Terminal olefins palladium

Selective aromatic functionalization has been a permanent object of research since the ninenteenth century. Catalysis has offered a powerful tool to achieve this goal. Over the years we have worked out a complex catalytic system consisting of an inorganic compound such as a palladium salt and an organic molecule containing a strained double bond such as norbomene (1,2). We have seen that these two catalysts cooperatively react with an aromatic iodide, an alkyl iodide and a terminal olefin. The following equation reports an example (L = solvent and/or olefin) (3). 

In order to rationalize the catalyst-dependent selectivity of cyclopropanation reaction with respect to the alkene, the ability of a transition metal for olefin coordination has been considered to be a key factor (see Sect. 2.2.1 and 2.2.2). It was proposed that palladium and certain copper catalysts promote cyclopropanation through intramolecular carbene transfer from a metal carbene to an alkene molecule coordinated to the same metal atom25,64. The preferential cyclopropanation of terminal olefins and the less hindered double bond in dienes spoke in favor of metal-olefin coordination. Furthermore, stable and metastable metal-carbene-olefin complexes are known, some of which undergo intramolecular cyclopropane formation, e.g. 426 - 427 415). 

The Fukuyama indole synthesis involving radical cyclization of 2-alkenylisocyanides was extended by the author to allow preparation of2,3-disubstituted derivatives <00S429>. In this process, radical cyclization of 2-isocyanocinnamate (119) yields the 2-stannylindole 120, which upon treatment with iodine is converted into the 2-iodoindole 121. These N-unprotected 2-iodoindoles can then undergo a variety of palladium-catalyzed coupling reactions such as reaction with terminal acetylenes, terminal olefins, carbonylation and Suzuki coupling with phenyl borate to furnish the corresponding 2,3-disubstituted indoles. 

Catalytic asymmetric hydrosilylation of prochiral olefins has become an interesting area in synthetic organic chemistry since the first successful conversion of alkyl-substituted terminal olefins to optically active secondary alcohols (>94% ee) by palladium-catalyzed asymmetric hydrosilylation in the presence of chiral monodentate phosphine ligand (MOP, 20). The introduced silyl group can be converted to alcohol via oxidative cleavage of the carbon-silicon bond (Scheme 8-8).27... 

The electrochemical Wacker-type oxidation of terminal olefins (111) by using palladium chloride or palladium acetate in the presence of a suitable oxidant leading to 2-alkanones (112) has been intensively studied. As recyclable double-mediatory systems (Scheme 43), quinone, ferric chloride, copper acetate, and triphenylamine have been used as co-oxidizing agents for regeneration of the Pd(II) catalyst [151]. The palladium-catalyzed anodic oxidation of... 

SCHEME 136. Palladium-catalyzed oxidation of terminal olefins to methyl ketones by TBHP or H2O2... 

Terminal olefins may be oxidatively cleaved by hydrogen peroxide, catalyzed by a palladium ) complex, to give methyl ketones in almost quantitative yields (equation 38)167. This methodology is an alternative to the well established Wacker protocol using palladium ) complexes. 

Catalytic asymmetric hydrosilylation of terminal olefins has been developed, using palladium coordinated to the novel binaphthyl ligands (MOP). In all cases (MOPa-d), the enantioselectivity is excellent ( 90% e.e.). The products can be converted into the corresponding secondary alcohols with retention of configuration446. 

The subsequent. mi-elimination, yielding a 1,2-substituted alkene and hydrido-palladium halide, is, however, reversible, and therefore the thermodynamically more stable (E )-alkene, ( )-RCH=CHZ, is generally produced when the coupling reaction is performed with a terminal olefin. It is worth mentioning that the formation of this 1,2-disubstituted alkene is accompanied with the formation of 1,1-disubstituted alkene, CH2 = C(Z)R, the amount of which depends on the kind of catalyst and the reaction conditions, a lower reaction... 

The oxidation of terminal olefins has been developed into a useful reaction for producing methyl ketones in good yields 6>. Again, cupric chloride and oxygen are employed to allow the palladium chloride to be used in only catalytic amounts. The method uses aqueous dimethylform-amide as solvent and a reaction temperature of 65° C. 

