Olefin complexes asymmetric

The seminal report of an asymmetric homogeneous metal-catalyzed reaction described the copper-catalyzed group-transfer reaction from a diazoester to an alkene, Eq. 3 (2). This article provided experimental verification of the intervention of copper carbenoid olefin complexes in the catalytic decomposition of diazo com-... 

Arylation, olefins, 187, 190 Arylketimines, iridium hydrogenation, 83 Arylpropanoic acid, Grignard coupling, 190 Aspartame, 8, 27 Asymmetric catalysis characteristics, 11 chiral metal complexes, 122 covalently bound intermediates, 323 electrochemistry, 342 hydrogen-bonded associates, 328 industrial applications, 8, 357 optically active compounds, 2 phase-transfer reactions, 333 photochemistry, 341 polymerization, 174, 332 purely organic compounds, 323 see also specific complexes Asymmetric induction, 71, 155 Attractive interaction, 196, 216 Autoinduction, 330 Axial chirality, 18 Aza-Diels-Alder reaction, 220 Azetidinone, 44, 80 Aziridination, olefins, 207... 

The results do not prove that in the reaction conditions used the alkyl formation is not reversible, but only that, if it is reversible, the carbon monoxide insertion on both diastereomeric rhodium-alkyls must be much faster than the rhodium-alkyls decomposition. Restricting this analysis of the asymmetric induction phenomena to the rhodium-alkyl complexes formation, two 7r-olefin complexes are possible for each diastereomer of the catalytic rhodium complex (see Scheme 11). The induction can take place in the 7r-olefin complexes formation (I — II(S) or I — II(R)) or in the equilibrium between the diastereomeric 7r-olefin complexes (II(r) and... 

II(S)) and/or to a different reaction rate of the two diastereomeric 7r-olefin complexes to the corresponding diastereomeric alkyl-rhodium complexes (VI(s) and VI(R)). For diastereomeric cis- or trans-[a-methylbenzyl]-[vinyl olefin] -dichloroplatinum( II) complexes, the diastereomeric equilibrium is very rapidly achieved in the presence of an excess of olefin even at room temperature (40). Therefore, it seems probable that asymmetric induction in 7r-olefin complexes formation (I — II) cannot play a relevant role in determining the optical purity of the reaction products. On the other hand, both the free energy difference between the two 7r-olefin complexes (AG°II(S) — AG°n(R) = AG°) and the difference between the two free energies of activation for the isomerization of 7r-com-plexes II(S) and II(R) to the corresponding alkyl-rhodium complexes VI(s) and VI(R) (AG II(R) — AG n(S) = AAG ) can control the overall difference in activation energy for the formation of the diastereomeric rhodium-alkyl complexes and hence the sign and extent of asymmetric induction. 

The model can be identified with the diastereoisomeric 7r-olefin complexes, and thus AG° should, at least qualitatively, control the sign of the asymmetric induction. In this case, if AGhi(S) > AG ii(R), AG° must be larger than AAG. If AG°nasymmetric induction will correspond... 

We first established that hydrocarbonylation reactions occur with cis-stereochemistry (29, 16) and that asymmetric induction occurs before or during the formation of the metal alkyl intermediate (5, 6). This means that is either during the 7r-olefin complex formation between catalyst and substrate or during the insertion of the 7r-complexed olefin into the M-H bond. Therefore, the model should focus on the interactions between the substrate double bond and the catalytically active metal atom of the catalyst. 

The fact that a model for the transition state controlling asymmetric induction based on steric interactions allows us to correctly predict the type of prevailing regio- and stereoisomer for about 85% of the asymmetric hydrocarbonylation experiments (including hydroformylation and hydrocarbalkoxylation) is an indication that asymmetric induction in these catalytic reactions is based mainly on steric interactions. The data obtained so far do not allow us to establish whether the more stable or the less stable 7r-olefin complex intermediate is the one that reacts preferentially. However, the regularities that we observed indicate that the kinetic features are the same, at least in most of the experiments. 

Rotational spectroscopy and ab initio calculations give an evaluation of the distance between donor and acceptor for ethene/halogen or interhalogen (BrCl) tt complexes73,96. Ab initio calculations indicate that a spiro transition state73 (43) is involved in the transfer of positive halogens to olefins an asymmetrical 1 1 halogen-ion/olefin CT complex (42)... 

