Halide scavengers

Phosphoms halides are subject to reactions with active hydrogen compounds and result in the elimination of hydrogen halide. They are convenient reagents in the synthesis of many esters, amides, and related compounds. However, because the involved hydrogen halide frequendy catalyzes side reactions, it is usually necessary to employ a hydrogen halide scavenger to remove the by-product. 

Reductive methods form B—B bonds from B—X bonds. For B2X4 (X = Cl, Br, I) from BXj, an electric discharge is supplemented by the presence of a metal, or metal atoms, as halide scavenger. The passage of BX3 at low pressure through a rf discharge in the presence of Hg produces the diboron tetrahalides B2X4 at 300 mg h ... 

Almost 10 years later, Giidel et al. proposed a general synthetic method for facial tris cyclometalated Ir3+ and Rh3+ complexes by reaction of dichloro-bridged dimers of Ir(ppy)2 with excess ppy ligand using AgCF3S03 as a halide scavenger and promoter (Scheme 3.76) [289]. 

The cationic pathway is followed for unsaturated triflate substrates, or for unsaturated halide substrates in the presence of halide scavengers such as Ag(I) or Th(I) salts [2,3], and is shown in Scheme 8G.2. The individual steps are similar to the neutral pathway, but the difference in the two mechanisms lies in the nature of the Pd(II) intermediates 2.1-2.3, which are now cationic. As will be discussed in more detail later in this review, this difference has a marked effect on both reactivity and enantioselectivity. 

Soulages, N. L. Simultaneous determination of lead alkyls and halide scavengers in gasoline by gas chromatography with flame ionisation detection. Anal. Chem. 38, 28(1966). 

The yields of these reactions are often improved by the addition of thallium(I) or silver(I) salts to act as halide scavengers and so generate the gold phosphine fragment in situ. This modification has been most extensively utilized in the addition of Au(PR3)+ to anionic and neutral cluster compounds, and a number of examples are given below ... 

On the other hand, silver salts can act as a mild Lewis acid, promoting various reactions such as allylations, aldolizations, cycloadditions, and cyclizations. Silver salts can also be used as halide scavengers, acting as cocatalysts in cross-coupling reactions catalyzed by other metals, especially palladium. In the latter context, the exact role of silver salts is far from clear and may be more complex than just halide... 

Tellurium alkoxides were also prepared from the stoichiometrically required amounts of tellurium tetrachloride and alcohol in tetrahydrofuran, with triethylamine serving as the hydrogen halide-scavenger. ... 

When the substrate is a triflate, or the reaction of halide substrates is carried out in the presence of halide scavengers, a variant of this mechanism in which the palladium(II) intermediates are cationic is followed [Ig, 3]. The cationic pathway is illustrated in Scheme 6-2. Oxidative addition yields the palladium(II) intermediate 9 and loss of the... 

The first examples of asymmetric Heck cyclizations that form quatemaiy carbon centers with high enantioselectivity came from our development of an asymmetric synthesis of the pharmacologically important alkaloid (—)-physostigmine (184) and congeners (Scheme 6-31) [68]. In the pivotal reaction, (Z)-2-butenanilide iodide 182 was cyclized with Pd-(5)-BINAP to provide oxindole 183 in 84% yield and 95% ee after hydrolysis of the intermediate silyl enol ether. With substrates of this type, cyclizations in the presence of halide scavengers took place with much lower enantioselectivity [68]. 

Studies suggest that the mechanism of AHRs is quite similar to the achiral version, which includes the same fundamental steps in the catalytic cycles that we have seen already. The major difference is the presence of chiral bidentate ligands, such as bisphosphines [R2P-Y-PR2], phosphine-phosphites [R2P-Y-P(OR)2], bisphospites [(RO)2P-Y-P(OR)2], aminophosphines [R2N-Y-PR2], or diamines (such as (-)-sparteine), which we encountered earlier in Chapter 12. The bidentate variation of the cationic cycle, shown in Scheme 12.15b, seems to explain most aspects of the mechanism of AHR when X = OTf or I (in the presence of halide scavengers). Throughout the cycle, the ligand remains bidentate, and this factor seems to enhance enantioselectivity. The neutral cycle variant, which ought to be... 

Both simulated exhaust apparatus and single-cylinder engines were used with tetraethyllead (TEL) and tributyl o-phosphate (TBP) as the test poisons. No halide scavengers were present in the fuel used for the engines to enable correlation with the findings from the simulated tests. 

The findings can be interpreted only in terms of the presence of both toxic and nontoxic lead in the exhaust. The ratio of these largely determines catalyst durability, and this ratio appears to be very sensitive to the many parameters which affect combustion. Particulate lead products are nontoxic, whilst toxic forms may be organo lead and/or partially oxidized forms of TEL and/or lead halides or oxyhalides if halide scavengers are present in the fuel. The toxic forms are probably present only as a very minor portion of the total lead in the exhaust. 

Xiao et al. [213] recently reported that the intermolecular Heck arylation with several electron-rich olefins can be effected with both aryl bromides and iodides in a highly regioselective manner by the use of ionic liquids. Regioselectivity towards the branched cr-olefin was up to 99/1 and there was no need for additional halide scavenger. In this case the ionic liquid seems to serves not only as a quite polar solvent but also as halide scavenger. 

Schuler (1962) who, using the i C(n,2n) C reaction, observed 3-5% yields of methyl halides in halide-scavenged alkanes. 

In fact, in the synthesis of imidoyl halides derived from N-alkylsubstituted amides, the corresponding iminium salts are obtained as the sole products. The hydrogen halide can be readily removed using an organic base, such as triethylamine, as the hydrogen halide scavenger. 

However, clear evidence of the IL effect was found for the regioselective arylation of olefins. It is generally accepted that the Heck reaction may proceed via two pathways, a neutral pathway leading to the preferential formation of linear olefins and an ionic counterpart more likely to give rise to branched olefins. Thus, Pd(OAc)2 and l,3-bis(diphenylphosphino)propane (DPPP) immobilized in [G4GiIm]BF4 promote the exclusive a-arylation of several classes of electron-rich olefins with a wide range of aryl iodides and bromides in the absence of halide scavengers (Scheme 3). ... 

When the substrate is a triflate, or when a halide is subjected to appropriate reaction conditions in the presence of a halide scavenger (such as silver or thallium salts), the reaction proceeds via the cationic manifold. After oxidative addition, dissociation of X yields cationic intermediate 67. Alkene coordination provides 68 and migratory insertion delivers 69. /S-Hydride elimination then yields the desired product 66. Of importance in the pathway 62- 7— 68— 69— 66 is that both phosphines maintain contact with the metal throughout the process. This is ostensibly the factor responsible for the high enantioselectivities observed for reactions that are thought to proceed along this pathway. This contrasts with the course of events in the neutral pathway, where phosphine dissociation is thought to be responsible for low enantioselectivities. 

Uses Intermediate hydrogen halide or inorg. halide scavenger for pharmaceuticals and specialty chems. 

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