Mercury compounds transmetallation

It can be seen that the pseudocyclic intermediate (84a) strongly resembles the stable alkylperoxy-mercury compound (84b) prepared from the reaction of TBHP with an alkene in the presence of mercury(II) carboxylate.238 The X-ray structure of the similar BrHg CH(Ph)CH(Ph)(OOBu1) compound has clearly shown the pseudocyclic nature of this adduct by the interaction existing between mercury and the OBu1 group.259 The transmetalation of mercury by palladium in (84b) produces acetophenone in 95% yield, presumably via the formation of the pseudocyclic intermediate (85 equation 85).42... 

Transmetallation of silylmercury compounds, which are most easily available via the reaction of a silane with (/-Bu)2llg, appears to be the most convenient method for the preparation of cyclic potassium oligosilyl compounds c-(MegSis)K (19)83c as well as c-(MenSi6)K (20)84 were obtained by Hengge and coworkers from the corresponding mercury compound by means of a Na/K alloy in THF (equation 33). 

Well-characterized 2-(chloromercurio) derivatives of 1-acetyl- and l-(phenylsulfonyl)pyrrole were prepared by reaction of HgCl2. These can be converted to />/s-(pyrrol-2-yl (mercury compounds or into ruthenium and osmium complexes by transmetallation. In all of these complexes there is coordination between the acetyl or sulfonyl oxygen and the metal, but the chelation is much tighter in the Ru and Os complexes <95JOM(491 )219> To date, the reactivity of these compounds has not been explored. 

In the initial studies about the reaction of /V.zV-disubstituted formamides with alkaline metals to give glyoxylic amides, the participation of carbamoyl metal derivatives as intermediates was postulated83. The first preparation of the carbamoyllithium 77 was described two years later by a mercury-lithium transmetallation from compound 76 at —75 °C (Scheme 20)84. The authors proposed also an aminocarbene structure 78 and studied its reactivity with methanol, methyl iodide, carbonyl compounds, esters, acyl chlorides, mercury(II) chloride and tri-n-butyltin chloride providing compounds 79. 

Vinyl complexes are typically prepared by the same methods used to prepare aryl complexes. Vinyl mercury compounds, like aryl mercury compoimds, are easily prepared (by the mercuration of acetylenes), and are therefore useful for the preparation of vinyl transition metal complexes by transmetallation. The use of vinyl lithium reagents has permitted the s rnthesis of homoleptic vinyl complexes by transmetallation (Equation 3.35). Reactive low-valent transition metal complexes also form vinyl complexes by the oxidative addition of vinyl halides with retention of stereochemistry about the double bond (Equation 3.36). Vinyl complexes have also been formed by the insertion of alkynes into transition metal hydride bonds (Equation 3.37), by sequential electrophilic and nucleophilic addition to alkynyl ligands (Equation 3.38), and by the addition of nucleophiles to alkyne complexes (Equation 3.39). The insertion of alkynes into transition metal alkyl complexes is presented in Chapter 9 and, when rearrangements are slower than insertion, occurs by s)m addition. In contrast, nucleophilic attack on coordinated alkynes, presented in Chapter 11, generates products from anti addition. 

One of the earliest methods for preparing aromatic boronic acids involved the reaction between diaryl mercury compounds and boron trichloride [198]. As organomer-curial compounds are to be avoided for safety and environmental reasons, this old method has remained unpopular. In this respect, trialkylaryl silanes and stannanes are more suitable and both can be transmetallated efficiently with a hard boron halide such as boron tribromide [199]. The apparent thermodynamic drive for this reaction is the higher stability of B-C and Si(Sn)-Br bonds of product compared to the respective B-Br and Si(Sn)-C bonds of substrates. Using this method, relatively simple arylboronic acids can be made following an aqueous acidic workup to hydrolyze the arylboron dibromide product [193]. For example, some boronic acids were synthesized more conveniently from the trimethylsilyl derivative than by a standard ortho-metallation procedure (entry 11, Table 1.3). 

For transmetallations with a metal (metallo-de-metallations, Scheme 10-95) arylmercury compounds are particularly suitable due to the position of mercury as a noble metal in the electrochemical series of standard potentials (for examples see Makarova, 1970). 

Vinyl selenides have been lithiated at the a-position by LDA983,984 at —78 °C in THF to give a-(arylselanyl)vinyllithiums 680, a-(methylselanyl)vinyllithiums 681 being obtained by selenium-lithium transmetallation from l,l-bis(methylselanyl)alkenes with n-BuLi in THF or t-BuLi in ether at —78 °C985 986. These intermediates reacted with alkyl halides, epoxides, carbonyl compounds and DMF985, the final deprotection being performed by mercury(II) salts986. 

Organoaluminum compounds, particularly AlEt3, are commercially very important as activators for olefin polymerization catalysts and are produced on a large scale, in spite of their pyrophoric nature and violent reaction with water. In the laboratory organoaluminum compounds can be made from mercury alkyls by transmetallation ... 

A general method for the synthesis of organomercurials is the transmetallation of mercury(n) salts with other organometalhc species. Organohthium and -magnesium compounds continue to be widely employed for the preparation of complexes with formula RHgX or HgR2 (see equation 1). 

Transmetalation using tin-lithium exchange is an excellent method — even better than mercury-lithium exchange — for the preparation of organolithium compounds, but it works only for compounds being more stable than n-butyllithium. This is also true for the synthesis of polylithiumorganic compounds. Examples are ( , )-1,5-dilithio-1,4-pentadiene 99the (Z,Z)-l,5-dilithio-l,4-pentadiene derivative 101 —... 

First, coordinatively unsaturated active palladium catalyst, PdL2, is produced via dissociation of the ligands, which then reacts with acyl halide to give the acylpalladium intermediate. Since deinsertion of CO of the acylpalladium derivatives may occur simul-taneously, the next step, transmetallation (so-called metathesis), is the most crucial for the efficiency of the overall reaction. A variety of organometallic compounds, such as boron, aluminum, copper, zinc, mercury, silicon, tin, lead, zirconium, and bismuth, are used as the partner in this coupling reaction without loss of CO. In this section, the important features of the cross-coupling reactions of a variety of organometallic compounds with acyl halides and related electrophiles are discussed. 

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