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Transmetallation nucleophilic substitution

While neopentyl-type and related cr-alkylpalladium complexes are stable toward dehy-dropalladation, all ordinary alkylpalladium complexes formed by carbopalladation of alkenes do undergo /S-hydride eUminalion more or less rapidly. However, under certain structural precondilions and also, to a certain extent, controlled by the external conditions, rearranganents, further carbopalladations (Sect. IV.3), transmetallations, car-bonylations, or nucleophilic substitutions (e.g., by hydride) can occur prior to /3-dehy-dropalladation. [Pg.1126]

Aryl exchange occurs before transmetallation. Thus, the transmetal-lation rate constant which is low for steric and electronic reasons results in increasing the coupling product of phosphine-bound aryls (Eq. 51). Transmetallation is slowed down when electron-rich haloarenes and weak bases are used and accelerated with electron-deficient haloarenes because the transmetallation shown in Eqs. 16 and 17 involves nucleophilic substitution of Pd-X. The reported equilibrium ratio of 3/5 is 4/96 at 60 °C when Ar is p-methoxyphenyl. Thus, it is quite reasonable that the reaction accompanies a large amount of 6 when the rate constant of transmetallation ifej is lower than ky A strong base, polar solvent, and a sterically less hindered bidentate ligand, such as dppf, increase k. The formation of yields of 9 in p-iodoanisole higher than the bromo derivative... [Pg.214]

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. [Pg.453]

In general, the syntheses of these complexes are achieved through (i) nucleophilic addition/substitution reactions of silver(i) fluoride or (ii) transmetallation reactions with other metal alkyl, alkenyl, and aryl complexes. [Pg.198]

Although there is a kinetic barrier to the direct deprotonation of tertiary amines, Ahlbrecht and Dollinger showed in 1984 that the Schlosser superbase, i c-BuLi/f-BuOK, can deprotonate A-methylpiperidine selectively on the methyl group (Scheme 3). This superbase probably yields an a-amino-organopotassium species (and f-BuOLi), but treatment with LiBr effects transmetalation to the more nucleophilic, and less basic, a-amino-organolithium species. Electrophilic quench with several aldehydes and ketones gives substitution products in good yields as typified by the example in Scheme 3. Similarly,... [Pg.1006]


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