Investigations of Benzyl and Aryl Palladium Complexes with Pendant Hydroxy Substituents and Their Transformation into Benzolactones on Carbonylation

Reactions of 2-hydroxymethylbenzyl halides with Pd(PPh3)4 afford complexes Pd(CH2C6H4-2-CH2OH-κ2C1α,O)(PPh3)X (X = Br, 3; X = Cl, 4) containing bidentate 2-hydroxymethylbenzyl
ligands. Further reactions of 3 or 4 with NaH produce the binuclear cyclocondensation
product Pd2(μ-2-OCH2C6H4CH2)2(PPh3)2, 7, containing a central planar Pd2O2 unit incorporated into a system of five fused rings. Compound 7 undergoes phosphine substitution to
form related binuclear products Pd2(μ-2-OCH2C6H4CH2)2(PAr3)2 {Ar = p-MeC6H4, 8; p-(MeO)C6H4, 9} but is cleaved by more electron-donating phosphines and by diphosphines to give
mononuclear alkoxides Pd(OCH2C6H4CH2-κ2C1α,O)(P−P) (P−P = dppf, 10; dppe, 11).
Compounds 3, 4, and 7−9 react readily with carbon monoxide to liberate the lactone
3-isochromanone via carbonyl insertion, and the acyl intermediate PdCl(COCH2C6H4-2-CH2OH)(PPh3), 12b, has been studied spectroscopically. Reactions of 2-halogenobenzenealkanols
with Pd(PPh3)4 afford simple derivatives with monodentate aryl ligands, trans-Pd(C6H4-2-(CH2)nOH)(PPh3)2X (n = 1, X = Br, 15; n = 2, X = I, 17) and the known complex (n = 1, X
= I, 14), which on reaction with NaH are also converted into binuclear products Pd2(μ-2-O(CH2)nC6H4)2(PPh3)2 {n = 1, 19a (previously reported) and n = 2, 20}; compound Pd2(μ-2-OCH2C6H4)2{P(C6H4-4-OMe)3}2, 25, is formed from 19a by phosphine exchange, and
cleavage of 19a or 20 by diphosphines generates new mononuclear complexes 22−24
containing chelating alkoxide ligands. Carbonylations of these arylpalladium derivatives of
2-hydroxyalkylphenyl halides to form the respective benzolactones, phthalide and 3,4-dihydroisocoumarin, have been investigated. Crystal structures of mononuclear compounds
3, 4, 14, and 17, including intermolecular H-bonding interactions to halogen ligands in the
solid state, and of binuclear compounds 7, 8, and 25 are reported and discussed. The new
complexes are discussed in relation to the mechanism of Pd(0)-catalyzed syntheses of
benzolactones from aromatic halo alcohols; in production of 3-isochromanone, the results
support a process involving coordination of a hydroxyl group prior to reductive elimination
of organic product but with CO insertion into the Pd−C rather than the Pd−O bond.