Copper stereoselective

Amino-acid Complexes. A small, but reproducible, stereoselective effect has been observed in both the free energy and enthalpy changes associated with formation of [Ni(L,L-methioninate)2]. The meso-complex [Ni(o-Met)(L-Met)] is more stable in AH than the optically active NiL2 by 1.0 (0.1) kJ mol" L The stereoselectivity is attributed to terdentate co-ordination and so supports weak co-ordination of the thioether group. Formation constants of nickel(ii) and copper(ii) with N -benzyl-L-histidine and N N -dibenzyl-L-histidine and of the ternary complexes with d- and L-histidine have been measured. Stabilization of ternary complexes is small but significant stereoselectivity is found with ternary nickel complexes, when the meso configuration is preferred in each case with copper, stereoselectivity is small or absent. The i.r. spectra of tra s-[Ni(Gly)2(H20)2] and its 0-, N-, 1- C-, and 2- C-labelled... 

Stereoselective cis-dihydroxylation of the more hindered side of cycloalkenes is achieved with silver(I) or copper(II) acetates and iodine in wet acetic acid (Woodward gly-colization J.B. Siddall, 1966 L. Mangoni, 1973 R. Criegee, 1979) or with thallium(III) acetate via organothallium intermediates (E. Glotter, 1976). In these reactions the intermediate dioxolenium cation is supposed to be opened hydrolytically, not by Sn2 reaction. 

The formation of g-alkyl-a,g-unsaturated esters by reaction of lithium dialkylcuprates or Grignard reagents in the presence of copper(I) iodide, with g-phenylthio-, > g-acetoxy-g-chloro-, and g-phosphoryloxy-a,g-unsaturated esters has been reported. The principal advantage of the enol phosphate method is the ease and efficiency with which these compounds may be prepared from g-keto esters. A wide variety of cyclic and acyclic g-alkyl-a,g-unsaturated esters has been synthesized from the corresponding g-keto esters. However, the method is limited to primary dialkylcuprates. Acyclic g-keto esters afford (Zl-enol phosphates which undergo stereoselective substitution with lithium dialkylcuprates with predominant retention of stereochemistry (usually > 85-98i )). It is essential that the cuprate coupling reaction of the acyclic enol phosphates be carried out at lower temperatures (-47 to -9a°C) to achieve high stereoselectivity. When combined with they-... 

When vmyl silicon tnfluonde is treated with two equivalents of potassium fluonde, a new reagent, a dipotassium organopentafluorosilicate, is formed [101] This intermediate has found applicahon as a component in an efficient stereoselective copper chlonde-promoted couplmg reaction (equation 81)... 

The chiral BOX-copper(ll) complexes, (S)-21a and (l )-21b (X=OTf, SbFg), were found by Evans et al. to catalyze the enantioselective cycloaddition reactions of the a,/ -unsaturated acyl phosphonates 49 with ethyl vinyl ether 46a and the cyclic enol ethers 50 giving the cycloaddition products 51 and 52, respectively, in very high yields and ee as outlined in Scheme 4.33 [38b]. It is notable that the acyclic and cyclic enol ethers react highly stereoselectively and that the same enantiomer is formed using (S)-21a and (J )-21b as the catalyst. It is, furthermore, of practical importance that the cycloaddition reaction can proceed in the presence of only 0.2 mol% (J )-21a (X=SbF6) with minimal reduction in the yield of the cycloaddition product and no loss of enantioselectivity (93% ee). 

The absolute configuration of products obtained in the highly stereoselective cycloaddition reactions with inverse electron-demand catalyzed by the t-Bu-BOX-Cu(II) complex can also be accounted for by a square-planar geometry at the cop-per(II) center. A square-planar intermediate is supported by the X-ray structure of the hydrolyzed enone bound to the chiral BOX-copper(II) catalyst, shown as 29b in Scheme 4.24. 

