Substitution reactions metal alkene complexes from

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

Since the early work dealing with Zeise s salt, many complexes have been prepared with the formula [PtL(C2H4)X2], where L = quinoline, pyridine, or ammonia and X=C1 , Br , I, or N()2. Similar compounds have been prepared that contain other alkenes than C2H4. Many of the complexes containing dienes, trienes, and tetraenes as ligands also contain carbonyl ligands. In fact, metal carbonyls are frequently starting complexes from which alkene complexes are obtained by substitution reactions. 

The synthesis of polyhalide salts, R4NX , used in electrophilic substitution reactions, are described in Chapter 2 and H-bonded complexed salts with the free acid, R4NHX2, which are used for example in acid-catalysed cleavage reactions and in electrophilic addition reactions with alkenes, are often produced in situ [33], although the fluorides are obtained by modification of method I.I.I.B. [19, 34], The in situ formation of such salts can inhibit normal nucleophilic reactions [35, 36]. Quaternary ammonium chlorometallates have been synthesized from quaternary ammonium chlorides and transition metal chlorides, such as IrClj and PtCl4, and are highly efficient catalysts for phase-transfer reactions and for metal complex promoted reactions [37]. 

Other methods for obtaining complexes of ethylene and other alkenes include ligand substitution reactions, reduction of a higher valent metal in the presence of an alkene, and synthesis from alkyl and related species [reductive elimination, of an allyl or hydride, for example hydride abstraction from alkyls protonation of sigma-allyls from epoxides (indirectly)] [74a],... 

The catalytic enantioselective addition of aromatic C - H bonds to alkenes would provide a simple and attractive method for the formation of optically active aryl substituted compounds from easily available starting materials. The first catalytic, highly enantioselective Michael addition of indoles was reported by Jorgensen and coworkers. The reactions used a,fl-unsaturated a-ketoesters and alkylidene malonates as Michael acceptors catalyzed by the chiral bisoxazoline (BOX)-metal(II) complexes as described in Scheme 27 [98,99]. 

The formation of the sultone (160) probably involves addition of the complex across the alkene double bond, a 1,2-hydride shift and an intramolecular nucleophilic substitution reaction. The sultone (161) is formed by addition of sulfur trioxide to give the unstable p-sultone which rearranges to the more stable y-isomer (161). Another useful route to sultones is by metallation of alkanesulfonate esters for example, butane-1,3-dimethylsulfonate (162), prepared from butanel,3-diol, yields the 8-sultone, namely 6-methyl-l,2-oxathiin-2,2-dioxide (163) (Scheme 67). 

Throughout this chapter, stabilized cations have been used for a variety of transformations. The Wacker process involved alkenes but when the alkene has an allylic position, a new type of organometallic can be formed with palladium reagents. 1-Propene, for example, reacts to give a Jt-allyl metal complex such as 338, stabilized by back-donation from the metal atom. Substitution reaction of this Jt-allyl complex with a suitable nucleophile will generate the allylic species, 339, where X is the nucleophile. Early work by Hiittel and Christ lS and also Volger l established that jt-allyl complexes could be prepared, but often in very poor... 

AstraZeneca published the use of an immobilized transition metal carbonyl complex as a catalyst in the Pauson-Khand reaction [67]. This reaction is known to produce useful products but it also suffers from a number of drawbacks dicobalt octacarbonyl and its analogs are volatile, toxic, and unstable due to loss of carbon monoxide and aerial oxidation. These drawbacks can be avoided by the use of an immobilized metal carbonyl complex (Scheme 13), which is safe and convenient to handle (see also [68]). It offers the additional advantages of being reusable after recovery from the reaction medium and the product becomes less contaminated with metal carbonyl remnants. The reaction was applicable to a wide range of substrates with the exception of tetra-substituted alkenes. A typical reaction of enine 32 to the bicyclic enone 33 is depicted in Scheme 13. 

The use of both LIU and HIU has been shown to increase the efficiency of the P-K reaction, which involves the formation of cyclopentenone from the annulation of a cobalt alkynyl carbonyl complex and an alkene. The use of low-power ultrasound, as for example, from a cleaning bath, although capable of producing intramolecular P-K reactions, generated relatively low cyclization yields. The motivation for the use of high intensity came from its ability, as previously described, to effectively decarbonylate metal carbonyl and substituted metal carbonyl complexes. Indeed, HIU produced by a classic horn-type sonicator has been shown to be capable of facile annulation of norbornene and norbornadiene in under 10 min in the presence of a trimethylamine or trimethylamine N-oxidc dihydrate (TMANO) promoter, with the latter promoter producing cleaner product mixtures. This methodology also proved effective in the enhancement of the P-K reaction with less strained alkenes such as 2,5-dihydrofuran and cyclopentene, as well as the less reactive alkenes -fluorostyrene and cycloheptene. The mechanism has been postulated to involve decarbo-nylation of the cobalt carbonyl alkyne, followed by coordination by the amine to the vacant coordination sites on the cobalt. 

Functionalized benzenes preferentially induced ortho-para substitution with electron-donating groups and meta substitution with electron-withdrawing groups (see above). Additionally, the order of reactivity found with aromatics was similar to that of electrophilic aromatic substitution. These observations implicated an electrophihc metalation of the arene as the key step. Hence, Fujiwara et al. [4b] believed that a solvated arylpalladium species is formed from a homogeneous solution of an arene and a palladium(ll) salt in a polar solvent via an electrophilic aromatic substitution reaction (Figure 9.2). The alkene then coordinates to the unstable arylpalladium species, followed by an insertion into the aryl-palladium bond. The arylethyl-palladium intermediate then rapidly undergoes )8-hydride elimination to form the alkenylated arene and a palladium hydride species, which then presumably decomposes into an acid and free palladium metal. Later on, the formation of the arylpalladium species proposed in this mechanism was confirmed by the isolation of diphenyltripalladium(ll) complexes obtained by the C-H activation reaction of benzene with palladium acetate dialkylsulfide systems [19]. 

Alkenes and alkynes are capable of normal substitution reactions like other nucleophiles, but this is rarely an associative process with 18-electron systems because alkenes and alkynes are poor nucleophiles. Such systems require prior dissociation of a ligand to allow coordination of the alkene, which can then form strong complexes because of the 7C back bonding from the metal to the Jt orbital of the C—C multiple bond. The bonding n electrons have relatively low nucleophilicity. However, in coordinatively unsaturated systems, prior dissociation is not a problem. The expected initial reactions are shown in Scheme 5.21. 

The formation of cyclopropanes from o(,P-unsaturated esters occurs under conditions at least as severe as those employed in the phosphine substitution and CO exchange reactions of carbene complexes. Coordinatively unsaturated carbene complexes are therefore reasonable intermediates for these reactions. Complexation of an alkene to the metal complex provides a means of bringing the carbene and alkene ligands into close proximity. Formation of a metallocyclobutane and reductive elimination of a cyclopropane complete our suggested mechanism (see Scheme 8). 

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