Orthopalladation

The specific ortho functionalization of arylamines is obviously important in quinoline synthesis (cf. the rc-allyl procedure devised for the preparation of o-allylanilines used as indole and quinoline precursors).76 Recently acetanilides have been subjected to orthopalladation and the ensuing complexes converted into useful precursors of 2-substituted quinoline derivatives (Scheme 143).215... 

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... 

Orthopalladated chromophores, preparation, 8, 287 OSIPs, see Outer-sphere ion pairs Osmium r 2-alkenes... 

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. 

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. 

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 " ... 

The orthopalladation of N-arylamides gives six-membered ring chelates ... 

Palladium(II) derivatives also are commonly used in these transformations (orthopalladation) [5, 6] (Scheme VIII.2). 

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

New orthopalladated NLO chromophores based on a parallel alignment of two different push-pull ligands have been synthesized by Centore et al. [153] and their second-order NLO activity has been measured by the EFISH technique working in CHCI3 with an incident wavelength of 1.907 pm. The maximum value of p Si.9i(EFISH) (610 x 10 " esu) was obtained for the NLO chromo-... 

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. 

McFarlane has studied the orthopalladated complexes of enantiomeri-cally pure l-phenyl-l-(A, A-dimethylamino)ethane and Chiraphos, shown in... 

Figure 4.10 An orthopalladated complex with a phosphine chiral at the P atom.

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. 

Three new orthopalladated chromophores 87 (A,B = N02, NR2 A B) based on push-pull 7r-conjugated ligands have been prepared and characterized.Second-order molecular NLO activity has been determined by EFISH measurements. A related study with OH-functionalized orthopalladated Schiff bases was also published by the same group in 2004. ... 

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. ... 

Palladation of the ketone function of j ,j -dimethylhydrazonesr rearrangement of palladacycles derived from 2-oxopropionaldehyde phenylhydrazones, " orthopalladation of acetophenonephenylhydrazone, " and cyclopalla-dated compounds derived from pinacolone-j ,iV-dimethylhydrazone were reported. " ... 

An example of a doubly orthopalladated aryl bis(phosphinite) ligand 95 has been reported. " Aryloxopalladium(ii) complexes were synthesized using the Herrmann-Beller palladacycle. Coordination of the 2-acylphenolates to palladium results in a significant perturbation of the delocalization within the aromatic ring. Another example involves a doubly orthometallated complex from o-G6H4(PBu )2 and Pd(OAc)2. ... 

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