Shift reagents halides

Certain metal complexes endowed with weak donor groups, such as ( -C5Fl5)2TiCl2 and (C6H5)3SnCl, also show shifts in their spectra in the presence of Ln(fod)3 chelates. In this case it appears likely that the halide ion associates with the lanthanide chelate to create an anionic species Eu(fod)3X that then forms an ion pair with the metal cation. A review article on the utilization of lanthanide shift reagents for the study of metal complexes has been published. ... 

The analysis of isothiouronium salts, which are conveniently formed by reacting alkyl halides with thiourea, provide a way to study alkyl halides . The H NMR spectrum of s-butylisothiouronium chloride and bromide exhibited enantiomeric discrimination in the presence of [Eu(tfc)3(fod)] The enantiomeric purity of several alkyl methyl phenyl sulfonium ions (73) under conditions of slow inversion was determined using either [Eu(tfc)3(fod)] or [Eu(tfc)4] It was possible to study the inversion barrier by variable-temperature studies in the presence of the shift reagent. 

Studies of Equilibria, Shift Reagents, and Solvent Effects.—Several studies of halogen-exchange equilibria have been reported, e.g. the exchange between phosphoryl, phosphonyl, and phosphinyl halides, P and P halides (27)... 

Tetrakis chelate anions formed when either a silver or potassium P-diketonate is mixed with a lanthanide tris P-diketonate are effective shift reagents for organic-soluble salts. Adding an organo-halide or fluoroborate salt leads to precipitation of silver halide or potassium fluoroborate, resulting in an organic-soluble ion pair. These reagents have been used to enantiomerically resolve the spectra of chiral isothiouronium and sulfonium ions. 

The lanthanide tetrakisOS-diketonate) complexes are effective NMR shift reagents for organic salts (Wenzel and Zaia, 1985, 1987). The active [R( S-diketonate)4] species are formed in situ by reaction between an oiganic salt and Ag[R(jS-diketonate)4]. For instance when organic ammonium halide Q+X is added to a solution of Ag[R(fod)4] in chloroform, the silver halide AgX precipitates from the solution and Q[R(fod)4] is formed ... 

The Stille coupling reaction is very versatile with respect to the functionality that can be carried in both the halide and the tin reagent. Groups such as ester, nitrile, nitro, cyano, and formyl can be present, which permits applications involving masked functionality. For example, when the coupling reaction is applied to l-alkoxy-2-butenylstannanes, the double-bond shift leads to a vinyl ether that can be hydrolyzed to an aldehyde. 

Allylic halides usually give both SN2 products and products of substitution with an allylic shift (SN2 products) although the mixed organocopper reagent RCu— BF3 is reported to give mainly the SN2 product.22 Allylic acetates undergo displacement with an allylic shift (SN2 mechanism).23 The allylic substitution process may involve initial... 

Oxidation of the heterocycles with common reagents such as MCPBA, sodium periodate or hydrogen peroxide cleanly affords the sulfoxides and sulfones, and it is clear that the sulfur atom is the principal centre of reaction for electrophiles. While the sulfone is a quite inert functionality, the sulfoxides may be reduced to the sulfides with phosphorus pen-tasulfide as for the tetrahydro systems (78CJC1423). Positive halogen sources likewise react at sulfur, and the intermediate sulfonium halide rearranges, usually by 1,2-shift to the a-halo product. 

---