Orthopalladation reactions

Under similar oxidative conditions, with activation of the aromatic C-H bond, some arenes could be used directly as aryl sources [41]. Unfortunately, by analogy with the Friedel-Crafts acylation, this reaction is regioselective for very few substrates only. High regioselectivity was, however, obtained if coordinating substituents on the arenes facilitate an orthopalladation reaction by a Pd(II) species [42]. After carbometallation and reductive elimination, Pd(0) is released, which has to be converted into the initial Pd(II) species in an extra oxidation step. Usually, quinines are used for this purpose, but in combination with certain heteropolyacids as cocatalysts even molecular oxygen can be employed as the oxidant. 

Scheme Vni,2. Examples of orthopalladation reactions. Reference numbers are in brackets.

Albisson DA,Bedford RB, Lawerence SE, Scully PN (1998) Orthopalladated triaryl phosphite complexes as highly active catalysts in biaryl coupling reactions. Chem Commun 2095-2096... 

Avis, M.W., van der Boom, M.E., Elsevier, C.J., Smeets, W.J.J. and Spek, A.L. (1997) Reactions of bis (iminophosphoranes) with palladium(II) dichloride metal-induced tautomerization orthopalladation and unexpected platinum-assisted [2 + 2]cycloaddition of an aryl-nitrile with a phosphinimine moiety./. Organomet. Chem., 527, 263. 

The orthometallation reaction with N donor atoms in the side chains usually forms five-membered chelated rings. There are exceptions, such as in the orthopalladation of N-arylamidines where six-membered rings are formed " ... 

As illustrated in Scheme 9.9, the proposed mechanism of forming phthalides and iso-coumarins involves orthopalladation of the carboxylic acid, subsequent alkenylation and nucleophihc cyclization or Wacker-type oxidative cyclization. The observed differences in product distributions from the reactions using n-butyl acrylate and styrene suggest that the electronic nature of the alkene plays an important role in product formation, although the exact origin of the difference remains unclear. 

Similar search results for cyclometalation, including orthometalation and cyclopalladation and orthopalladation (i.e keywords cyclometalation or orthometalation or cyclopalladation or orthopalladation), showed 240,422 studies. These results indicate that the number of articles related to cyclometalation reactions is more than six times that concerning any of the three recent synthetic organic reactions shown above. 

Pd/trifurylphosphane catalyst Albisson et al. [27] carried out orthopalladation of the inexpensive, commercially available tris(2,4-di-tert-butylphenyl)phosphite the product, a dimeric complex, was very effective in biaryl couphng reactions with turnover numbers of up to 830 000. 

Cyclometalations proceed as shown in eqs. (20.35)-(20.39). With palladium compounds, these reactions are called cyclopalladation or orthopalladation. As the palladium bonded to an aromatic ring is very reactive, alkenes, alkynes, CO and halogen are easily replaced. For example, as shown in eq. (20.61), ortho-aryl derivatives are seleetively prepared in high yield [212]. 

Orthopalladation of (3 )-a-methyl-4-nitrobenzylamine has been reported. The reaction of [PdGl(G6H4GH2-NMe2)]2 with the anion of A-methyl-3-hydroxy-2-methyl-4-pyridone afforded 67The first six-membered palladacycles such as 68 (X = Br, Gl, OAc Y = G1, F, NO2) synthesized from primary phenethylamines with electron-withdrawing substituents on the aryl ring were reported by Vicente et al ... 

The cyclopalladated complex (naphthyl)ethylamine-6, iV]-dipalladium has been used as a chiral template to promote the intramolecular [4 -h 2] Diels-Alder reaction between diphenyl-vinylphosphine and 1-phenyl-3,4-dimethylphosphole. Halide-bridged palladium(ii) dimers of orthopalladated (S)-(-h)-7V,7V-dimethyl-Q -methylbenzylamine and (naphthyl)ethyl]amine were reported by the same authors.Solution and solid-state structures were determined as well as reactions with 3,4-dimethyl-1-phenylphosphole and allyldiphenylphosphine carried out. 

Orthopalladated complex [Pd(/i-Gl)(G6H4-2-PPh2=NPh-/ -6 ,A)]2 and related complexes have been obtained by the reaction of Pd(OAc)2 with iminophosphoranes. ... 

An orthometallated complex was obtained from dibenzylamine and Pd(OAc)2, subsequent reaction with NaBr gives a bridged dimer [Pd(/x-OAc) G6H4(GH2NHGH2Ph)-2 ]2 Insertion reactions with alkynes were carried out. It is noted that double insertion can occur. Furthermore, reactions with Tl(acac), PPh3NH(GH2Ph)2, and AgX were studied. Internal acetylenes can be inserted into orthopalladated a-methylbenzylamine. Double and tri-insertion may occur depending on the alkyne. 

Metal-induced tautomerization and orthopalladation was found to occur in the reaction of bis(iminophosphoranes) with palladium(ii) dichloride.Four-membered Pd-N-P-G palladacycles, for example, 148, were obtained from the reaction of sodium bis(/ -aryliminophosphoranyl)alkanides with [Pd2X4(PR3)2]. At elevated temperatures, 148 converts into rare G,G -palladacycles 149 and 150 (Scheme Although strictly speaking not a four-membered... 

---