Chiral metal complexes isomerization

Allylic double bonds can be isomerized by some transition metal complexes. Isomerization of alkyl allyl ethers 480 to vinyl ethers 481 is catalysed by Pd on carbon [205] and the Wilkinson complex [206], and the vinyl ethers are hydrolysed to aldehydes. Isomerization of the allylic amines to enamines is catalysed by Rh complexes [207]. The asymmetric isomerization of A jV-diethylgeranylamine (483), catalysed by Rh-(5)-BINAP (XXXI) complex to produce the (f )-enaminc 484 with high optical purity, has been achieved with a 300 000 turnover of the Rh catalyst, and citronellal (485) with nearly 100% ee is obtained by the hydrolysis of the enamine 484 [208]. Now optically pure /-menthol (486) is commerically produced in five steps from myrcene (482) via citronellal (485) by Takasago International Corporation. This is the largest industrial process of asymmetric synthesis in the world [209]. The following stereochemical corelation between the stereochemistries of the chiral Rh catalysts, diethylgeranylamine (483), diethylnerylamine (487) and the (R)- and (5)-enamines 484... 

Nonetheless, and despite the high number of possible addition sites, the chiral metal complex proved to be able to afford only eight optically pure isomers. As resulted from UV analysis, the addition occurred site-selectively onto only two double bonds, identified as C(13)-C(14) and C(27)-C(28) by theoretical calculations (see atoms marked in black in Fig. 34.7). For each double bond, two possible regioisomers are formed and, finally, the racemic nature of the endohedral starting material leads to eight optically active isomeric compounds since all of them present a cis stereochemistry. 

To date, direct asymmetric synthesis of optically active chiral-at-metal complexes, which by definition leads to a mixture of enantiomers in unequal amounts thanks to an external chiral auxiUary, has never been achieved. The most studied strategy is currently indirect asymmetric synthesis, which involves (i) the stereoselective formation of the chiral-at-metal complex thanks to a chiral inductor located either on the ligand or on the counterion and then (ii) removal of this internal chiral auxiliary (Fig. 4). Indeed, when the isomerization of the stereogenic metal center is possible in solution, in-... 

The metal center in (i9 -C5H5)M(L)(CO)(allyl) can be considered pseudotetrahedral and characterized by the (R) and (5) nomenclature. Neutral chiral complexes of this type have been prepared previously, e.g., phosphine carbonyl derivatives (21, 75) and nitrosyl halide derivatives prepared by carbonyl displacement from nitrosyl cations by iodide (57, 60). Each of the diastereomers produced by the addition of a prochiral allyl moiety to the chiral metal center exhibit endo-exo isomerism. The endo-exo equilibration depends on charge and the neutral nitrosyl iodides rearrange rapidly compared to the cationic nitrosyls. 

Optical activity may be achieved in at least three ways (a) by the asymmetric arrangement of ligands about the central metal ion (b) by the use of a ligand which is itself optically active and (c) by the use of a ligand in which an atom (or atoms) becomes chiral upon attachment to the metal. The second of these is illustrated by propylenediamine and the tetradentate (7) and the last by (8) and by (9). Neither the nitrogen atoms nor the arsenic atoms in these last two ligand molecules are chiral, but the ones near the centers of their respective molecules become so when they are coordinated to a metal ion. In each case, they can be of the same or of opposite chirality. Thus complexes with a six-coordinate metal can theoretically exist in a large number of isomeric forms these differ in stability and not all of them have been isolated. 

The ophcally active Pd complex with a chiral allenyl ligand undergoes epimer-izahon in the presence of a catalytic amount of Pd(0) complex. This reaction does not involve the isomerization to the propargyl complex, but takes place via a dinuclear intermediate as depicted in Scheme 5.39. The -allenyl ligand in the dinuclear palladium intermediate may racemize via a vinyl-vinyidene intermediate. This type of reaction is prohahly involved in a kinetic resolution of racemic propargyl alcohols promoted hy chiral transihon metal complex [203]. The intermolecular allyl ligand transfer from Pd to Ee complexes occurs under... 

