Metal-promoted eliminations

Heterogeneous dehalogenations of vicinal dihalides can be achieved smoothly with zinc in protic solvents or magnesium, lithium or sodium in ethereal solution. In general the elimination occurs at the surface of the metal and the reactivity of this surface is maintained by the presence of a solvent which redissolves any produced olefin and any metallic salt. However, in one case, catalysis by the zinc dihalide deposited on the metal surface during the elimination has been reported .  

It seems unlikely that the influence of the metal is of an electrophilic nature, inducing a carbonium-ion mechanism, as the products of substitution and dehydrohalogenation are never found simultaneously. More plausibly the metal donates electrons, and carbanion or concerted processes with carbanion character rather than radical processes are usually favoured. Radical additions occur very easily to olefinic double bonds and l-bromo-l,2-diphenylethane undergoes radical bromination with N-bromosuccinimide without accompanying dehalogenation.  

Elimination from meio-dideuterodibromoethane and the diastereoisomeric 2,3-dibromobutanes with zinc shows anti stereospecificity - . However, such stringent stereochemical behaviour is not shown in elimination from the higher homologues such as the dibromo-pentanes, -hexanes or -octanes. In the cyclodecyl series a marked preference foriy/i-elimination is observed . These findings have a parallel in base-catalysed Hofmann eliminations, and explanations in terms of steric interactions which cause preferential substrate conformations to be assumed adequately account for the experimental results. An alternative approach is to invoke carbanion intermediates in which rotation about the Cg-Ca bond is more rapid or of a similar rate to the ejection of bromide ion . As products characteristic of a solvent interaction on a carban- 

The absence of solvolytic products can be explained if any intermedate carbanion is associated with the metal surface. An extension of this view is to consider these metal catalysed eliminations as proceeding through organo-metallic compounds (BrMCR.,Br M = Mg or Zn) which break down by a concerted process . 

Radical processes seem more probable with the monovalent alkali metals than with the divalent magnesium and zinc and complete non-stereospecificity is often observed - . Of course, initial radical formation could be followed by a rapid electron transfer to give a cabanion , which subsequently undergoes a unimolecular elimination to give the olefin. 

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Intramolecular Cyclizations.— Theoretical calculations have been reported for the conversion of 1,3-disubstituted propanes into cyclopropanes. Metal-promoted eliminations in 1,3-dihalogenoalkanes afford cyclopropanes, generally in good yields. ... 

Metal-promoted elimination of any pair of halogens (except fluorine [F]) has also been used to produce double and triple bonds. The use of zinc (Zn) metal (with formation of zinc(II) dibromide [ZnBr2]) is common and, occasionally, salts of other metals, for example, tin(II) chloride [SnCy, have also been used as shown in Scheme 7.43. There is some evidence that when these Lewis acids are used, the products form by single-electron transfer or radical process. 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

This preparation illustrates an efficient two-step process for the transformation of a cycloalkenone to the corresponding a-substituted derivative. The first step involves the installation of an a-iodo substituent by a process thought to involve nucleophilic addition of pyridine, iodine capture of the resulting enolate, and pyridine-promoted elimination of pyridine.5 The resulting vinyl iodides are superior to other vinyl halides as participants in a variety of transition-metal catalyzed coupling reactions, illustrated here by the Suzuki coupling with an arylboronic acid. Other coupling partners that... 

To explain the formation of the expected product of insertion into the H—Si bond of HSiEt3 in similar yields from different precursors, it was suggested that t-Bu2Si also arises by ultrasound-promoted lithio-dehalogenation of t-Bu2SiX2 (X = Cl, Br, I) see also Section II.E. This would represent the first example of a metal-promoted a-elimination of dihalosilanes. In a related study of the chemistry of diadamantylsilylene Ad2Si the reactive species formed from Ad2Sil2 upon treatment with lithium under ultrasonic irradiation was compared with that formed upon pyrolysis of a silirane, as shown in equations 36 and 3757. 

These salts can be used for the synthesis of both transition metal and main group element thionyl imides by metathetical reactions, for example, Cp2Ti(NSO)2 and Ph3 As(NSO) (x = 1, 2), respectively. " The M NSO group invariably adopts a cis geometry in these derivatives. A characteristic reaction of thionyl imides is the thermal or base-promoted elimination of SO2 to give the corresponding sulftir dimide. ... 

Almost all of the reactions of metals can be classified into just a few typical reactions, and the reactions that metals promote in organic chemistry are simple combinations of these typical reactions. If you learn these typical reactions, you will have no trouble drawing metal-mediated mechanisms. The typical reactions of metal complexes are ligand addition/ligand dissociation/ligand substitution, oxidative addition/reductive elimination, insertion/j8-elimination, a-insertion/ a-elimination, cr-bond metathesis (including transmetallations and abstraction reactions), [2 + 2] cycloaddition, and electron transfer. 

A preparative method for the generation of di-/-butylsilylene via metal promoted a-elimination of dihalosilanes has been developed. The silylene could be trapped with alkenes to give silacyclo-propanes in 65-70% yield (Scheme 35) <64AG(E)226,88AG(E)1355>. 

While main group organometallics are thermodynamically unstable with respect to oxidation, their kinetic stabilities vary widely. Transition metal organometallics generally ofler a greater thermodynamic stability because the transition metals are less electropositive, but kinetic stabilities stUl vary widely, particularly due to the tendency toward metal hydride elimination. Transition metal polymer systems can be made by the direct route. As expected, high bond polarity promotes attack by reagents. Also, compormds with empty or half-filled orbitals will be more reactive than those with filled orbitals. 

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