Palladium-catalyzed cyclization-carboalkoxylation of alkenyl indoles tolerated substitution along the alkenyl chain and at the internal and tfr-terminal olefinic position. In addition to 2-(4-alkenyl)indoles, 2-(3-alkenyl)-, 2-(5-hexenyl)-, 3-(3-butenyl)-, and 3-(4-pentenyl)indoles also underwent efficient palladium-catalyzed cyclization-carboalkoxylation to form the corresponding tricyclic indole derivatives in moderate to good yield with excellent regioselectivity. By employing this procedure, efficient palladium-catalyzed cyclization-carboalkoxylation of 2-(4-pentenyl)indole with ethanol, 1-octanol, 2-propanol, and cyclohexanol was achieved. 

These reactions are commonly interpreted to be composed of three main steps, namely a) oxidative addition of an aryl-X species to palladium(0) with formation of an arylpalladiumffi) bond b) insertion of a terminal olefin and c) reductive elimination regenerating palladium(0). To achieve a catalytic cycle, the rates of these steps have to match each other. The basic process was discovered by Heck in 1968. The mechanism has not yet been well defined and several variants have been proposed. A widely accepted scheme is reported in Figure 6. 

A pyridine-2-carboxylato (N-0) complex of palladium(II) with a labile tosylato ligand has been shown to act as an efficient carbonylation catalyst for a series of alcohols and olefins. The catalyst precursor, [Pd(N-0)(0Ts)(PPh3)] (24), in conjunction with promoters (e.g. Lil, LiCl, TsOH), is active for the carbonylation of primary, secondary and tertiary alcohols as well as functionahzed terminal olefins with good selectivity and turnover frequency. This reaction has been further discussed in a report dealing with the kinetic modeling of this and other catalytic-carbonylation reactions. 

Ellman utilized the Suzuki coupling twice between a support-bound vinyl bromide and an alkyl 9-BBN derivative in a solid-phase synthesis of E- and F-series prostaglandins. The Suzuki reaction was performed in situ, with the hydroboration of a terminal olefin being followed by the palladium-mediated step. This sequence is attractive in library synthesis because of the wide range of suitable commercially available alkenes. The inspiration behind this chemistry was the solution-phase work of Johnson and Braun, where the couplings of 35 with 2-iodo-4-(silyloxy)cyclopent-2-enone 36 went well at room temperature with PdCljCdppO-AsPhj as catalyst (Scheme 41). The modular chemistry demonstrated in this paper was clearly amenable to adaptation to a solid-phase strategy. 

Complexes of many transition metals including cobalt, rhodium, iridium, iron, nickel, palladium, and platinum have been found to catalyze double-bond migration in terminal olefins. Evidence for a mechanism of the following type, which is probably also applicable to some of the other catalysts, has been obtained by Cramer 24, 27) for the rhodium chloride-catalyzed reaction (Reaction 37). 

Choi, K.-M., Mizugaki, T., Ebitani, K., Kaneda, K. Nanoscale palladium cluster immobilized on a Ti02 surface as an efficient catalyst for liquid-phase Wacker oxidation of higher terminal olefins. Chem. Lett. 2003, 32, 180-181. 

Palladium-based catalysts also bring about cyclopropanations in high-yield. With palladium acetate/CHjNj, styrene , unactivated terminal olefins strained olefins , 1,3-dienesan enamine , as well as a,3-unsaturated carbonyl compounds have been cyclopropanated (Table 1). Contrary to an earlier report, the reaction also works well with cyclohexene if the conditions are chosen appropriately it seems that the notniyst is rapidly deactivated in the presence of this olefin >. Trisubstituted a,p-unsaturated carbonyl compounds were found to be unreactive, and the same is true for the double bonds in diethyl fumarate, maleic anhydride, coumarin and 1,3-dimethyluracil. Whereas the latter two were totally unreactive, [3-1-2] cycloaddition of diazomethane gave pyrazolines in the former two cases. The last entry of Table 1 shows that an allyl alcohol function can still be cyclopropanated, but methylene insertion into the O—H bond is a competing process. 

