De-insertion reactions

The oxidative addition of (ArS) 2 to Pd(0) and coordination of 73 to the resultant Pd(II) both lower the total energy [state (C) from (A) via (B)]. Both the insertion of isocyanide into Pd-S of 78 giving state (D) and the reductive elimination of 74 from 79 affording state (E) are reversible. The equilibrium of the insertion and de-insertion of the isocyanide favors the formation of the product of the de-insertion reaction. [State (C) is lower than state (D).] Although state (C) is more stable than state (E), the short-lived Pd(0) can be trapped by (ArS)2 to give 77 [state (E) from state (E)]. 

Insertion takes place between a Jt-bonded fragment and a a-bonded fragment in mutual cis-positions, as was described above. The de-insertion reaction can only proceed if there is a vacant site cis to the acyl group. The experiment outlined in Figure 2.12 proves this point. A manganese acetyl complex which is labelled with 13C at the acyl carbonyl group was synthesised and heated to give de-insertion of CO. The result was that the only product formed contained the methyl substituent in a position cis to the labelled 13C. 

Multi]de insertion reactions of isocyanates have been observed in the presence of Ni catalysts. Pyri-midinediones are obtained in low yield from reaction of diphenylacetylene with excess alkyl isocyanates in the presence of Ni(COD). Similarly, alkyl and aryl isocyanates undergo simple cyclotrimerization to form symmetrical triazinetriones in the presence of both low-valent Ni and Ti catalysts. 

These materials main limitation is their great volume change during insertion/de-insertion reactions (for example, the transformation from Sn to Li4 4Sn is combined with a more than four-fold increase in the initial lattice volume) which is responsible for a loss of contact between the grains and the... 

Baggetto L, Notten PHL (2009) Lithium-ion (de)insertion reaction of germanium thin-film electrodes an electrochemical and in situ XRD study batteries and energy straage. J Electrochem Soc 156 A169-A175... 

McKervey and Ye have developed chiral sulfur-containing dirhodium car-boxylates that have been subsequently employed as catalysts for asymmetric intramolecular C-H insertion reactions of y-alkoxy-ot-diazo-p-keto esters. These reactions produced the corresponding ci -2,5-disubstituted-3(2H)-furanones with diastereoselectivities of up to 47% de. Moreover, when a chiral y-alkoxy-a-diazo-p-keto ester containing the menthyl group as a chiral auxiliary was combined with rhodium(II) benzenesulfoneprolinate catalyst, a considerable diastereoselectivity enhancement was achieved with the de value being more than 60% (Scheme 10.74). 

The rate also varies with butadiene concentration. However, the order of the rate dependence on butadiene concentration is temperature-de-pendent, i.e., a fractional order (0.34) at 30°C and first-order at 50°C (Tables II and III). Cramer s (4, 7) explanation for this temperature effect on the kinetics is that, at 50°C, the insertion reaction to form 4 from 3, although still slow, is no longer rate-determining. Rather, the rate-determining step is the conversion of the hexyl species in 4 into 1,4-hexadiene or the release of hexadiene from the catalyst complex. This interaction involves a hydride transfer from the hexyl ligand to a coordinated butadiene. This transfer should be fast, as indicated by some earlier studies of Rh-catalyzed olefin isomerization reactions (8). The slow release of the hexadiene is therefore attributed to the low concentration of butadiene. Thus, Scheme 2 can be expanded to include complex 6, as shown in Scheme 3. The rate of release of hexadiene depends on the concentra-... 

A very impressive application of this chemistry is the total synthesis of (—)-ephedradine A 102.222 The key intermediate /rcarboxylic acid ester 101 was synthesized by intramolecular C-H insertion reaction. Upon treatment with a catalytic amount of Rh2(Y-DOSP)4, aryl diazo ester 100 possessing a chiral auxiliary underwent a C-H insertion reaction to give 101 in 63% yield and 86% de (Equation (83)). 

The Claisen rearrangement of allyl vinyl ethers is a classic method for the stereoselective synthesis of y,J-unsaturated esters. The allylic C-H activation is an alternative way of generating the same products [135]. Reactions with silyl-substituted cyclohexenes 197 demonstrate how the diastereoselectivity in the formation of 198 improves (40% to 88% de) for the C-H insertion reactions as the size of the silyl group increases (TMS to TBDPS) (Tab. 14.14). Indeed, in cases where there is good size differentiation between the two substituents at a methylene site, high diastereo- and enantioselectivity is possible in the C-H activation. 

In many of these systems, the postulated olefin complex intermediate would be labile. Therefore, its role as a pre-equilibrium intermediate is not terribly relevant to the kinetic problem. I think the relevant feature is whether the favorable paths in these insertion reactions involve the first or second type of transition state. This perhaps de-emphasizes the question of whether or not a 7r-bonded intermediate is involved but certainly does focus attention on the question of whether a coordinated unsaturated species is involved as a reactant. This is because the first type of transition state will require two coordination positions and hence involve the elimination of some other ligand before it can form, whereas the second will not. I don t know the answer to this question but this is how I would formulate the problem. 

