Carbon-palladium bond

The preparation of helically well-ordered polymers with stable screw-sense, which is able to be transmitted to newly formed polymer main-chains effectively, is highly desired for the development of new methodology for the synthesis of optically active helical polymers. An aromatizing polymerization of 1,2-diisocyanobenzenes is promoted by methylpalladium(II) complexes, producing poly(quinoxaline-2,3-diyl)s.146-148 The polymerization proceeds with successive insertion of the two isocyano groups of the diisocyanobenzene to the carbon palladium bond of... 

As described in the preceding sections, many domino reactions start with the formation of vinyl palladium species, these being formed by an oxidative addition of vinylic halides or triflates to Pd°. On the other hand, such an intermediate can also be obtained from the addition of a nucleophile to a divalent palladium-coordinated allene. Usually, some oxidant must be added to regenerate Pd11 from Pd° in order to achieve a catalytic cycle. Lu and coworkers [182] have used a protonolysis reaction of the formed carbon-palladium bond in the presence of excess halide ions to regenerate Pd2+ species. Thus, reaction of 6/1-386 and acrolein in the presence of Pd2+ and LiBr gave mainly 6/1-388. In some reactions 6/1-389 was formed as a side product (Scheme 6/1.98). 

These precatalysts invariably contain sp2 carbon-palladium bonds and usually require temperatures in excess of 120°C for their transformation into the active Pd(0) nanoparticle catalysts. 

A comparison of sp2 and sp3 carbon-palladium-bonded PdCys incorporating heterocycles... 

Dimethyl-4-methylene-l,3-dioxolan-2-one reacts with isonitriles in the presence of a palladium catalyst to afford iminofurans (Equation (121)).480 Successive insertion of isonitriles to the carbon-palladium bond of 7r-allylpalladium intermediate is postulated. 

In analogy to the mechanism of the palladium-catalyzed enyne cyclization, it is postulated that exposure of palladium(O) to acetic acid promotes in situ generation of hydridopalladium acetate LnPd"(H)(OAc). Alkyne hydrometallation affords the vinylpalladium complex A-10, which upon r-carbopalladation of the appendant alkyne provides intermediate B-7. Silane-mediated cleavage of carbon-palladium bond liberates the cyclized product along palladium(O), which reacts with acetic acid to regenerate hydridopalladium acetate to close the cycle (Scheme 33). 

The reaction mechanism was considered to be oxidative cyclization, and pal-ladacyclopentene 32 was formed. Reductive elimination then occurs to give cyclobutene 33, whose bond isomerization occurs to give diene 28. The insertion of alkyne (DMAD) into the carbon palladium bond of 32 followed by reductive elimination occurs to give [2+2+2] cocyclization product 27. Although the results of the reactions of E- and Z-isomers of 29 with palladium catalyst 26a were accommodated by this pathway, Trost considered the possibility of migration of substituents. Therefore, 13C-labeled substrate 25 13C was used for this reaction. 

The allylic 2-alkynoates 77 are a group of special enynes with an ester linkage between their double bond and triple bond. When halopalladation of the triple bond is followed by C-C double bond insertion and the cleavage of the carbon-palladium bond, a series of y-lactones (78-81) can be obtained. 

Carbon monoxide insertion into the carbon-palladium bond of 4, followed by nucleophilic displacement with methoxide, gives a 4 1 mixture of trans and cis-dimethyl hex- endioate which is the desired l, 4-dicarbonylaton precursor to adipic acid. 

A very promising synthesis of /3-lactones has been recently reported, involving the palladium-catalyzed carbonylation reaction of halogeno alcohols. For example, 3-phenyl-2-oxetanone was obtained in 63% yield from 2-phenyl-2-bromoethanol in DMF solution at room temperature under 1 atmosphere pressure of carbon monoxide (equation 115). A proposed mechanism, in which palladium metal inserts into the carbon-halogen bond, followed by insertion of a molecule of carbon monoxide into the carbon-palladium bond and then ring closure, fits kinetics data (80JA4193). 

The reactions shown in Scheme 1 require activation of the aromatic C-H bond by a metal and subsequent insertion of an alkene or alkyne in the aryl-carbon palladium bond (Chapter III.1.3.2.5). C-H activation has been the topic of many studies since the 1960s and several metal complex systems are known to induce... 

A second important palladium-catalyzed process is cross coupling of an aryl or alkenyl tiaUcyltin reagent with an aUcyl or aryl halide. In this reaction, one carbon-palladimn bond is generated by transmetalation of Pd -X with BU3 Sn-R, while the other carbon-palladium bond is formed by oxidative addition of an organic halide with a Pd°L species. This reaction, which is tolerant of many organic functional groups, is normally known as the Stille Reaction (Scheme 4). ... 

The carbon-metal systems considered appropriate for this section (and not covered elsewhere in this series) are carbon-tin, carbon-mercury and, to a restricted extent, carbon-palladium bonds. Although or-ganomercurials and organostannanes have been studied for over a century, the former continue to attract attention because of their controlled formation by the Markovnikov oxymercuration reaction (OM) and its variants, and the latter because of newer and regiospecific methods for forming C—Sn bonds and oxidatively cleaving them. In both cases, signiticant examples of synthetically useful oxidations have... 

Organopalladium compounds—compounds that contain a carbon-palladium bond—are... 

Organopalladium compound (Section 26.2) An organometallic compound that contains a carbon-palladium bond. 

Vinylpalladation of alkenes, if applied to norbornadiene, leads to a nortricyclene adduct 43, which is converted via carboxylation of the carbon-palladium bond to the corresponding ester 44 with retention of configuration. ... 

Carbon monoxide, and isonitriles, will insert into a carbon-palladium bond, subsequent reaction with a nucleophile generates the product. 

As a facile method of carbon-carbon bond formation, the insertion of carbon-carbon multiple bonds into carbon-transition-metal bonds is a very important fundamental reaction in organotransition-metal chemistry. However, in contrast to the tremendous number of reports about the insertion of carbon-carbon multiple bonds into carbon-transition-metal bonds, direct insertion of carbon-heteroatom multiple bonds, such as carbonyl and nitrile groups, without using stoichiometric organometallic reagents, has received scant attention.111 A palladium(II)-catalysed cycliza-tion reaction of alkynes with carbon-heteroatom multiple bonds under mild conditions has been developed, using insertion of carbon-heteroatom multiple bonds into the carbon-palladium bond as the key step.[2]... 

Carbon monoxide insertion is another widely observed reaction. The following carbon monoxide insertion into an azobenzene palladium complex is typical lu). Azobenzene-palladium chloride complex has a carbon-palladium bond at the ortho-position, the carbonylation of which gives indazolinones by carbon monoxide insertion. Further carbonylation of indazolinone catalyzed by Co2(CO)8 produces quinazolinedione, hydolysis of which gives aniline and anthranilic acid 172). Thus this method is useful for the synthesis of substituted anthranilic acid. 

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