Hie products of this catalytic enanlioselective 1,4-addition still contain an enone moiety, prone lo subsequenl 1,4-addition [73]. An inlriguing queslion regarding stereoconlrol was posed would the stereoselectivity in the second addition step be governed by the catalyst or would there be a major effect fToni the stereocenlers already present Sequential 1,4-addition lo dimetlioxy-substiltiled cyclobexadienone 66 (Scheme 7.18) using the copper catalyst based on IS, R, i j-ligand 18 both in the... 

The lithium enolate 2a (M = Li ) prepared from the iron propanoyl complex 1 reacts with symmetrical ketones to produce the diastercomers 3 and 4 with moderate selectivity for diastereomer 3. The yields of the aldol adducts are poor deprotonation of the substrate ketone is reported to be the dominant reaction pathway45. However, transmetalation of the lithium enolate 2a by treatment with one equivalent of copper cyanide at —40 C generates the copper enolate 2b (M = Cu ) which reacts with symmetrical ketones at — 78 °C to selectively produce diastereomer 3 in good yield. Diastereomeric ratios in excess of 92 8 are reported with efficient stereoselection requiring the addition of exactly one equivalent of copper cyanide at the transmetalation step45. Small amounts of triphcnylphosphane, a common trace impurity remaining from the preparation of these iron-acyl complexes, appear to suppress formation of the copper enolate. Thus, the starting iron complex must be carefully purified. 

The reversal of the stereoselectivity is attributed to the ability of chlorotrimethylsilane to trap the initially formed cuprate-enone complex, thereby suppressing equilibration of the diastereomeric complexes. The copper-catalyzed 1,4-addition of Grignard reagents to 5-substituted 2-cyclo-hexenone also proceeded with very high trans diastereoselectivity22. 

The reaction used to test these solid catalysts was the aziridination of styrene with AT-tosyliminophenyliodinane (Phi = NTos) (Scheme 10). In most cases, enantioselectivities were low or moderate (up to 60% ee). The loss of enantioselectivity on changing from ligand 11a to ligand 12 was attributed to the fact that ligand 12 is too big for the copper complex to be accommodated into the zeolite supercages. Further studies carried out with hgands 11a and 11b [62] demonstrated that the reaction is more enantioselective with the supported catalyst (82% ee with 11a and 77% ee with 11b) than in solution (54% ee with 11a and 31% ee with 11b). This trend supports the confinement effect of the zeolite structure on the stereoselectivity of the reaction. 

Section B gives some examples of metal-catalyzed cyclopropanations. In Entries 7 and 8, Cu(I) salts are used as catalysts for intermolecular cyclopropanation by ethyl diazoacetate. The exo approach to norbornene is anticipated on steric grounds. In both cases, the Cu(I) salts were used at a rather high ratio to the reactants. Entry 9 illustrates use of Rh2(02CCH3)4 as the catalyst at a much lower ratio. Entry 10 involves ethyl diazopyruvate, with copper acetylacetonate as the catalyst. The stereoselectivity of this reaction was not determined. Entry 11 shows that Pd(02CCH3) is also an active catalyst for cyclopropanation by diazomethane. 

The first example of a stereoselective substitution at tin was the reaction of (—)- -butylneophylphenyltin hydride (65) ([with diazomethane in the presence of copper in diethyl ether to form optically active methylneophylphenyl-t-butyltin (84) ([o g5 - 1.5) 20 44- >. 

As shown earlier in many examples, the Claisen rearrangement of allyl vinyl ethers also provides a very powerful method for carbon-carbon bond formation in domino processes. Usually, the necessary ethers are formed in a separate step. However, both steps can be combined in a novel domino reaction developed by Buchwald and Nordmann [306]. This starts from an allylic alcohol 6/4-102 and a vinyl iodide 6/4-103, using copper iodide in the presence of the ligand 6/4-104 at 120 °C to give 6/4-105 (Scheme 6/4.25). The reaction even allows the stereoselective formation of two adjacent quaternary stereogenic centers in high yield. 

Table 2. Stereoselectivities for cyclopropane formation from olefins and ethyl diazoacetate with representative copper catalysts (reproduced from reference 59, with the permission of the American Chemical Society)... 

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