Palladium complexes aromatic compounds

Following this pnblication, the anthors tested a series of Pd-NHC complexes (33-36) for the oxidative carbonylation of amino compounds (Scheme 9.8) [44,45]. These complexes catalysed the oxidative carbonylation of amino compounds selectively to the nreas with good conversion and very high TOFs. Unlike the Cu-NHC catalyst 38-X, the palladium complexes catalysed the oxidative carbonylation of a variety of aromatic amines. For example, 35 converted d-Me-C H -NH, d-Cl-C H -NH, 2,4-Me3-C H3-NH3, 2,6-Me3-C H3-NH3, and 4-Ac-C H3-NH3 to the corresponding nreas with very high TOFs (>6000) in 1 h at 150°C, in 99%, 87%, 85%, 72%, and 60% isolated yields, respectively (Pco,o2 = 3.2/0.8 MPa). 

Grignard reagents convert aromatic sulfonyl chlorides or aromatic sulfonates to sulfones. Aromatic sulfonates have also been converted to sulfones with organolithium compounds.1745 Vinylic and allylic sulfones have been prepared by treatment of sulfonyl chlorides with a vinylic or allylic stannane and a palladium-complex catalyst.1746 Alkynyl sulfones can be prepared by treatment of sulfonyl chlorides with trimethylsilylalkynes, with an AICL catalyst.1747... 

During their work on the arylation of aromatic compounds by substitution, Fujiwara, et al. observed biaryl formation when aromatic compounds were placed in the presence of olefin-palladium complexes and silver nitrate.80 Developing this reaction as a method for biphenyl synthesis, these authors showed that the more stable the olefin-palladium complex was, the lower the yield. Ethylene dichloropalladium proved to be the best choice, when used with silver nitrate. However, the reaction required stoichiometric amounts of both catalysts (Scheme 10.47). Benzene derivatives substituted by electron-donating or -withdrawing groups reacted as well, but a mixture of regioisomers was produced, except for nitrobenzene, which only gave m,m -dinitrobiphenyl. 

The presence of chelating groups in those complexes is necessary to stabilize the intermediate aryl-palladium complex for isolation but it does not seem necessary to cause palladation. The chelating group does, however, tremendously accelerate the palladation. Aromatic compounds reactive to electrophilic substitution apparently undergo palladation with palladium acetate in acetic acid solution fairly readily at 100 °C or above. Of course, the arylpalladium acetates presumably formed, are not stable under these conditions, and they decompose very rapidly into biaryls and palladium metal 34,35,36) ag do aryl palladium salts prepared by the exchange route 24>. If the direct palladation is carried out in the presence of suitable olefins, arylation can be achieved, so far, however, only in poor yields, arid with concurrent loss of stereospecificity and formation of isomers and other side products 37.38). 

In Section 16.5, a few other C,C coupling reactions of alkenes and of aromatic compounds, which contain an sp2—OTf, an sp2—Br, or an sp2—Cl bond, will be discussed because these C,C couplings and the preceding ones are closely related mechanistically. These substrates, however, react with metal-free alkenes. Palladium complexes again serve as the catalysts. 

Second order non-linear optical properties have been reported for a variety of TTF donor-acceptor compounds <02T7463> and the palladium complex 84 is a room-temperature semiconductor <02CL936>. Preparation of the zinc and cadmium compounds 85 has been reported <02CC1474> and aromatic fused TTFs such as 86 form thin films with useful electrical properties <02JAP265466>. A ferromagnetic interaction occurs in the salt of a TTF... 

Phenylboronates [ArB(OR)2] react with electron-deficient aromatic compounds, such as acetophenone, to give the biaryl." " Arylboronates also react with 7i-allyl palladium complexes to form the alkylated aromatic compound." ... 

