobalt complexes isomers

Upon treatment with suitable cobalt complexes, methylbutynol cyclizes to a 1,2,4-substituted benzene. Nickel complexes give the 1,3,5-isomer (196), sometimes accompanied by linear polymer (25) or a mixture of tetrasubstituted cyclooctatetraenes (26). 

Quite recently, Ciampolini and coworkers have reported the synthesis of two isomeric mked oxygen-phosphorus macrocycles and the crystal structures of their cobalt complexes. Synthesis of macrocycle 27 was accomplished by condensation of 1,2-bis-(phenylphosphino)ethane dianion with 2,2 -dichlorodiethyl ether in THE. The two isomers of 27 were isolated in 1.5% and 2% yield. The synthesis is formulated in Eq. (6.17), below. 

In a more recent and improved approach to cyclopropa-radicicol (228) [ 110], also outlined in Scheme 48, the synthesis was achieved via ynolide 231 which was transformed to the stable cobalt complex 232. RCM of 232 mediated by catalyst C led to cyclization product 233 as a 2 1 mixture of isomers in 57% yield. Oxidative removal of cobalt from this mixture followed by cycloaddition of the resulting cycloalkyne 234 with the cyclic diene 235 led to the benzofused macrolactone 236, which was converted to cyclopropa-radicicol (228). 

A desymmetrizing reduction of a dicarbonyl has also been achieved as a route to flMfi-aldol adducts. Yamada and coworkers have shown that a chiral cobalt complex catalyzes the desymmetrization of diaryl-1,3-diketones in excellent yield and enantioselectivity, greatly favoring the anti isomer [Eq. (10.65)]. Anti selectivity is rationalized using a Felkin-Anh model ... 

The reaction using 11a as a substrate in the presence of several oxides as additives revealed that addition of tributylphosphine oxide, hexamethylphos-phoric triamide, and dimethyl sulfoxide all accelerate the reaction considerably. Furthermore, when about 10 molar amounts of N-methylmorpholine M-oxide (NMO) is added to the alkyne-cobalt complex 12b in THF,the reaction proceeds even at room temperature and cyclopentenone 13 b is obtained in 37% yield accompanied by another rearranged product, the methylenecyclobutanone 35, obtained in 23% yield as a mixture of ( )-and (Z)-isomers (Scheme 14). These facts indicate that dissociation of the carbonyl ligand of the alkyne-cobalt complex 12 is the rate-determining step in this rearrangement. This is also supported by the fact that under a CO atmosphere in refluxing THF the reaction is completely suppressed. 

Electronic effects become apparent in the M—NCS/M—SCN linkage switches observed in a series of Pd11 (iso)thiocyanate complexes. Ligands positioned trans to the pseudohalide and that are suited to accept electron density from the metal into empty orbitals (backbonding) stabilize the Pd—NCS linkage isomer. However, this rationale is contradicted by the trend in Co—(NCS) bonding in a series of cobalt complexes (see ref. 204b for a review). 

Geometrical isomerism Geometrical isomerism is possible only in hexacoordinate complexes and in the case of 2 1 metal, e.g. chromium and cobalt, complexes arises from coordination of the ligand in a meridional (81) or a facial (82) mode in an octahedral complex. In the former case only an enantiomorphic pair of isomers is possible, but in the latter the possibility exists of four enantiomorphic pairs and a centrosymmetric isomer (Figure 1). 

Extension of the NMR studies described in Section 58.2.3.3(i)(b) to the compounds (136 R = N02 and R = Bu ) led the authors to conclude that one Na- and two No-coordinated cobalt complexes were produced. These were assigned the partial structures (137), (138) and (139), which involve coordination of the ligand in its azo form and in the two alternative hydrazone forms. In general (Section 58.2.3.2), azophenols exist exclusively in the azo form and serious doubts must exist regarding structure (139). Similarly, in view of the reported equilibration between the various isomers in solution, assignment of discrete tautomeric structures must be open to question. As-... 

It must be concluded, therefore, that the ligands do not become completely detached from the metal ion in isomerization reactions. Comparable results have been observed in the isomerization95 of potassium diaquodioxalatochromium(III) and the racemization96 of optically active potassium tris(oxalato)chromium(III) when no exchange with free ligand in solution occurs. Thus, although it is not practicable to take advantage of the desirable properties of individual isomers of 2 1 chromium and cobalt complexes of tridentate azo compounds because of the facility with which such compounds isomerize in solution, the technically important unsymmetrical 2 1 complexes are capable of practical application because they show little or no tendency to disproportionate in solution. 

The equilibrium between metal-quinone redox isomers has been found to be extremely sensitive to the properties of nitrogen-donor coligands. The redox isomers, reported in Ref. 159, can exist (5.14) in the cobalt complexes containing semiquino-late (SQ) and catecholate (Cat) ligands derived from 3,5-di-/-butyl-l,2-benzoquinone (3,5-DBBQ) ... 

Fundamentally, optical isomers of the spiroheterocyclic cobalt complex must exist. Since these compounds could not be separated, we tried to synthesize the dicyano derivative under CO substitution by reaction with KCN in liquid NH3 in the hope that a separation into the optical antipodes might be possible. At 120°C, however, the reaction gave, besides K2[Co(NO)(CO)(CN)2], a complex in which the C(CH2PPh2)4 ligand is tridentate (139) ... 

The cobalt complexes are all diamagnetic and behave as 1 1 electrolytes in methanol.7 The infrared spectra of the complexes contain sharp, strong N—H stretching absorptions (Table I). The electronic spectral data are also shown in Table I. Isomers I and II differ in the chiralities of nitrogen atoms in the macro-cyclic ligands.7 Both chemical and physical properties are dependent on ring... 

Occasionally, solubility properties are such that it is possible to apply the ideas of the less soluble diastereoisomer to diastereoisomeric nonelectrolytes. For example, among the four possible (D- and L-1,2,3, D- and L-1,2,6) tris-L-alaninato complexes of cobalt(III) and rhodium(III) one in each case is utterly insoluble in water, the others being rather soluble (149). The insoluble cobalt complex is the D(+)-l,2,3 isomer (sometimes called / , fac, or cis), and the isomorphous insoluble rhodium complex is almost certainly D(—)-l,2,3-[Rh(Lala)3],... 

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