Phosphine-metal complexes Diels-Alder reaction

Helmchen and co-worker investigated the use of phosphinooxazolines as ligands for copper(II) catalyzed Diels-Alder reactions (Scheme 19) (214). Optimal selectivities are found for a-naphthyl-substituted phosphinooxazoline (299). These catalysts require 2.5 h to induce complete conversion to cycloadduct, compared to 18 h using the triflate complex 269c under identical conditions. Helmchen invokes a square-planar metal geometry to explain the stereochemistry of the adducts, similar to the model proposed by Evans. He suggests that the bulky phosphine substituents are required to orient binding of the dienophile in such a way as to place the olefin directly below the terf-butyl substituent on the oxazoline. 

The mechanism of the Arbuzov rearrangement of (allyloxy)phosphines PhJP-(0C3H5)34 (n = 0-2) at room temperature catalysed by [CpRu(AN>3]PF4 has been investigated. A new procedure has been developed for the preparation of enantiomerically pure sulphoxides and ruthenium complexes such as [CpRu((X>)(FI%3)(t-BuMeSO)]SbF4 by a combination of asymmetric oxidations of dialkylsulphides and metal resolution of the products. The Diels Alder reaction of coordinated dienophiles in [CpRu(dnq]p)3 (dienophile)JPF( have been investigated for a range of complexes. ... 

Further progress has been made in the chemistry of 1,3-azaphospholes and related systems. Heinicke s group has developed a route to the new bulky A-substituted 1,3-benzazaphospholes (200). These workers have also carried out studies of the C-lithiation and subsequent phosphination of these systems. " The first examples of pyrido[6]-annelated azaphosp-holes (201) have been prepared" and a study of diastereo-and regio-se-lective Diels-Alder reactions of the 2-phosphaindolizines (202) has been described." Various alkali metal" and samarium" complexes of 1,2,4-diazaphospholide anions (203) have also been characterised. 

Schemes 58-62. A new non-rigid phosphine ligand was synthesized and reacted with Fe(CO)5 to form the mononuclear iron complex (Equation (81)). Phosphino-oxazoline ligands were used as assembling ligands for hetero-metallic complexes, where the phosphorus atom binds to iron and the nitrogen atoms act as donor atoms to copper, cobalt, or palladium (Scheme 58). The copper complex catalyzes cyclopropanation and Diels-Alder reactions. When 2-(A -diphenylphosphinomethyl-A -cyclohexyl)aminopyridine (NNP) reacts with Fe(CO)5 in ethanol, /ra .r-(OC)3Fe(NNP)2 is formed (Scheme 59). This monometallic complex can then be reacted with a copper salt in CH2GI2 to form a complex having an Fe-Cu dative bond. The complex was demonstrated to be an efficient catalyst for the cyclopropanation of styrene by ethyl diazoacetate and for the Diels-Alder reaction of cyclopentadiene and methacrolein. No other heterometallic complexes have been shown to have such reactivity. Previously known...

Chiral metallacycles have been employed as auxiliaries for the promotion and control of asymmetric reactions such as Diels-Alder cycloadditions involving phospholes [19]. This synthetic methodology can be extended to the synthesis of diphosphine motifs via asymmetric hydrophosphination of vinylic and other tmsat-urated phosphine-functionalized substrates. The advantages offered by these metal complexes are listed below ... 

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