Phosphane oxides unsaturated

The stereochemical trends discussed above are not limited to a, yS-unsaturated carbonyl compounds other Michael acceptors such as nitroalkenes and unsaturated phosphane oxides display similar behavior. A representative example for the nitroalkene class of Michael acceptors is shown with substrate 70 in Scheme 6.13 [28]. The best results were thus obtained for arylcuprates. Other organocuprates were much less selective, which severely restricts their application in organic synthesis. 

Similar observations were made in a related series of unsaturated phosphane oxides (such as 73, Scheme 6.14) [29]. Whereas dialkylcuprates mostly reacted non-selectively, the best diastereoselectivities were observed for disilylcuprates (74). 

Scheme 6.14. Diastereoselective cuprate addition to ,/ -unsaturated phosphane oxide 73 (TBS = t-butyidimethylsilyl).

Lewis acid and the oxygen atom of the phosphane oxide, respectively. With this catalyst system, N-allyl- and N-benzhydrylimines generally gave lower enantioselectivities. The addition of phenol was found to have a beneficial effect on the reaction rates. The JandaJEL -supported bifunctional catalyst of 14 has also been shown by Shibasaki and co-workers to promote the Strecker-type reaction of aromatic and a,/ -unsaturated imines in excellent yields with 83-87% ee in the presence of tert-butanol (110%) [11]. The reactivity of the Janda/EL catalyst was comparable to the homogeneous analogue 14, and the catalyst could be recycled at least four times. 

A coordinatively unsaturated 14-electron palladium(O) complex, usually coordinated with weak donor ligands (usually tertiaiy phosphanes), has meanwhile been proven to be the catalytically active species [5]. This complex is mostly generated in situ. Tetrakis(triphenylphosphane)palladium(0) [6J, which exists in an equilibrium with tris(triphenylphosphane)palladium(0) and free triphenylphosphane in solution, is frequently employed. The endergonic loss of a second phosphane ligand [7] leads to the catalytically active bis(triphenylphosphane)palladium(0). However, palladium(D) complexes such as bis(triphenylphosphane)palladium dichloride or palladium acetate, which are easily reduced (e.g. by triarylphosphanes see below) in the reaction medium, are more commonly employed for convenience, as they are inherently stable towards air. The mechanistic situation is a little more complicated with palladium acetate, in that anionic acetoxypalladium species Pd(PPh3) (AcO ) (n = 2, 3) are formed in the presence of acetate ions [5], and these actually participate in the oxidative addition step and the following coupling reaction. 

A coordinatively unsaturated 14-electron palladium(0) complex, usually containing two tertiary phosphanes as weakly electron-donating ligands, has been proved to be the catalyUcaUy active species. It is conunonly generated in situ from either from a palla-dium(0) complex or by reduction of relatively inexpensive paUadium(II) acetate or chloride.In the first step of the catalytic cycle ( in Scheme 1), a haloalkene, a haloarene, or a similar substrate is oxidatively added to the coordinatively unsaturated paUadium(O) species to generate a tr-alkenyl- or (r-arylpalladium(II) complex.f t This then coordinates an alkene molecule, and when the latter and the alkenyl (aryl) residue on the palladium are in a cis orientation, the cr-alkenyl- or o--arylpalladium complex can... 

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