Pericyclic Reactions involving Metals

Pericyclic Reactions involving Metals.—Further data on the Diels-Alder addition of dienophiles to the cyclopentadiene ring in the complexes [Pt(Tj -C5H5XYXcod)] have appeared reaction occurs from the side remote from the platinum atom. Rate studies reveal that metal carbonyls accelerate the 1,3-dipolar cycloaddition of diazo-esters to norbornadiene. Cycloreversion occurs under extremely mild conditions, affording 3-ethoxycarbonylpyrazole, for example, from the reaction of 

The ease of isomerization and the product distribution are greatly influenced by the catalyst employed, (290) becoming the major product by use of the catalyst system derived from bis(cyclo-octa-l,5-diene)nickel(0) and tetracyanoethylene. The origins of the peri- and regio-selectivities have been explained in terms of a MO treatment extended-Huckel calculations reveal that among the strained C—C bonds the C-2—C-5 and structurally equivalent bonds have MO shapes suitable for edge-on 

The 1 1 complex derived from phenyltungsten trichloride and aluminium trichloride is an effective catalyst for diene-cyclobutane metathetical interconversions. Thus, the tetracyclic compounds (291) and (292) were respectively isomerized to the dienes (293) and (294). Rather more surprising was the virtually quantitative formation of the cyclobutanoid compound (296) from (295). Reaction of norbomadiene with 2,2 -bipyridyl(cyclo-octa-l,5-diene)nickel at 25°C yielded the exo-trans,endo-metal o-carbocyclic (297) which, on treatment with an activated olefin (e.g. maleic anhydride), afforded the cyclo-dimer (298 predominantly exo-trans,endo) in good yield by displacement of the hydrocarbon moiety. Catalytic conversions can also be achieved. 

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Pericyclic Reactions involving Metals.—Migratory aptitudes in the Mo(CO)g-pro-moted isomerizations of [4,4,2]propella-2,4,ll-trienes (329) are reported in two papers. The metal atom blocks the intramolecular Diels-Alder reaction by... 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

The mechanisms of these reactions are still a matter of controversy and some at least proceed via organometallic intermediates formed by replacement of hydrogen atoms in the reactant by metals. However it seems very likely that the original examples [equation (5.325)] represent a true catalysis by a transition metal of a pericyclic reaction involving an antiaromatic transition state. 

Allenylidenes could be considered as divalent radicals derived from allenes. In a similar way to vinylidenes, allenylidenes can be stabilized by coordination with transition metals and again ruthenium is one of the most widely used metals. Metal-allenylidene complexes can be easily obtained from terminal propargylic alcohols by dehydration of the initially formed metal-hydroxyvinylidenes, in which the reactivity of these metal complexes is based on the electrophilic nature of Ca and Cy, while Cp is nucleophilic. Catalytic processes based on nucleophilic additions and pericyclic reactions involving the it system of ruthenium allenylidenes afford interesting new structures with high selectivity and atom economy. 

There is a large variety of polar and radical reactions, transition metal-catalyzed and pericyclic conversions, that have been carefully developed with regard to scope, selectivity, and yield. They are compiled in large compendia, for example, in [16-19], and in series, for example [20, 21], and are continuously improved and extended in timely research papers. This literature should be consulted in parallel with suggestions taken from electrosynthesis. Electrosynthesis is a clear alternative to chemical synthesis, when reactive intermediates (see Sect. 3.3) such as radical ions, radicals, carbanions, or carboca-tions are involved. The more advantageous are summarized in the following sections. 

This chapter examines reactions that involve molecular rearrangements and cycloadditions. The use of these terms will not be restricted to concerted, pericyclic reactions, however. Often, stepwise processes that involve a net transformation equivalent to a pericyclic reaction are catalyzed by transition metals. The incorporation of chiral ligands into these metal catalysts introduces the possibility of asymmetric induction by inter-ligand chirality transfer. The chapter is divided into two main parts (rearrangements and cycloadditions), and subdivided by the standard classifications for pericyclic reactions e.g., [1,3], [2,3], [4-1-2], etc.). The latter classification is for convenience only, and does not imply adherence to the pericyclic selection rules. Indeed, the first reaction to be described is a net [1,3]-suprafacial hydrogen shift, which is symmetry forbidden if concerted. 