Compounds of the class I SiMnlKCOyCp) have been mentioned already in this connection (117-119, 225, 259) (Table X, entries 12, 13, 63, and 87). Reactions with tertiary phosphines, chlorine, methyl-lithium, or an excess of HC1 all lead to elimination of RgSiH ("de-insertion ), as in... 

Diazoacetamides are also exceptional substrates for dirhodium carboxamidate-catalyzed reactions, although with these substrates a mixture of /3-lactam and y-lactam products are formed [8]. The rhodium carboxamidate catalyst can have a major effect on the ratio of products formed. A good synthetic example is the Rh2(4S-MPPIM)4)-catalyzed synthesis of (-)-hcliotridanc 11 (Scheme 5) [9]. The key C-H insertion step of 9 generated the indolizidine 10 in 86 % yield and 96 % de, whereas reaction of 9 with achiral catalysts tended to favor the opposite diaster-eomer. 

Novel methods for functionalizing piperidines at the 3- and 4-positions were also introduced. Mete and co-worker synthesized 3-diazo-piperidin-2-one and characterized its reactivity in transition-metal catalyzed reactions, particularly H-X insertion reactions and cyclopropanation reactions <02T3137>. Christoffers and co-workers developed an asymmetric Michael addition reaction with a chirally modified 4-piperidone-enamine. They were able to create a quaternary carbon center in >95% de and elaborate the compound on through classical means to the functionalized piperidine 107 (Scheme 21) <02EJ01505>. 

Dirhodium(ll) tetrakis[methyl 2-pyrrolidone-5(R)-oarboxylate], Rh2(5R-MEPV)4, and its enantiomer, Rh2(5S-MEPY)4, which is prepared by the same procedure, are highly enantioselective catalysts for intramolecular cyclopropanation of allylic diazoacetates (65->94% ee) and homoallylic diazoacetates (71-90% ee),7 8 intermolecular carbon-hydrogen insertion reactions of 2-alkoxyethyl diazoacetates (57-91% ee)9 and N-alkyl-N-(tert-butyl)diazoacetamides (58-73% ee),10 Intermolecular cyclopropenation ot alkynes with ethyl diazoacetate (54-69% ee) or menthyl diazoacetates (77-98% diastereomeric excess, de),11 and intermolecular cyclopropanation of alkenes with menthyl diazoacetate (60-91% de for the cis isomer, 47-65% de for the trans isomer).12 Their use in <1.0 mol % in dichloromethane solvent effects complete reaction of the diazo ester and provides the carbenoid product in 43-88% yield. The same general method used for the preparation of Rh2(5R-MEPY)4 was employed for the synthesis of their isopropyl7 and neopentyl9 ester analogs. 

After the discovery of this unique transformation, the scope was explored. When certain 1,2-dihydronaphthalene substrates 158 were utilized, it was found that the direct C-H insertion products 161 were formed in extraordinarily high diastereose-lectivity and enantioselectivity (>98 de and >95 ee, Scheme 38), much higher than was typically observed for insertion reactions with aryldiazoacetates (see Sect. 3.2). Thus, it was reasoned that these compounds were actually being produced via a combined C-H activation/Cope rearrangement, through a transition state such as 162, followed by a retro-Cope. Furthermore, appropriate substrate modification allowed isolation of the C-H activation/Cope adduct 160, providing products in >98% de and up to >99% ee [112, 113]. In certain cases, a double C-H insertion into dihydronaphthalene substrates was also observed [112],... 

The insertion of acetylene into the Pd-CH3 bond of the complex PdCl(NH3)(CH3) has been studied by de Vaal and Dedieu [63] by using the valence double-zeta basis sets. The geometries of the prereaction complex [PdCl(NH3)(CH3)(C2H2)], the transition state, and product have been optimized at the SCF level, and their energetics has been improved at the CASSCF and Cl level. It has been shown that acetylene is quite weakly bound (5.8 kcal/mol) in the square-planar Pd(II) complexes because of weak 7T back-donation from Pd to the tt orbital of C2H2. The insertion barrier calculated relative to the acetylene complex is 20.5, 22.6, and 17.1 kcal/ mol at the SCF, CASSCF, and Cl levels of theory, respectively, and the transition state corresponding to this barrier displays monohapto coordination of acetylene. The entire insertion reaction is calculated to be exothermic by 26.0, 19.3, and 22.4 kcal/mol at the SCF, CASSCF, and Cl levels, respectively, relative to the acetylene complex. 

Ion insertion/de-insertion during doping/de-doping reactions... 

The electrode materials currently being used in lithium ion batteries are based on lithium intercalation/de-intercalation reactions. Such reactions are inherently tied to crystallographic considerations. The insertion of high concentrations of lithium ions is usually limited to one lithium atom per host atom because hthium concentrations above this level result in phase transformations that may lead to the formation of irreversible phases. Recently, however, another approach has been devised in which the material is not constrained by intercalation and instead accomplishes energy storage through a process based on conversion reactions. 

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