Oxidations tetrakis palladium

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

Dialkylindolines and 1,3-dialkylindoles are formed in poor yield (<10%) from the reaction of ethyl- or phenymagnesium bromide with 2-chloro-N-methyl-N-allylaniline in the presence of catalytic quantities of (bistriphenylphosphine)nickel dichloride.72 In a modification of this procedure, the allyl derivatives can be converted by stoichiometric amounts of tetrakis(triphenylphosphine)nickel into 1,3-dialkylindoles in moderate yield72 (Scheme 43) an initial process of oxidative addition and ensuing cyclization of arylnickel intermediates is thought to occur. In contrast to the nickel system,72 it has proved possible to achieve the indole synthesis by means of catalytic quantities of palladium acetate.73 It is preferable to use... 

Several factors can influence the effectiveness of a metal scavenger, one of which is the oxidation state of the metal. In these studies the removal of Pd(II) as trans-di( j-acetato) bis [o-(di-o-tolylphosphino) benzyl] dipalladium (II) (3) and Pd(0) as tetrakis-(triphenylphosphine)palladium(0) (4) was investigated. 

Tetrakis(triphenylphosphine)palladium(0) (VI) is regarded as coor-dinatively saturated yet it is potentially unsaturated since it undergoes dissociation in solution thereby allowing oxidative addition to take place. A new convenient method for preparing VI (38) in addition to the fact that it readily undergoes oxidative addition (39, 40, 41, 42, 43, 44) make this reaction attractive for studying the stereochemistry of the process. 

The oxidative addition is quite general with alkyl, allyl, benzyl, vinyl, and aryl halides as well as with acyl halides to afford the palladium (II) complex VII. The frans-bis( triphenylphosphine )alkylpalladium halides can also be carbonylated in an insertion reaction to give the corresponding acyl complexes, the stereochemistry of which (17, 18) proceeds with retention of configuration at the carbon bonded to palladium. The acyl complex also can be formed from the addition of the corresponding acid halide to tetrakis (triphenylphosphine) palladium (0). 

Lithium diisopropylamide, 163 Phenyliodine(III) diacetate, 242 Tetrakis(triphenylphosphine)-palladium(O), 289 Trimethylamine N-oxide, 325 Containing one nitrogen—indoles Hexamethyldisilazane-Chlorotri-methylsilane, 141 Iron carbonyl, 152... 

We should review the basic chemistry of palladium, as you will be seeing many more cxampL of these steps in specialized situations. Palladium chemistry is dominated by two oxidation state The lower, palladium(O), present in tetrakis(triphenylphosphine)palladium, for example, is nom nally electron-rich, and will undergo oxidative addition with suitable substrates such as halidt and triflates (TfO- = CFsSC CT), resulting in a palladium(II) complex, Oxidative addition thought to occur on the coordinatively unsaturated 14-electron species, formed by ligand dissocia tion in solution. 

More recently, methods based on the use of mild reductants, able to transfer a single electron to the polyhaloalkyl halide, have been described. Various metals or their derivatives have been employed ruthenium, platinum and their complexes in low oxidation state, iron" and its carbonyl complexes, or tetrakis(triphenylphosphane)palladium. Sodium arcncsul-finate, sodium dithionite" and various oxidants have also been used. Other examples of polyhaloalkyl halide additions to simple alkenes are summarized in Table 1. Typical examples are the formation of diiodide 6, chloro iodide 7, and iodo steroid 8. ... 

Salt 91 cleanly reacts at 0°C with 1 equiv of tetrakis(triphenylphosphane)palladium to complex 167, which was isolated as a yellow oil in 70% yield. Obviously, the reactive P-P bond of the diphosphirenium ion was oxidatively added onto the zerovalent metal of the coordinatively unsaturated [Pd(PPh3)2] unit. When a CH2CI2 solution of 91 was heated with Z equiv of [Pd(PPh3)4], binuclear complex 168 was obtained (Scheme 56) <19950M3614, 1996PS53>. 

In general, the process is easy for coordinatively unsaturated metal species, in particular 16-electron (d and d ) metals [Ni(0), Pd(0)], since these can attain a stable 18-electron configuration as a consequence. Additionally, two metal oxidation states must be sufficiently stable. For example, tetrakis(triphenylphosphine)palladium inserts readily at 80 °C into the C-Br bond of bromobenzene to give the organometallic complex PhPd(PPh3)4Br. 

