Dimethyl sulfoxide cobalt complexes

Molecules having only a sulfoxide function and no other acidic or basic site have been resolved through the intermediacy of metal complex formation. In 1934 Backer and Keuning resolved the cobalt complex of sulfoxide 5 using d-camphorsulfonic acid. More recently Cope and Caress applied the same technique to the resolution of ethyl p-tolyl sulfoxide (6). Sulfoxide 6 and optically active 1-phenylethylamine were used to form diastereomeric complexes i.e., (-1-)- and ( —)-trans-dichloro(ethyl p-tolyl sulfoxide) (1-phenylethylamine) platinum(II). Both enantiomers of 6 were obtained in optically pure form. Diastereomeric platinum complexes formed from racemic methyl phenyl (and three para-substituted phenyl) sulfoxides and d-N, N-dimethyl phenylglycine have been separated chromatographically on an analytical column L A nonaromatic example, cyclohexyl methyl sulfoxide, did not resolve. 

The relatively minor alteration in reaction conditions can be seen to drastically alter the nature of the product (468). In addition many sulfoxide complexes are thermally degraded, and in consequence the extent of drying can alter the nature of the product. Thus, the complex [Co(0-Me2SO)8][I]2 is isolated from a cobaltous iodide-dimethyl sulfoxide system, but extensive drying in vacuo causes degradation to yield [Co(0-Me2SO)6][CoI4] (128). 

Several reports have appeared on the effect of additives on the Pauson-Khand reaction employing an alkyne-Co2(CO)6 complex. For example, addition of phosphine oxide improves the yields of cyclopentenones 119], while addition of dimethyl sulfoxide accelerates the reaction considerably [20]. Furthermore, it has been reported that the Pauson-Khand reaction proceeds even at room temperature when a tertiary amine M-oxide, such as trimethylamine M-oxide or N-methylmorpholine M-oxide, is added to the alkyne-Co2(CO)6 complex in the presence of alkenes [21]. These results suggest that in the Pauson-Khand reaction generation of coordinatively unsaturated cobalt species by the attack of oxides on the carbonyl ligand of the alkyne-Co2(CO)6 complex [22] is the key step. With this knowledge in mind, we examined further the effect of various other additives on the reaction to obtain information on the mechanism of this rearrangement. 

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. 

In order to show that the origin of this difference is not a function of the particular substrate analogue used, similar NMR relaxation studies have been performed with dimethyl sulfoxide (DMSO)1401 since the crystal structure of the enzyme-NADH-DMSO ternary complex is well resolved.1366 From the relaxation data, the distance between the methyl protons of DMSO and Co11 was calculated to be 8.9 0.9 A, again too great for direct coordination of the sulfoxide group to the metal ion. Since the cobalt enzyme appears to be functionally similar to the native enzyme, the difference is unlikely to be a direct result of substitution. One possibility is that there may actually be a difference between the solution and crystalline structure of the enzyme ternary complex, particularly since it is well established that the crystalline enzyme is 1000 times less active than in solution.1402... 

Ogino and his students have observed that the use of dimethyl sulfoxide as a solvent allows the formation of very large chelate rings on cobalt(III) complexes. Rings of intermediate size do not form.7 The conformations of these large rings, and application of the method to the synthesis of complexes of other metals, await investigation. 

The complications which result from the hydrolysis of alkali metal cyanides in aqueous media may be avoided by the use of non-aqueous solvents. The one most often employed is liquid ammonia, in which derivatives of some of the lanthanides and of titanium(III) may be obtained from the metal halides and cyanide.13 By addition of potassium as reductant, complexes of cobalt(O), nickel(O), titanium(II) and titanium(III) may be prepared and a complex of zirconium(0) has been obtained in a remarkable disproportion of zirconium(III) into zirconium(IV) and zirconium(0).14 Other solvents which have been shown to be suitable for halide-cyanide exchange reactions include ethanol, methanol, tetrahydrofuran, dimethyl sulfoxide and dimethylformamide. With their aid, species of different stoichiometry from those isolated from aqueous media can sometimes be made [Hg(CN)3], for example, is obtained as its cesium salt form CsF, KCN and Hg(CN)2 in ethanol.15... 

Coordinated a-amino amides can be formed by the nucleophilic addition of amines to coordinated a-amino esters (see Chapter 7.4). This reaction forms the basis of attempts to use suitable metal coordination to promote peptide synthesis. Again, studies have been carried out using coordination of several metals and an interesting early example is amide formation on an amino acid imine complex of magnesium (equation 75).355 However, cobalt(III) complexes, because of their high kinetic stability, have received most serious investigation. These studies have been closely associated with those previously described for the hydrolysis of esters, amides and peptides. Whereas hydrolysis is observed when reactions are carried out in water, reactions in dimethyl-formamide or dimethyl sulfoxide result in peptide bond formation. These comparative results are illustrated in Scheme 91.356-358 The key intermediate (126) has also been reacted with dipeptide... 

The cobalt(III) complex [Co(2,3-Me2[14]-l,3-diene-l,4,8,l l-N4)Br2] Br is six-coordinate and diamagnetic. Analysis of the visible spectrum (absorptions occur near 15.8 and 26.2 kK) leads to a value for Dq of 2630 cm" . The NMR spectrum in dimethyl sulfoxide shows a methyl singlet at 3.32 ppm. The infrared spectrum is very similar to that given for the nickel(II) complexes. A wide variety of cobalt(lll) complexes has been prepared by metathetical reactions on the dibromo and dichloro complexes. The imine functions can be hydrogenated, producing cobalt(III) complexes of 2,3-Me2 [14] ane-1,4,8,11-N4 or 2,3-Me2-[14]-l-ene-l,4,8,ll-N4. ... 

Figure 5. Absorption spectrum of pentaammine (S)-2-chloropropionato cobalt-(III) tetraphenylborate in ("lower set) water (as ClOf salt), hexamethylphosphor-triamide (hmpa), dimethyl sulfoxide, N-metkylformamide (nmf), N,N-dimethyl-formamide (dmf) ("upper set) acetonitrile, methanol, ethanol, acetone, pyridine (py) and tetrahydrofuran (thf). The reference contained tetraphenylborate (as Na salt) at the same concentration as the solution of the complex.

The PMR spectra of the o - and tt -allyl complexes correspond very well with the spectra of the corresponding manganese and cobalt carbonyl complexes (9). Although the exact location of the tt -allyl group with respect to the metal is not known, the reaction with cyanide ion indicates that the TT -allyl group may be considered to be bidentate, a conclusion in full accord with the displacement of carbon monoxide in the conversion of a -to-tt-allyl cobalt and manganese carbonyls (9), and with the coordination of dimethyl-sulfoxide in the conversion of 7r-to-o -allyl palladium chloride (10). Structure(I) is tentatively proposed for the tt -allyl cyanocobaltate complex. 

Activation parameters and deuterium isotope effects indicate that there is a difference of mechanism between [Co(acac)3] and the other three complexes. In all cases it is thought that the mechanism involves the generation of a monodentate acac intermediate, but this rate-determining step is thought to be dissociative for cobalt (AH = 36.4 kcal mol" A5 positive) but associative for the other metals (27.4 A// 28.8 kcal mol A5 negative).The exchange of acetylacetone with [Co(acac)3] has also been studied in acetonitrile and in dimethyl sulfoxide and dioxan. In the latter investigation effects of added protons were studied by addition of chloroacetic acid. 

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