Yasuyuki Ura

A benzylpalladium complex with a tridentate NNP ligand containing 2,2′‐bipyridine and a phosphoryl moiety, [PdBn(MeNNP(O))], was synthesized by reacting [{PdBn(MeNNP(O))} ₂ H](OTf) with DBU. [PdBn(MeNNP(O))] readily reacted with O ₂ to afford [PdOOBn(MeNNP(O))], which was formed by O ₂ insertion into the Pd–C(sp ³ ) bond. Although O ₂ insertion proceeded slowly in the dark, ambient light significantly accelerated insertion. Experimental results and density functional theory calculations showed that O ₂ insertion proceeded in the dark solely via a radical chain mechanism, whereas insertion under ambient light proceeded via both photoexcitation and radical chain mechanisms. By contrast, the reaction of [PdBn(MeNNP(O))] with O ₂ under blue LED irradiation afforded PhCHO as the major oxygenated product. Irradiation with blue LED light promoted the homolysis of the Pd–C(sp ³ ) bond to afford a Pd(III) species and benzyl radical. The benzyl radical reacted with O ₂ , and the subsequent hydrogen abstraction and dehydration afforded PhCHO. Moreover, the addition of acid accelerated the reaction of [PdBn(MeNNP(O))] with O ₂ in the dark, indicating that the phosphoryl moiety assisted the reaction with O ₂ .

Oxidative cyclization of 4-penten-1-ols using a Pd catalyst and n -BuONO or n -BuONO/ p -benzoquinone afforded 3-hydroxy- and 3-methoxytetrahydropyrans via terminal attack.

The Wacker-type oxidation of aliphatic terminal alkenes proceeds using a Pd/Fe catalyst system under mild reaction conditions using 1 atm O₂ without other additives. The use of 1,2-dimethoxyethane/H₂O as a mixed solvent was effective. The slow addition of alkenes is also important for improving product yields. Fe(III) citrate was the most efficient cocatalyst among the iron complexes examined, whereas other complexes such as FeSO₄, Fe₂(SO₄)₃, Fe(NO₃)₃, and Fe₂O₃ were also operative. This method is also applicable to aliphatic internal alkenes, which are generally difficult to oxidize using conventional Pd/Cu catalyst systems. The gram-scale synthesis and reuse of the Pd catalysts were also demonstrated.

The dynamical functions of molecular metal chains, that could be derived from unique physical and chemical properties coupled with self-assembly of metal chain components, have largely been unexplored, and this review paper focuses on the dynamic behaviours of Pd chains supported by linear tetraphosphines, meso- and rac-Ph₂PCH₂P(Ph)CH₂P(Ph)CH₂PPh₂ recently reported, and the current development on fine-tuning of metal chains by introducing heterometal atoms, i.e. heterometallic molecular metal chains, that are considered as promising dynamical components in the next generation.

With the hope of tuning structural and electronic properties of the metal chains by changing the N‐substituent group and P‐chiral configurations, a series of PNP bridged tetraphosphines, rac‐ and meso‐Ph₂PCH₂P(Ph)N(Ar)P(Ph)CH₂PPh₂ (Ar=4‐RC₆H₄, R=F (rac‐dpmppanF), CH₃ (rac‐dpmppanMe), H (meso‐dpmppan), OMe (meso‐dpmppanOMe); Ar=2,4‐xylyl (rac‐dpmppanMe₂)), were synthesized, and utilized to prepare the Pd₄ and Pt₂Pd₂ chains with XylNC terminal ligands, [M₄(rac‐dpmppanR)₂(XylNC)₂](PF₆)₂ (M₄=Pd₄, R=F (1_F), Me (1_Me); M₄=Pt₂Pd₂, R=F (4_F), Me (4_Me)) and [M₄(meso‐dpmppanR)₂(XylNC)₂](PF₆)₂ (M₄=Pd₄, R=H (6), OMe (6_OMe); M₄=PdPt₂Pd, R=OMe (9_OMe)). While the M₄ metal chains with rac‐dpmppanR (R=H, F, Me) ligands showed only marginal effects on the tetranuclear metal chains, the meso stereoisomer of meso‐dpmppanOMe afforded a new series of Pd₄ (6_OMe), PtPd₃ (7_OMe), PtPd₂Pt (8_OMe), PdPt₂Pd (9_OMe), Pt₃Pd (10_OMe), and Pt₄ (11_OMe) chains that exhibited a characteristic electronic absorption band tunable by the number and positions of incorporated Pt atoms from 628 nm to 473 nm. Especially, 11_OMe is the first example of Pt₄ chains supported by tetraphosphines. The coordinatively unsaturated Pd₄ strings, [Pd₄(rac‐dpmppan)₂(CH₃CN)](BF₄)₂ (12) and [Pd₄(meso‐dpmppanOMe)₂](BF₄)₂ (13), were also synthesized and 12 was utilized in axial ligand exchange reactions with PPh₂Me, and PPh₃ to disclose their successive incorporation into [Pd₄(rac‐dpmppan)₂(L)₂](BF₄)₂ (L=PPh₂Me (14 a), PPh₃ (14 b)) and remarkable red‐shift (713–718 nm) of the HOMO‐LUMO transition of the Pd₄ chains. The linear Pd₄, Pt_nPd_4‐n (n=1–3), and Pt₄ complexes obtained in the present study could be useful 1D building blocks for assembling through metal−metal interaction.

2-Hydroxytetrahydrofurans were synthesized by Wacker-type oxidation of 3-methyl-3-buten-1-ols with high stereoselectivity, using a PdCl₂(MeCN)₂/NO/BQ catalyst system under O₂.

The aerobic dehydrogenation of saturated carboxylic acids to α,β‐unsaturated carboxylic acids proceeded by a simple Pd(OAc)₂/pyridine catalyst system under 1 atm of O₂. β‐Acyloxylated carboxylic acids were also produced from propionic acid and butyric acid by the conjugate addition of the substrates to the formed α,β‐unsaturated carboxylic acids. When propionic acid was used as a substrate, trans‐Pd(OCOEt)₂(pyridine)₂ was formed in situ as a precatalyst. The kinetic experiments indicated that Pd(OCOR)₂(pyridine) is a catalytically active species and that β‐C(sp³)−H bond activation in the carboxylate ligand is the turnover‐limiting step. The DFT calculations agreed with the mechanism.

The Pd₈string supported by linear tetraphosphine ligands reacted with H⁺to afford a Pd₄H complex with a terminal hydride, and was applied to the electrocatalytic H₂formation from H⁺by making a chemically modified electrode with Nafion film.