The stereoselective allylation of aldehydes was reported to proceed with allyltrifluorosilanes in the presence of (S)-proline. The reaction involves pentacoordinate silicate intermediates. Optical yields up to 30% are achieved in the copper-catalyzed ally lie ace-toxylation of cyclohexene with (S)-proline as a chiral ligand. The intramolecular asymmetric palladium-catalyzed allylation of aldehydes, including allylating functionality in the molecules, via chiral enamines prepared from (5)-proline esters has been reported (eq 15). The most promising result was reached with the (S)-proline allyl ester derivative (36). Upon treatment with Tetrakis(triphenylphosphine)palladium(0) and PPh3 in THF, the chiral enamine (36) undergoes an intramolecular allylation to afford an a-allyl hemiacetal (37). After an oxidation step the optically active lactones (38) with up to 84% ee were isolated in high chemical yields. The same authors have also reported sucessful palladium-catalyzed asymmetric allylations of chiral allylic (S)-proline ester enamines" and amides with enantiomeric excesses up to 100%. 

A coordinatively unsaturated 14-electron palladium(O) complex, usually coordinated with weak donor ligands (usually tertiaiy phosphanes), has meanwhile been proven to be the catalytically active species [5]. This complex is mostly generated in situ. Tetrakis(triphenylphosphane)palladium(0) [6J, which exists in an equilibrium with tris(triphenylphosphane)palladium(0) and free triphenylphosphane in solution, is frequently employed. The endergonic loss of a second phosphane ligand [7] leads to the catalytically active bis(triphenylphosphane)palladium(0). However, palladium(D) complexes such as bis(triphenylphosphane)palladium dichloride or palladium acetate, which are easily reduced (e.g. by triarylphosphanes see below) in the reaction medium, are more commonly employed for convenience, as they are inherently stable towards air. The mechanistic situation is a little more complicated with palladium acetate, in that anionic acetoxypalladium species Pd(PPh3) (AcO ) (n = 2, 3) are formed in the presence of acetate ions [5], and these actually participate in the oxidative addition step and the following coupling reaction. 

By building the quinone molecule into a macrocycle, a more efficient palladium-catalyzed aerobic 1,4-oxidation was developed [63], Thus, with catalytic amounts of 40 and Pd(OAc)2, 1,3-cyclohexadiene was oxidized to 1,4-diacetoxycyclohex-2-ene at a rate which was more than twice that with the system having quinone and porphyrin as separate molecules. The trans selectivity with tetrakis(hydroquinone)poiphyrin 40, however, was moderate (trans/cis = 70 30). 

Reaction of bis(disilanyl)dithiane 32 with the corresponding palladium(O)-isonitrile complex affords a four-membered cyclic bis(silyl)palladium(II) complex 34 quantitatively together with the formation of a disilane (Eq. 15) [30]. The formal intramolecular metathesis of the two Si-Si bonds of 32 may proceed through initial formation of tetrakis(silyl)Pd(IV) complex, corresponding to the platinum complex 33. The double oxidative addition of the two Si-Si bonds may be followed by reductive elimination of the disilane with accompanying formation of four-membered bis(silyl)palladium complex 34, due to difficulty in reductive elimination leading to formation of a three-membered cyclic disilane. 

Oxatrimethylenemethanepalladium complexes can also be generated by oxidative addition of palladium(O) to 5-methylene-l,3-dioxolan-2-ones and subsequent decarboxylation. Again, reaction with norbornene, norbornadiene and dicyclopentadiene yields polycyclic cyclopropyl ketones in medium to high yield (Table 19). In this case, tetrakis(triphenylphosphane)pal-ladium(O) was the best catalyst found, whereas tris(dibenzylideneacetone)palladium(0)-chloro-form/triphenylphosphane (see above) and bis(cycloocta-l,5-diene)nickel/triphenylphosphane (used in stoichiometric amounts) proved less efficient. 

Deoxygenation of heteroaromatie A-oxides 181 to the corresponding amine 182 was achieved under mild conditions. Polymethylhydrosilaoxane (PMHS) in the presence of either tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4], titanium (IV) isopropoxide [Ti(z-PrO4)4], or palladium on carbon (Pd/C) [89] were employed in this transformation. 

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