In 1953, Robert s experiments on the conversion of C-labeled chlorobenzene with KNH2 into aniline gave strong support to the intermediacy of ortho-benzyne in this and related reactions. Additional direct evidence for the existence of ortho-benzyne was provided by the observation of its IR spectrum, sohd-state dipolar NMR spectrum, and NMR in a molecular container, and by UV photoelectron spectroscopy. Even at low temperatures, arynes are extraordinary reactive. The reactions of arynes can be divided into three groups (i) pericyclic reactions, (ii) nucleophilic additions, and (iii) transition-metal catalyzed reactions. The pericyclic reactions can be divided into several categories such as Diels-Alder reactions, [2-f2] cycloadditions, 1,3- and l,4-dipolar cycloadditions, and the ene reactions. Arynes react with practically aU kinds of nucleophiles. More recently, the transition-metal catalyzed reactions of arynes have been studied, in particular those involving palladium. 

The second group of reactions involves the (4- -3) and (5 + 2) cycloadditions, in some cases being also pericyclic and therefore subject to the Woodward-Hoffmann rules, or transition metal catalyzed (Fig. 7). We use the (x+y) descriptor for the topological presentation of the number of atoms. 

The mechanism of olefin metathesis does not involve the classic reactions we have covered—namely, oxidative addition, reductive elimination, (3-hydride elimination, etc. Instead, it simply involves a [2+2] cycloaddition and a [2+2] retrocycloaddition. The [2+2] terminology derives from pericyclic reaction theory, and we will analyze this theory and the orbitals involved in this reaction in Chapter 15. In an organometallic [2+2] cycloaddition, a metal alkylidene (M=CR2) and an olefin react to create a metal lacyclobutane. The metalla-cyclobutane then splits apart in a reverse of the first step, but in a manner that places the alkylidene carbon into the newly formed olefin (Eq. 12.83). Depending upon the organometallic system used, either the alkylidene or the metallacycle can be the resting state of the... 

There is evidence that electrostatic catalysis works for a number of pericyclic reactions (Table 1) and we may speculate that potentially similar effects arise for rearrangements in water or other polar solvents where the initial state of the reaction involves less polar species than the transition state. In such cases metal or other small ions may stabilize the transition state by electrostatic interactions which may or... 

A combined system formed from Co(acac)3, 4 equiv of diethylalu-minum chloride, and chiral diphosphines such as (S,S)-CHIRAPHOS or (/ )-PROPHOS catalyzes homo-Diels-Alder reaction of norbomadiene and terminal acetylenes to give the adducts in reasonable ee (Scheme 109). Use of NORPHOS in the reaction of phenylacetylene affords the cycloadduct in 98.4% ee (268). It has been postulated that the structure of the active metal species involves noibomadiene, acetylene, and the chelating phosphine. The catalyzed cycloaddition may proceed by a metallacycle mechanism (269) rather than via simple [2+2 + 2] pericyclic transition state. 

Many types of pericyclic and cycloaddition reactions have been documented. Although there are no general guidelines for the asymmetric preparation, reagents such as chiral catalysts are providing more general routes. Many of the reactions discussed rely on the use of low temperature. Although it is expensive to conduct low temperature reactions on an industrial scale, reactions that need temperatures of down to -105°C can be conducted. It should be noted that at such temperatures only stainless steel vessels, which require neutral or basic conditions, can be used at these extreme temperatures. In addition, reactions that involve the use of a metal can cause contamination problems in wastewater or the product. 

Pericyclic electrochemical reactions are increasingly developed. They involve chain reactions with a radical cation as chain transferring step or the generation of reactive dienophiles (see Chapter 22, Sec. V). Transition metal complexes are increasingly applied in electrochemistry as electrocatalysts for reductive carboxylation [47], acylation or alkylation [41], or activation [51]. 

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