Phosphino-oxazoline copper

Phosphino-oxazoline)copper complex 28 was found by Helmchen et al. to be an excellent Diels-Alder catalyst [37] (Scheme 1.47, Table 1.20). The nitrogen atom acts as an electron-donating ligand, whereas phosphorus is a cr-donor-Tt-acceptor ligand. The copper complex of this phosphino-oxazoline ligand is therefore expected to have... 

The phosphino-oxazoline copper(II) complex (55) has also been found to be an effective catalyst[136] as have some titanium complexes, such as the extensively researched titanium-TADDOL system (56)[137]. A modified Ti(IV)-TADDOL compound is the catalyst of choice to promote Diels-Alder cycloaddition reactions between cyclopentadiene and alk-2-enyl phenylsulfonylmethyl ketones[138]. 

Two general approaches to aza-Diels-Alder reactions have been reported. One incorporates the requisite nitrogen atom into the 2rt component (imine), while the other incorporates the requisite nitrc en in the 4rt component (azadiene). Chiral copper Lewis acids have been used with success in both approaches. Jorgensen and coworkers reported enantioselective imino Diels-Alder reactions catalyzed by CuC104 MeCN in the presence of phosphino-oxazoline (287) or BINAP (290) (Scheme 17.64) [93]. Phosphino-oxazoline (287) proved to be the ligand of choice in the aza-Diels-Alder reaction of N-tosyl a-imino ester (142) with Danishefsky s diene (286), while BINAP (290) gave the highest selectivity when dimethyl-substituted Danishefsky s diene (289) was used. 

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...

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