The investigation of platinum(II)-chiral olefin complexes has shown that, when the diastereomeric equilibrium is reached, which diastereoface of the olefin is preferentially bound to the metal depends on the type of chirality of the olefin used61-63. When an optically active asymmetric ligand is present in the complex and a racemic olefin, is used, one diastereoface will be preferred for complexation and correspondingly one of the antipodes is preferentially complexed61 63). Let us suppose that with a certain catalytic system (e.g., Rh/(—)-DIOP), the re-re enantioface of a prochiral a-olefin reacts preferentially. With the same catalytic system the same face of all a-olefins, including the racemic a-olefins, is expected to react preferentially. However, when a racemic olefin is used, two diastereomeric transition states (e.g. a and b in Fig. 11) can form for each of the transition states shown in Fig. 7, depending on which one of the antipodes of the racemic monomer approaches the catalyst. 

As shown in the asymmetric hydroformylation of 1- and 2-butene with Rh/(--)-DIOP or Pt/(—)-DIOP catalytic systems, it seems likely that asymmetric induction occurs mainly in the step in which the n-olefin complexes, which are assumed to be formed in the first reaction step, are transformed into the corresponding metal-alkyl intermediate 15). 

The model we have used for the description of the transition states implies that also the corresponding n-olefin complexes contain an asymmetric metal atom and that changes of configuration at the metal occur more slowly than the intramolecular transformation of the 7i-complex into the metal-alkyl complex. 

Asymmetric Hydrogenation. Rhodium complexes of the type Rh(diene)(diphos )+, where diphos is a chiral bidentate diphosphine ligand, are catalyst precursors for the asymmetric hydrogenation of certain prochiral olefins (15). Asymmetric hydrogenation of a-acylaminoacrylates, for example, affords chiral amino acid derivatives, some of which have medicinal utility such as L-DOPA. 

Polymer-supported salen catalysts were also developed by employing poly (norbornene)-immobihzed salen complexes 139 of manganese and cobalt (Scheme 3.40) [77]. The poly(norbornene) complexes are highly active and selective catalysts for the epoxidation of olefins. The asymmetric epoxidation of cis-P-methylstyrene 132 occurred smoothly at -20 °C to give the chiral epoxide 133 in 100% conversion with 92% ee. Under the same reaction conditions, Jacobsen s catalyst (an unsupported salen complex) afforded the same product with 93% ee. 

A prerequisite for effective asymmetric hydrogenation is that the prochiral olefin is bound stereoselectively to metal at the rate-determining transition-state (Scheme 1). It is therefore of interest to consider stable metal-olefin complexes which may exist as diastereomers by virtue of alternative modes of prochiral olefin complexation. Most work has been done with comparatively simple asymmetric sulfur or nitrogen ligands, and selectivity is usually low. With simple olefins this is not surprising, since discrimination depends on rather small differences in steric bulk in the absence of polar interactions. 

A detailed mechanism of asymmetric hydrovinylation is discussed in order to explain the pathways of the asymmetric induction4- 51314. The 7t-allylnickel complex, as the catalyst precursor, is activated by phosphanes and ethylaluminum chloride and reacts with an olefin to give a catalytically active nickel hydride-olefin complex. The olefin then inserts into the metal hydride bond and after coordination and insertion of ethene a new alkylnickel compound is... 

The analogous five-coordinate olefin complexes show the presence of isomers because of an asymmetric environment above and below the equatorial plane. The isomer distribution depends on the steric requirement of the olefin substituent. In a number of fiuoroolefins, a direct through-space H-F coupling was observed (10). With allenes, Pt is coordinated to one of the double bonds, usually the less substituted one, and the uncoordinated double bond is probably bent backward by some 38°, by analogy with the bending found in PtP(C6H5)3(CH2=C=CH2) (11). 

The Zr—C bond distances in olefin complexes have in general been computed to be asymmetric. The Zr—C bond distances in the one observed olefin complex (ref 119) are also inequivalent. If one assumes that the closer C is bonded to Zr and the one further away not bonded, then a Zr—C nonbond distance can be estimated by averaging the longer of the two Zr—C ethylene bond distances for the eight zirconium ethylene structures of Table 1 and the six from Table 8 this yields a Zr—C nonbond distance of 2.86 A. Eor reference the longer of the two olefin bond... 

The chiral anisole derivative 119 has been used in the synthesis of several asymmetric functionalized cyclohexenes (Fig. 27) [55]. In a reaction sequence similar to that employed with racemic anisole complexes (see above), 119 adds an electrophile and a nucleophile across C4 and C3, respectively, to form the cy-clohexadiene complex 120. The vinyl ether group of 120 can then be reduced by the tandem addition of a proton and hydride to C2 and Cl respectively, affording the olefin complex 121. Direct oxidation of 121 liberates the cyclohexenes of the type 122 having the initial asymmetric auxiliary still intact. Alternatively, the auxiliary may be cleaved under acidic conditions to afford q -allyl complexes, which can undergo reactions with nucleophiles regioselectively at Cl. Oxidative decomplexation liberates the cyclohexenes 123-127. 

The first olefin complex and a modern chiral diene for asymmetric catalysis. 

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