Aromatic hydrocarbons, such as benzene add to alkenes using a ruthenium catalyst a catalytic mixture of AuCVAgSbFs, or a rhodium catalyst, and ruthenium complexes catalyze the addition of heteroaromatic compounds, such as pyridine, to alkynes. Such alkylation reactions are clearly reminiscent of the Friedel-Crafts reaction (11-11). Palladium catalysts can also be used to for the addition of aromatic compounds to alkynes, and rhodium catalysts for addition to alkenes (with microwave irradiation). " Note that vinyhdene cyclopropanes react with furans and a palladium catalyst to give aUylically substituted furans. ... 

The C—I bond is very unstable and more reactive than C—Br, C—Cl and C—F bonds. Iodine is the most expensive of the common halogens and is much less frequently used in synthesis than bromine, chlorine or fluorine. Organometallic reactions proceed with iodinated aliphatic or aromatic compounds more easily than with the other halogens. Noble metal catalysis with palladium complexes is most effective with iodinated compounds. A useful synthetic procedure is the facile reduction of iodinated derivatives under mild conditions. Replacement of iodine by hydrogen at an sp carbon is an exothermic reaction with A// = -25 kJ mol . ... 

Allylation of aromatic compounds with allylic alcohols and esters through C-0 bond cleavage catalyzed by molybdenum, tungsten, and palladium complexes has been reported recently [25,26]. In addition,molybdenum-catalyzed aromatic substitution with alcohols has been achieved [27]. 

The area of catalytic activation of most unreactive C-Cl bonds has flourished tremendously over the last decade. Because of its considerable practical importance the field keeps growing at an impressive pace. Numerous novel techniques have been developed for the synthesis of various functionalized aromatic compounds from the corresponding chloroarenes. A series of new palladium, nickel, and rhodium catalysts have been synthesized for C-Cl activation and much information has been accrued on the mechanism of catalysis with these complexes. 

Aromatic compounds react with palladium(II) salts such as PdCOAc) and Na2PdCl4 via an electrophilic aromatic substitution process to give arylpalladium complexes. This type of reaction is most commonly observed with aromatic rings bearing a substituent that makes a fi e- or six-membered chelate ring with palladium in the metallation products (eq (87)) [118]. In this case, the electrophilic substitution occurs only at the ortho position to the chelating substituent. 

An aromatic nitro function hydrogenates to the corresponding amine in high yield over palladium-on-carbon in a process that is slightly superior to that using platinum. Homogeneous reduction of aromatic compounds (1) by a triphenylphosphine complex of palladium yields anilines 2 along with minor quantities of azobenzene and azoxyben-zene derivatives . 

Ketone synthesis. Ketones can be prepared in 65-85% yield by the reaction of aromatic or aliphatic acyl halides with diaryl- or dialkylmercury(II) compounds in HMPT with this palladium complex as catalyst. ... 

C-H o-bond activation of hydrocarbons by transition metal complexes is of considerable importance in modern organometallic chemistry and catalytic chemistry by transition-metal complexes [1], because a functional group can be introduced into alkanes and aromatic compounds through C-H o-bond activation. For instance, Fujiwara and Moritani previously reported synthesis of styrene derivatives from benzene and alkene via C-H o-bond activation of benzene by palladium(ll) acetate [2]. Recently, Periana and his collaborators succeeded to activate the C-H o-bond of methane by the platinum(ll) complex in sulfuric acid to synthesize methanol [3], Both are good examples of the reaction including the C-H o-bond activation. 

Ort/io-metallated palladium complexes of azo and hydrazobenzene catalyze the reduction by H2 of nitroaromatics, alkenes, alkynes, and aromatic carbonyl compounds. A palladium-aryl or bond in the precursor complex is a requirement for catalytic activity. The ligands are themselves susceptible to reduction. The kinetics of the reaction under 1 atm H2 have been measured. Palladium(O) complexes catalyze the hydrostannolysis of allyl and allyloxy carbonyl groups. The reaction can be applied to the selective protection-deprotection of aminoacid derivatives see equation (9). Alkenyl cyclopropanes carrying electron-withdrawing substituents are selectively hydrogenolyzed by Pd(0)/PBu3 catalysts... 

---