Scrambling reactions

Although both arsenic and phosphorus are soft, arsenic is softer. Likewise, Cl is softer than F, so we predict that a scrambling reaction would take place as shown earlier. 

Scrambling reactions occur between R4Sn and SnX4 to give mixed alkyl halides. 

The complex TpPtMeH2 was synthesized by reacting TpPtMe(CO) with water (66). While it is stable towards reductive elimination of methane at 55 °C, deuterium incorporation from methanol-c/4 solvent occurs rapidly into the hydride positions and subsequently, more slowly, into the methyl position (Scheme 15). The scrambling into the methyl position has been attributed to reversible formation of a methane complex which does not lose methane under the reaction conditions (75,76). Similar scrambling reactions have been observed for other metal alkyl hydrides at temperatures below those where alkane reductive elimination becomes dominant (77-84). This includes examples of scrambling without methane loss at elevated temperature (78). 

An obvious, but important criterion for selecting a scrambling reaction is that equilibration must occur under conditions compatible with the target. 

For self-selection experiments and selection in the presence of an organic guest, this is generally a simple criterion to satisfy. However, biomolecules dramatically narrow the available conditions the reaction must ideally occur at room temperature and in buffered aqueous solution. Both of these conditions can be (at least in principle) attained by physically separating the scrambling reaction from the biomolecule against which the library is selected. 

Results obtained for Rg = He, Ar, and Kr are shown in Figures 6, 7, and 8, respectively. Because He is ineffective in quenching 02(a nj and only weakly binds to Oj, the most important effect of addition of He to the ion source is the decreased importance of the scrambling reaction (13). This consideration accounts for the increasing EF(66) with increasing He partial pressure at low and indicates... 

Similar reactions applied to transition metal-acetylene complexes appear capable of separating the 2 carbon atoms originally linked by the acetylenic triple bond 18). Thermal isomerization of metal-acetylene complexes may achieve the same result, showing how metal clusters can catalyze scrambling reactions of acetylenes, e.g.. 

Ni[R-dtp]2 complexes form paramagnetic five-co-ordinate adducts with hexamethylphosphoramide Xo date no phosphate esters have been found to form adducts. However, the NMR spectra of solutions of Ni(R-dtp>2 complexes after long standing or gentle heating clearly show that scrambling reactions of the type ... 

The number of sulfur atoms In the bridge are also between 3 and 4. When any of these dlalkylpolysulfides are reacted with carbanlons such as secondary butylllthlum, In excess, one observes the gradual disappearance of the Initial compound and the formation of the dlsymetrlc dlalkylthloether resulting from a scrambling reaction. 

NMR studies of the mixed carboxylates containing acetates and trichloroacetates have indicated that ligand scrambling reactions occur in solution although all of the species were isolated, and isomerization possibilities occur, no discrete isomers were identified.329 Gas... 

The "scrambling reaction of POCl3 with the crystalline P2Os-net-work provides an excellent example of the applicability of the Lp.g.f. approach (72). POCl3 causes a partial degradation of the network and the equilibrium state resembles exactly a threefunctional polycondensation. The ratio R = [C1]/[P] is known from the experiment and from stoichiometry one gets... 

One is generally interested in the enthalpies of scrambling reactions since these may be measured thermochemically and thus brought into relationship with equilibrium constants. Real scrambling equilibria, however, deviate more or less from the ideal random case. And, in order to be able to calculate enthalpies, it is assumed that for large values of the enthalpy, (JS)real (JS)rand. Accordingly, we may now estimate the enthalpy of a real scrambling reaction. 

Until relatively recently, less attention has been given to catalysis of redistribution reactions by transition metal complexes. Redistributions have been observed during the course of platinum-catalyzed hydrosila-tion hence, the scrambling reaction can be a nuisance by decreasing the yield of desired hydrosilation products. A noteworthy example is the H/Cl exchange that occurs during the hydrosilation of allyl chloride [Eq. (10)]. 

Other synthetic routes to alkene-metal complexes reported in the literature involve bridge cleavage reactions of halogeno complexes (method F), facile intermolecular scrambling reactions involving metal-alkene and metal-PF3 complexes (18, 81) (method G) (Scheme 4), and the coupling together of two alkyne units to afford a metallo-cyclopentadiene derivative (method H). 

The enolates derived from a-benzylselanylcyclopentanone and a-tetralone were treated with allyl bromide producing the corresponding a-allylcyclanones in 82 and 53 % yield, respectively. No scrambling reaction concerning the benzyl moiety was observed [37] (Scheme 30). 

Only EtSb(TePh)2 was prepared on a larger scale and isolated as crystals in 78% yield. This type of reaction could not be applied to (MesSb)4. Reactions of this stibetane with dichalcogenides resulted in scrambling reactions giving product mixtures . ... 

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