Neutral manifold

Scheme 6-29 The cationic and neutral manifold.s of the Heck reaction with bidentate phosphines.

A counterpart to the splitting lemma of elementary catastrophe theory is the centre manifold theorem (also called a neutral manifold), generalizing the Grobman-Hartman theorem to the case of occurrence of sensitive states (5.28). The centre manifold theorem allows us to establish an equivalence (nonequivalence) of two autonomous systems. In this sense it is also a generalization to the case of autonomous systems of the equivalence relationship introduced in Chapter 2 for potential functions. 

The most common Mizoroki-Heck reaction mechanism is called the neutral mechanism, because its intermediates are uncharged. The catalytic cycle for the neutral manifold of the intramolecular Mizoroki-Heck reaction of alkenyl and aryl halides is shown in Scheme... 

Silanes. Group IVB metallocenes catalyze the stepwise polymerization (dehydrocoupling) of primary silanes, yielding H—(SiHR) —H and n H2 (232). The mechanism appears to he strictly metathetical for such molecules as (Cp)(C5(CH3)5)Hf(X) (R = H, alkyl X = H, alkyl, chloride), wherein the entire process takes place on the neutral manifold (Scheme 11) (233). 

Asymmetric intramolecular carbopalladation is an effective reaction for producing enantioenriched polycycles. Most examples are intramolecular Heck reactions. One seminal example from natural product synthesis is the 5-exo carbopalladation of eneamide 201 to oxindole 202 (after acid treatment) from a total synthesis of ( )-physostigmine 203 (Scheme 31).The reaction occurs in 84% yield with 95% ee, which is remarkably efficient for the construction of a quaternary center. Reaction conditions that favor the neutral manifold of the Heck reaction are employed. Examination of the scope of the oxindole synthesis and mechanistic analysis have appeared. " Group selective reactions are also powerful reactions in carbopalladation asymmetric synthesis.From a synthesis of (+)-vemolepin 206, alkenyl triflate 204 is... 

Figure 7. Potential energy diagram for HI, showing the two lowest ionization states (2n3//2 and 2 IT j, ) coupled to a neutral dissociative continuum (3Ao) at the three-photon (3

Although Ni(CO)4 was discovered many years ago, no neutral Ni2(CO)x compound has ever been synthesized in macroscopic amounts. However, several communications report ionic species such as [Ni2(CO)8l+, [Ni2(CO)7], and [Ni2(CO)6]+, where structures with one or two bridging carbonyls are proposed.2418 Plausible structures for neutral Ni2(CO)x (x = 5, 6, 7) have been investigated by theoretical methods, and decomposition temperatures well below room temperature have been predicted.2419,2420 Tetra-, penta-, and hexanuclear nickel carbonyl clusters have been investigated by means of molecular orbital theory. It is found that the neutral forms are more stable than the corresponding anionic forms but the anionic forms gain in stability as the nuclearity rises.2421 Nickel carbonyl cluster anions are manifold, and structural systematics have been reviewed.2422,2423 An example includes the anion [Ni9(CO)i6]2- with a close-packed two-layer metal core.2424... 

Detailed aspects of the catalytic mechanism remain unclear. However, influence of basic additives on the partitioning of the conventional hydrogenation and reductive cyclization manifolds coupled with the requirement of cationic rhodium pre-catalysts suggests deprotonation of a cationic rhodium(m) dihydride intermediate. Cationic rhodium hydrides are more acidic than their neutral counterparts and, in the context of hydrogenation, their deprotonation is believed to give rise to monohydride-based catalytic cycles.98,98a,98b Predicated on this... 

The analogous Z aryl triflate 19.1 reacts under the cationic manifold to give, ultimately, oxindole (/ )-17.3a in 72% yield and 43-48% ee (Scheme 8G.19) [38]. An important synthetic advance is the observation that Heck cyclization of this substrate could be diverted to the more selective neutral pathway by addition of halide salts. For example, Heck cyclization of triflate 19.1 in the presence of 1 equiv. of n-Bu4NI gave (/ )-17.3a in 62% yield and 90% ee, which is similar to the enantioselectivity obtained for cyclization of the corresponding iodide 18.1c under neutral conditions (see entry 6, Table 8G, 1). Conversely, cyclization of iodide 18.1c in the... 

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