Especially noteworthy is the field of asymmetric catalysis. Asymmetric catalytic reactions with transition metal complexes are used advantageously for hydrogenation, cyclization, codimerization, alkylation, epoxidation, hydroformylation, hydroesterification, hydrosilylation, hydrocyanation, and isomerization. In many cases, even higher regio- and stereoselectivities are required. Fundamental investigations of the mechanism of chirality transfer are also of interest. New chiral ligands that are suitable for catalytic processes are needed. 

The insertions of the monomers are believed to occur in two steps [268, 269]. In the first one, the incoming monomer coordinates with the transition metal. This results in formation of a short-lived a-allylic species. In second one, the metal-carbon bond is transferred to the cocrdinated mmiomer with formation of a 71-butenyl bond. Coordination of the diene can take place through both double bonds, depending upon the transition metal [270] and the structure of the diene. When flie mmiomer coordinates as amonodentate ligand, then asyn complex forms. If however, it coordinates as abidentate ligand, then an anti complex results [271]. In the syn complex, carbons one and four have the same chirality while in the anti complex they have opposite chiralities [268]. Due to lower thermodynamic stability the anti complex isomerizes to a syn complex [268]. If the aUyUc system does not have a substituent at the second carbmi, then the isomerization of anti to syn usually occurs spontaneously even at room temperature [268]. 

The radical mechanism of OA occurs only for polar substrates. A free radical initiator (I) is made, typically by photolysis or electrochemical means. The initiator reacts with the metal complex to oxidize it by one electron, as shown in Figure 19.10. The species can then react with RX to generate R-. The R- radical undergoes a chain reaction with a second metal complex to make R-M " -X and another R- radical. This continues until chain termination by two R radicals coupling together or by radical trapping. The propagation step in the mechanism competes with isomerization or racemization of R-, so that the product is almost always a racemic mixture of optical isomers when a chiral C atom is used. Unlike the S 2 mechanism, the rate of the reaction is independent of steric bulk on the transition metal. Furthermore, the reaction sequence with respect to 3°>2°> I >CH3 (which maps with the... 

Reactions of 2-alkenes, 3-alkenes, etc., with monohydrosilanes lead predominantly to alkylsilanes with terminal silyl group, which means that in the presence of transition metal complexes, hydrosilylation is accompanied by the isomerization of olefins. The formation of adducts with an internal silyl group is also possible, especially in the presence of chiral platinum and palladium complexes (8). 

A new protecting group for amines has been studied and used in the first chiral synthesis of anticapsin. The route involves initial bis-allylation to give a base-stable group resistant to nucleophiles, but which can be easily deprotected using the known allyl-to-propenyl isomerization reaction promoted by transition-metal complexes (Scheme 11). 

Chiral molecules in the role of hgand add a variety to the optical isomerism in metal complexes. 1,2-diaminopropane (pn) is an unsymmetrical biden-tate hgand possessing chiral carbon. The complex [Co(en)(pn)(N02)2] can form two cis- and two trans-isomers. Both the ds-forms give two optical isomers, as shown in Figures 51 and 52. 

One of the first functionalized substrates subjected to the asymmetric P-H addition promoted by metal complexes were phosphine-functionalized alkenols, viz., 3-diphenylphosphinobut-3-en-l-ol and 2-diphenylphosphinoprop-2-en-l-ol (Scheme 7) [54]. The target was the diphosphine ProPhos which had previously been prepared by tedious organic manipulations extending to 14 steps from a chiral pool consisting of malic and L-ascorbic acid [55, 56]. The hydrophosphination reaction employing (/ )- was carried out as shown in scheme 7 and showed excellent selectivity in the case of 3-diphenylphosphinobut-3-en-l-ol (four isomeric products in the ratio 2 18 1 4) and moderate selectivity in the case of 2-diphenyl-phosphino prop-2-en-l-ol (1 2 5 8). Isomer 12a was the major product in the case of 3-diphenylphosphinobut-3-en-l-ol (n = 1), and for 2-diphenylphosphinoprop-2-en-l-ol (n = 2), 11a and 11b co-crystallized out. The two analogous substrates gave products that differ in the chirality at the newly formed carbon center. 

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