Ortho alkenes, reactivity

Several arylations involving reactive alkenes such as norbomene or allenes have been reported. Togni and coworkers have shown that norbomene is selectively added to the ortho positions of phenols to produce a mixture of 30 and 31 in 69% and 13% yield, respectively, after 72 hours at 100°C (22) [108, 109]. 1,1-dimethylallene also reacts with aromatic carboxamides (33) to produce prenylation products (34) in the presence of cationic iridium complexes (23) [110]. In both cases, initial ortho C-H bond activation in arenes directed by coordinating groups followed by olefin insertion has been proposed. 

Co(ni) alkyl peroxides have been prepared and used by Mimoun and coworkers in the hydroxylation of hydrocarbons with this metal a Haber-Weiss type of reactivity is suggested. Square-planar Pt(II) complexes, of the type [(dppe)Pt(CF3)(solv)], used by Strukul in the epoxidation of alkenes and in Baeyer-Villiger oxidations of ketones (Schemes 8 and 9), are effective catalysts also in the direct hydroxylation of aromatics with hydrogen peroxide. The reactivity increases in the presence of electron releasing substituents in the aromatic ring. Ortho and para derivatives are practically the only products observed and interesting selectivity toward the ortho products has been detected (equation 85). 

The effect of monofluorination on alkene or aromatic reactivity toward electrophiles is more difficult to predict Although a-fluonne stabilizes a carbocation relative to hydrogen, its opposing inductive effect makes olefins and aromatics more electron deficient. Fluorine therefore is activating only for electrophilic reactions with very late transition states where its resonance stabilization is maximized The faster rate of addition of trifluoroacetic acid and sulfuric acid to 2-fluoropropene vs propene is an example [775,116], but cases of such enhanced fluoroalkene reactivity in solution are quite rare [127] By contrast, there are many examples where the ortho-para-dueeting fluorine substituent is also activating in electrophilic aromatic substitutions [128]... 

Pyridylarenes undergo Cu(II)-catalysed diverse oxidative C-H functionalization reactions. The tolerance of alkene, alkoxy, and aldehyde functionality is a synthetically useful feature of this reaction. A radical-cation pathway (Scheme 4) has been postulated to explain the data from mechanistic studies. A single electron transfer (SET) from the aryl ring to the coordinated Cu(II) leading to the cation-radical intermediate is the rate-limiting step. The lack of reactivity of biphenyl led to the suggestion that the coordination of Cu(II) to the pyridine is necessary for the SET process. The observed ortho selectivity is explained by an intramolecular anion transfer from a nitrogen-bound Cu(I) complex.53... 

Another reaction in which the excited alkene is the reactive species in ortho photocycloaddition is that of dichlorovinylene carbonate with benzene [82-84] (Scheme 15). 

In the last 15 years, Wagner and his co-workers have developed the intramolecular ortho photocycloaddition of alkenes to the benzene ring of the acetophenone chromophore. In these photoreactions, the ring opening products of ortho photocycloadducts, 1,3,5-cyclooctatriene derivatives (220), and their secondary photocycloadducts, tricyclic compounds (221), were often produced [271,272] (Scheme 63). The triplet exciplexes generated from ir, ir -triplet states of acetophenone derivatives were postulated as the reactive intermediates [273],... 

A variety of four-membered ring compounds can be obtained with photochemical reactions of aromatic compounds, mainly with the [2 + 2] (ortho) photocycloaddition of alkenes. In the case of aromatic compounds of the benzene type, this reaction is often in competition with the [3 + 2] (meta) cycloaddition, and less frequently with the [4 + 2] (para) cycloaddition (Scheme 5.7) [38-40]. When the aromatic reaction partner is electronically excited, both reactions can occur at the 7t7t singlet state, but only the [2 + 2] addition can also proceed at the %% triplet state. Such competition was also discussed in the context of redox potentials of the reaction partners [17]. Most frequently, it is the electron-active substituents on the aromatic partner and the alkene which direct the reactivity. The [2 + 2] photocycloaddition is strongly favored when electron-withdrawing substituents are present in the substrates. In such a reaction, crotononitrile 34 was added to anisole 33 (Scheme 5.8, reaction 15) [41 ], and only one regioisomer (35) was obtained in good yield. In this transformation, the... 

Selectivity is predicted by examination of the orbital coefficients of the HOMO and LUMO for both diene and alkene. The transition state for a typical reaction is shown for the reaction of methyl acrylate and 2-phenyl-1,3-butadiene (see 67), where the orbitals with largest coefficients (HOMOdiene-LUMOalkene) combine (the absolute value of the orbital coefficient is used, -0.625 0.69l and -0.475 1-0.471)) to predict the cycloadduct produced in the greatest amount (the para product, 68). Just as the HOMOgy-LUMOaikene is [0 - (-8.77) = 8.77], and it predicts the relative reactivity of these reactants. The magnitude of the orbital coefficients in each partner is important for predicting selectivity. In this case, the orbital coefficients are 0.065 and 0.004, respectively, and there is selectivity for the para product. Similarly, the orbital coefficients for 1-methoxy-1,3-butadiene and acrolein correctly predict the major product is the ortho adduct. For simple cases, the orbital coefficients can be estimated by a simple Hiickel molecular orbital calculation (a very low level calculation but one that gives a first approximation that is useful for estimating relative differences). 

As a reactive dienophile ortbo-benzyne also participates in the ene reaction. Thus, alkenes with allylic hydrogen can undergo concerted reaction to give substituted benzenes. However, the yields are rarely good. Cycloaddition of ortho-benzyne to alkynes should in principle give benzocyclobutadienes. Such intermediates are highly unstable and not surprisingly are not isolated. Instead, the products, formed in low yield, derive from further reaction with another molecule of ortho-benzyne or by dimerization (Scheme 7.32). 

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]. 

As one would expect that increasing steric hindrance in the catalytically active rhodium complex will result in lower reaction rates, the results of Van Leeuwen and Roobeek seemed contradictory at first. They used the very bulky tris(ortho tert-butylphenyl)phosphite la (Figure 2) as a ligand and found high reaction rates in the rhodium catalyzed hydroformylation of otherwise unreactive alkenes like 1,2-and 2,2-dialkyalkenes (see table 1) [8]. The high reactivity was explained by the exclusive formation of... 

Chiral biaryl Schiff base-CuOTf complexes also efficiently catalyze asymmetric aziridination. ortho Substituents prove once more to be crucial since ligands derived from 2,6-disubstituted benzaldehyde and particularly from 2,6-dichlorobenzaldehyde provide, by reaction with CuOTf, monomeric species of high reactivity and selectivity. Under these conditions, asymmetric aziri-dination of trans- and c/s alkenes occurs with very good enantio-selectivities (eqs 92 and 93). 

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