Allyl cations Subject

The intramolecular 4 + 3-, 3 + 3-, 4 + 2-, and 3 + 2-cycloaddition reactions of cyclic and acyclic allylic cations have been reviewed, together with methods for their generation by thermal and photochemical routes.109 The synthetic uses of cycloaddition reactions of oxyallyl cations, generated from polybromo and some other substrates, have also been summarized seven-membered rings result from 4 + 3-cycloadditions of these with dienes.110 The use of heteroatom-stabilized allylic cations in 4 + 3-cycloaddition reactions is also the subject of a new experimental study.111 The one-bond nucleophilicities (N values) of some monomethyl- and dimethyl-substituted buta-1,3-dienes have been estimated from the kinetics of their reactions with benzhydryl cations to form allylic species.112 Calculations on allyl cations have been used in a comparison of empirical force field and ab initio calculational methods.113... 

The intramolecular version of this reaction has many applications. The stereochemistry of the starting material shown in Eq. (168) seems to influence the efficiency of the reaction [421,422]. The acceptor of the allylic cationic species is not limited to conjugated dienes. The following result shown in Eq. 169 indicates that a simple terminal olefin might serve as the reaction partner to give bicyclic cyclopentane skeleton [423]. In this formal [3 -n 2] cyclization, the reaction should proceed stepwise. Other results of inter- or intramolecular [3 -1- 4] or [3 -1- 2] cyclizations are collected in Table 15. The stereochemistry of the reactions has been studied in detail and is the subject of more extensive reviews [418,424]. 

When naphthalene oxides of general structure 194 are subjected to acid-induced ring opening, the generation of 2-substituted naphthols 196 were overwhelmingly favored over the 3-substituted naphthols (Scheme 11) [180]. The explanation put forward was that allylic cation 195 was significantly more stable than 197. 

The base-catalysed cis-trans isomerization of l-aryl-2-phenylcyclopropanes has been subjected to kinetic analysis and a complexed anion, e.g. (190), is thought to be produced initially. Configurational inversion of the cyclopropyl sulphoxide (173) can also be effected by base. A kinetic analysis of the hydrolysis of 2- and 2,2-di-substituted bromocyclopropanes suggests that for 2-vinyl-substituted compounds considerable progress towards an allyl cation has been been made at the transition state. Metallation studies show that, for nortricyclanes at least, the cyclopropyl hydrogen atoms are most readily attacked by pentylsodium in the presence of potas-... 

It is tempting to think that allylic cations would behave similarly to other stabilized cations in their reactions with alkyl azides. " In practice what happens is invariably an initial formal [3+3] cycloaddition of azide to the allylic cation, which is followed by a migration event (typically of hydride) or trapping by a nucleophile. Pearson and coworkers found that when sulfonylindole 27 was treated with SnCk at -78 °C, followed by basic workup, triazoline adducts 28 were obtained as mixture of chloride epimers (Scheme 7.24). When A -alkyl indoles (e.g. 29) were subjected to Lewis acidic conditions, tri-azines such as 30 were obtained as the sole products. In the former case involving an iV-sulfonyl indole, a [3+2] cycloaddition pathway explains the product, whereas the iV-alkyl indoles examined underwent [3+3] cycloaddition. 

In Summary Conjugated dienes are electron rich and are attacked by electrophiles to give intermediate allylic cations on the way to 1,2- and 1,4-addition products. These reactions may be subject to kinetic control at relatively low temperatures. At relatively higher temperatures, the kinetic product ratios may change to thermodynamic product ratios, when such product formation is reversible. 

Desmethylindolactam G (50, R=H, replace zPr by H) was subjected to the same sequence (50 54). The first allylation step succeeded with 52% yield, but the aza-Claisen rearrangement/cationic cyclization gave only 9.5% of the product mixture 52-54 (replace /Pr by H). 

When the ferf-butoxycarbonyl (Boc) carbamate-derived sulfone 5 was subjected to the Bi(OTf)34H20-catalyzed allylation conditions (Scheme 2), the cyclic carbamate 6 was obtained as the major product (36%, diastereoisomeric ratio (dr) = 82 18), along with the corresponding allylation product 7, although in low yield (16%). Such cyclic carbamate resulting from the internal capture of an intermediate (3-silyl cation with the Boc group and concomitant loss of isobutylene has already been reported in the literature [59]. 

