Cyclopropanation phosphite ligands

Nevertheless, the earbene rendition remains popular because it represents in the chemist s mind a relevant intermediate in reactions such as electrophilic cyclopropanation or [l,2]-sigmatropic shifts. In that respect, Toste showed that the yield of the gold-catalyzed cyclopropanation of cis-stilbene 115 with cyclopropene 114 was critically dependent on the ancillary ligand L on Au (Scheme 4.32). For instance, the use of phosphite ligands such as P(OMe)3 and P(OPh)3 led to poor yields. In the case of phosphines, which are less n-acidic than phosphites, the desired cyclopropane 116 was obtained with moderate yields. On the other hand, the use of IPr, which is strongly o-donat-ing and weakly n-acidic compared with a phosphite, provided 116 in 80% yield. 

The Lewis acid-catalysed 3 + 2-cycloaddition of cyclopropanes with aldehydes yields tetrahydrofurans with high diastereoselectivity.22 The enantiospecific Sn(II)-and Sn(IV)-catalysed formal 3 + 2-cycloadditions of aldehydes with donor-acceptor (g) cyclopropanes produce optically active tetrahydrofurans23 The bulky phosphites and phosphoramidites are excellent ligands which promote the Pd-catalysed 3 + 2-intramolecular cycloaddition between alkylidenecyclopropanes and alkynes24 The... 

With (l-methylethylidene)cyclopropane (1) and a naked nickel catalyst, cyclodimers are formed with methyl acrylate in a combined yield of 59% and an isomeric ratio (2/3) of 96 4. When the reaction is performed in the presence of tris(2-phenylphenyl) phosphite as modifying ligand, the combined yield increases to 89% but the selectivity drops to 85 15. ... 

The ligand substitution reactions of carbene complexes such as (CO)s-CrC(OCH3)CHj allow the synthesis of many phosphine- and phosphite-substituted carbene complexes. It is expected that these complexes will have modified reactivity and will provide a means of fine tuning reactions of carbene complexes. More importantly, the substitution reactions of carbene complexes proceed by a dissociative mechanism involving coordinatively unsaturated intermediates. Study of the ligand substitution reactions can give valuable information about these coordinatively unsaturated intermediates which are also involved in the important cyclopropanation, alkene scission, and thermolysis reactions of metal-carbene complexes. 

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