Diamines, chromium complexes

Diamino-substituted complexes of type 37 were first obtained by Fischer et al. [12] in two steps via the 1,2-addition-elimination product 34 from di-methylamine and 35 (Scheme 6). The (3-aminoallenylidene)chromium complexes 36, which can be prepared either from 33 [47,48] or directly from 35 [33], can also be transformed to l,3-bis(dialkylamino)-substituted complexes of type 37 (e.g., R2=z Pr) by treatment with dimethylamine in excellent yields [33]. Although the complex 37 is accessible by further reaction of the complex 34 with dimethylamine, and 34 itself stems from the reaction of 35 with dimethylamine, the direct transformation of 33 to 37 could not be achieved [12]. In spite of this, heterocyclic carbene complexes with two nitrogens were obtained by reactions of alkynylcarbene complexes 35 with hydrazine [49] and 1,3-diamines [50]. 

Fio. 18. Spectra of a series of chromium complex salts with successive replacement of ethylene diamine by the SON ion. 

Many tris(diamine)- and cw-diacidobis(diamine)chromium(III) complexes have been resolved into their enantiomers, thus providing proof of structure. Absolute configurations are frequently inferred from ORD and CD measurements, which have become of great importance because they provide quick structural information. The spectra-structure correlations are... 

An alternative procedure starts from a metal complex of o-phenylenedi-amine. For example, the bis(o-phenylenediamine) complex of nickel(II) chloride reacts with acetylacetone to give a mixture of a nickel complex of the diamine and diketone together with 2,4-dimethylbenzodiazepinium chloride (76CPB1934 91JPR327). Chromium complexes have been used in the same way [91JCS(D)2045]. 

The chromium-complexed benzylic radicals are also known and were employed in a variety of useful organic reactions. One electron reductive coupling of the tricarbonylchromium complexes of o-substituted benzaldehydes and benzaldi-mines with samarium iodide gave 1,2-diols and diamines, respectively, in good yields with extremely high threo diastereoselectivity (Eq. 7) [7]. Therefore, enantiomerically pure 1,2-diols and diamines were easily obtained by using the planar chiral chromium complexes of benzaldehyde and benzaldimine. 

An intramolecular reductive coupling of mono-chromium complexes of the biphenyls having carbonyl or imine groups at the both ortho-positions gave the trans-diolsy diamines or amino alcohols without formation of any stereoisomers (Eq.8) [8]. 

Although early studies by Nozaki examined (-)-sparteine 2 in the asymmetric lithiation of isopropylferrocene (as noted in Sect. 1.1 above), the first enantioselective generation of planar chirality in good ees using an organolithium (Clay-don, in this volume) was reported by Uemura in 1994 in the lithiation of tricar-bonyl(q -phenyl carbamate)chromium complexes using chiral diamines. After quenching with electrophiles, enantioenriched (o-substituted phenyl car-bamate)chromium complexes were obtained in up to 82% ee (Scheme 19) [61]. 

Improvement in the preparation of the trans-diamminebis-(ethylene-diamine)cobalt(III) ion has allowed a quantitative spectral evaluation of the products of the ammonatlon(acid ammonolysis). For example, one product of an inversion reaction has been found to consist of about 65% trans-diammine, 22%L -cis-diammine, and 13%D -cis-diammine. Extension of these studies to chromium complexes in ammonia has given no indication of comparable inversions. 

An interesting reversal of chiral induction in chromium(III)-salen complexes using a tartaric derived alicyclic diamine moiety (i.e., 7) has been observed by Mosset, Saalfrank, and co-workers <99T1063>. Thus, epoxidation of the chromene 8 using catalyst 7 and an oxidant consisting of MCPBA/NMO afforded the 3S,4S epoxide 9, whereas the Jacobsen catalyst (1) provided the corresponding 3R,4R enantiomer. A mechanistic rationalization for this curious crossover has not yet been proposed. 

Nearly all the reported studies have been on chromium(III) complexes (cf. Table XVI). The chromium (III) complexes L4Cr(0H)2CrL44+ with ammonia or aliphatic diamines (en and tn) deprotonate in strongly basic solutions to form blue /i-hydroxo-/i-oxo species, which in some cases have been isolated as stable and crystalline salts, e.g., A,A-... 

Tris(diamine) complexes of chromium(III) have been important from several viewpoints. First, the tris(ethylenediamine) complex is valuable as a synthetic intermediate,1 the action of heat on the chloride salt giving the cis-dichlorobis(ethylenediamine) complex and on the thiocyanate salt giving the irans-bis(iso-thiocyanato) complex. Second, the cations are resolvable, and studies2 of their optical activity have been fruitful in establishing relations between signs of Cotton effects and absolute stereochemistries. A large number of other studies, including kinetic and equilibrium measurements, have shown these complexes to be readily hydrolyzed to bis(ethylenediamine) complexes.3... 

Other reductions of 2,4,6-triphenylpyrylium ion to give 29 have been examined. Tetramethyl-p-phenylene diamine (TMPD) transfers one electron and ESR spectroscopy shows 29 and TMPD" " to result. Chromium(II) ion was shown to reduce 2,4,6-triphenylpyrylium ion and other related cations. A comparison of chemical reactivity with reduction potential suggested an outer-sphere activated complex. A scale of the relative stabilities of the various radicals was deduced. Since 29 results when 2,4,6-triphenyl-... 

As has been demonstrated in Section 3.1.2.4, complexation of amino groups offers a quite different concept for reversible protection. Making use of this principle, the coordination of chromium carbyne complexes with amino acids was described as a new amino protection. Similarly, bis(ethylene-diamine)cobalt(III) complexes of amino acids constitute a new interesting method for protection which recently was extended to the development of a new type of anchoring in solid-phase synthesis (Scheme 69). First, a t-BOC amino acid is condensed with the aquabis(ethylenediamine)cobalt(lII) complex of p-aminomethylbenzoic acid. The handle obtained in this way is then linked to aminomethyl polystyrene to give the anchored amino acid (69). Due to the sufficient stability of the complex toward... 

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