The exact mechanism of lattice oxygen incorporation and second hydrogen abstraction, and the precise sequence of elementary events is still a subject of speculation. Several authors assume that two distinct active sites are involved in the acrolein formation. The first, presumably a cation, participates in the formation of the initial allyl complex, while the second, which may contain a different cation and reactive oxygen anions, is the place where further hydrogen abstraction and oxygen incorporation take place. 

However, cationic allyl-Pd complexes are subject to attack by a nucleophile.16 Such an attack releases the catalytically active Pd(0)... 

The cationic Jt-allyl complex is often isolable and has been the subject of considerable study. X-ray structures of the 7t-allyl complexes with chiral ligands have been a primary source of structural information from which the design and predictive model of chiral catalysts derive. Chiral-metal-olefin complexes, which constitute another important class of intermediates have also been isolated, albeit few in number [31]. These static studies have been complemented by a growing number of NMR studies taking advantage of modem heteronuclear correlation and NOE techniques, which offer opportunities to monitor solution structures of the catalytic species [32-34],... 

Treatment of ethacrylate esters 1 with nitronium tetrafluoroborate in acetonitrile has been shown to give cyclopropanes 2 and the products of allylic nitration 3. Formation of 2 was postulated to proceed via an a-carbonyl cation. In an attempt to obtain evidence for the possible intermediacy of a-carbonyl cations in these reactions in terms of Wagner-Meerwein derived products, the more highly substituted substrates 4a, b were subjected to the same reaction conditions of NC>2BF4/MeCN followed by aqueous work-up. This gave 5a, b and 6a, b as shown. 

Fig. 19 Allyl enammonium salts are encapsulated within the supramolecular assembly, promoting electrocyclization to the corresponding iminium cation. This cation is subject to hydrolysis in basic solution, yielding the aldehyde products while promoting catalytic turnover... 

Allyl)Fp complexes are also subject to attack, at C-3, by radicals. The mechanism of allylic transposition of ()] -allyl)Fp complexes, as well as the mechanism of phosphite substitution for CO, has been ascribed to attack by Cp(CO)(L)Fe- on the original Fp-aUyl. The reaction of (12) with CCI4 proceeds by a radical chain mechanism, ultimately between CCI3 and the Fp-allyl. The substitution of a-halo ketones and esters most likely proceeds similarly. A radical cation coupling mechanism has been proposed for the dimerization of (jj -allyl)Fp and (j7 -propargyl)Fp complexes. ... 

Nucleophilic attack on ( -alkene)Fp+ cations may be effected by heteroatom nucleophiles including amines, azide ion, cyanate ion (through N), alcohols, and thiols (Scheme 39). Carbon-based nucleophiles, such as the anions of active methylene compounds (malonic esters, /3-keto esters, cyanoac-etate), enamines, cyanide, cuprates, Grignard reagents, and ( l -allyl)Fe(Cp)(CO)2 complexes react similarly. In addition, several hydride sources, most notably NaBHsCN, deliver hydride ion to Fp(jj -alkene)+ complexes. Subjecting complexes of type (79) to Nal or NaBr in acetone, however, does not give nncleophilic attack, but instead results rehably in the displacement of the alkene from the iron residue. Cyclohexanone enolates or silyl enol ethers also may be added, and the iron alkyl complexes thus produced can give Robinson annulation-type products (Scheme 40). Vinyl ether-cationic Fp complexes as the electrophiles are nseful as vinyl cation equivalents. ... 

Treatment of A-nitroso-iV-(cr5-2-phenylcyclopropyl)urea with excess sodium formate in methanol at 25 "C for 15 hours affords ( )-3-methoxy-l-phenylprop-l-ene (25%) and 3-methoxy-3-phenylprop-l-ene (60%, Table 15, entry 7). An almost identical result is obtained when the tran -isomer is used as substrate. Formation of the 2-phenylcyclopropyldiazonium ion, decomposition with loss of nitrogen to the 2-phenylcyclopropyl cation, followed by ring opening to form the phenylallyl cation, can explain the result. When [l- Hi]aminocyclopropane is subjected to deamination with nitrous acid in water at 0°C (Table 15, entry 8), allyl alcohol is obtained, which is isolated as the 4-(phenylazo)benzoate derivative in 44% yield. In this case, the deuterium is only located at the C2 allyl position. ... 

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