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2.4.5.3 Variation 3: Via Palladium-Catalyzed Reactions

DOI: 10.1055/sos-SD-002-00214

Kündig, E. P.; Pache, S. H.Science of Synthesis, (20032170.

The complexation of a haloarene by a tricarbonylchromium group activates the CArX bond. Chlorobenzene complex 6 (M=Cr; R1=Cl) undergoes oxidative addition to palladium(0)/triphenylphosphine readily at ambient temperature and is 75 times more reactive than free bromobenzene.[‌122‌] We recall that with palladium(0)/triphenylphosphine, free chlorobenzene requires temperatures in excess of 130°C, although this can be remedied by using the more basic bulky ligands tri-tert-butylphosphine.[‌123‌] The first report of a palladium-catalyzed reaction of 6 (M=Cr; R1=Cl) entailed a Sonogashira coupling with alkynes.[‌124‌] This reaction was subsequently modified and an example is shown in Scheme 11.[‌125‌] Another approach to (alkynylarene)tricarbonylchromium complexes involves a Stille cross-coupling reaction.[‌126‌] Palladium-catalyzed alkoxycarbonylation of 6 (M=Cr; R1=Cl) [25°C, palladium(0)/triphenylphosphine, carbon monoxide, an alcohol], leading to complexed benzoic acid esters, was studied in detail and it was shown that Cr(CO)3 activation is important in the oxidative addition step (the insertion of Pd into the CArCl bond).[‌122‌,‌127‌,‌128‌] With electron-poor arenes, ipso nucleophilic substitution is a competitive reaction.[‌129‌] Arylpalladium intermediates are also accessible via transmetalation (Li Zn Pd). This has been used successfully in the synthesis of moracin M, a phytoalexin of Morus alba Linn.,[‌130‌] and in the synthesis of aryl amino acid derivatives[‌131‌] (see also Section 2.4.5.1). Intramolecular Heck and related reactions of complexed ortho-alkenyl chloro­arenes have been realized with excellent diastereocontrol of the stereogenic center formed in the alkene carbopalladation step.[‌132‌,‌133‌] Suzuki coupling reactions have been carried out with tricarbonyl(fluorobenzene)chromium(0).[‌134‌] Arylaryl bond formation via Suzuki coupling between (bromoarene)tricarbonylchromium(0) complexes and arylboronic acids has been studied in detail by Uemura and co-workers.[‌135‌,‌136‌] The finding that planar chiral complexes couple with substituted arylboronic acids with high diastereoselectivity has been developed into an efficient and elegant route to enantiopure atropisomeric biaryls with applications in natural product synthesis [e.g., ()-steganone and O,O-dimethylkorupensamine A; see also Section 2.4.11].[‌135‌] Diastereoselectivity is determined in the cis-biaryl palladium intermediate, and the kinetic product is the atropoiso­mer with the substituent endo to the Cr(CO)3 group (50 endo51). On heating endo51 to 110150°C, isomerization to the thermodynamically favored exo isomer takes place. The exo diastereomer is formed directly under the reaction conditions with small R2 substituents (e.g., CHO; 50 52).[‌136‌]

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Meeeeeeeeee{η8-[(eeeeeeeeeeeeee)eeeeeee]eeeeeee}eeeeeeee(8) (88); Meeeeee Meeeeeeee:[‌888‌]

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