Optically active alcohols, amines, and alkanes can be prepared by the metal catalyzed asymmetric hydrosilylation of ketones, imines, and olefins [77,94,95]. Several catalytic systems have been successfully demonstrated, such as the asymmetric silylation of aryl ketones with rhodium and Pybox ligands however, there are no industrial processes that use asymmetric hydrosilylation. The asymmetric hydrosilyation of olefins to alkylsilanes (and the corresponding alcohol) can be accomplished with palladium catalysts that contain chiral monophosphines with high enantioselectivities (up to 96% ee) and reasonably good turnovers (S/C = 1000) [96]. Unfortunately, high enantioselectivities are only limited to the asymmetric hydrosilylation of styrene derivatives [97]. Hydrosilylation of simple terminal olefins with palladium catalysts that contain the monophosphine, MeO-MOP (67), can be obtained with enantioselectivities in the range of 94-97% ee and regioselectivities of the branched to normal of the products of 66/43 to 94/ 6 (Scheme 26) [98.99]. 

Recently, a palladium complex coordinated with an axially chiral, monoden-tate phosphine ligand, MeO-MOP (7a) or its analogs [21], has been reported to be highly effective for the enantioselective hydrosilylation of alkyl-substituted terminal olefins (Scheme 4) [22,23]. Simple terminal olefins 8 were transformed efficiently into the corresponding optically active 2-alkanols 11 with enantiose-lectivities ranging between 94% and 97% ee by the catalytic hydrosilylation-ox-... 

The axially chiral, monophosphine ligand, MeO-MOP (7a), was not as effective for styrene derivatives as for simple terminal olefins [31]. The palladium-catalyzed hydrosilylation of styrene (13) with trichlorosilane in the presence of the (H)-MeO-MOP ligand (7a) under standard conditions (without solvent) followed by oxidation gave (H)-l-phenylethanol (16) with only 14% ee (Scheme 8). Use of benzene as solvent for the hydrosilylation reaction improved the enanti-... 

Very recently, White and coworkers introduced the chiral Lewis acid Crm(salen) as cocatalyst into Ll/Pd11 catalytic system. The oxidative allylic acetoxyaltion of terminal olefins 1 afforded the corresponding branched allylic acetates 3 in high regioselectivity and moderate enantio-selectivities (up to 63% ee) (Scheme 6) [22], The asymmetric induction possibly results from the coordination between Cr salen) and BQ, and the adduct of Cr,n(salen) BQ promotes the acetoxylation of rc-allyl-palladium complex to form enantioenriched branched allylic acetates. 

Asymmetric hydrosilylations of terminal alkenes, 1-arylalkenes, norbomenes and dihydrofurans with HSiCIj have been successfully performed by Hayashi and coworkers [914, 915, 916, 1340, 1341]. These reactions take place at 40°C when catalyzed by chiral palladium complexes, and the most efficient ligand is monophosphine 3.51 (R = Me) (Figure 7.19). The regioselectivity of the hydrosilylation of terminal olefins is opposite to that usually observed after treatment with H2O2/KF, secondary alcohols are obtained as major products [752, 855, 1340], The regioisomeric primary alcohols are typically formed in only about 10% yield in these reactions. 

Other Metal Catalysts Palladium catalysts are most widely used for C—N bond formation, particularly in Buchwald-Hartwig coupling reactions [48], Halides or pseudohalides are generally used. Recently, direct aminationof C— H bonds has been developed with palladium catalysts. Pd(0)-catalyzed diamination of terminal olefins at allylic and homoallylic carbons takes place via formal sp3 C—H activation under solvent-free conditions [49]. More recently, an asymmetric version of allylic and homoallylic diamination has been successfully achieved using di-tert-butyldiaziridi-none as the nitrogen source (Equation 11.21) [50]. 

Fix et al. reported the palladium-catalyzed oxidative cyclization of aminoalkenes.132b The reaction of the aminoalkenes 176 having a methyl group on the olefin moiety gave the 2-vinylpyrrolidines 177, while the reaction of the aminoalkene 178 having a terminal olefin gave a mixture of the cyclic enamines 179 and 179 (Schemes 58 and 59). 

In the well-known Wacker process ethylene is converted to acetaldehyde by aerobic oxidation in an aqueous medium in the presence of PdCl2 as catalyst and CuCl2 as cocatalyst [7], Terminal olefins afford the corresponding methyl ketones. Oxidative acetoxylation of olefins with Pd(II) salts as catalysts in acetic acid was first reported by Moiseev and coworkers [8], The addition of an alkali metal acetate, e. g. NaOAc, was necessary for the reaction to proceed. Palladium black was also found to be an active catalyst under mild conditions (40-70 °C, 1 bar) in the liquid phase, if NaOAc was added to the solution before reducing Pd(II) to Pd black, but not afterwards [9,10]. These results suggested that catalytic activity... 

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