金屬有機(jī)化學(xué)ppt課件-資料下載頁(yè)
2025-05-12 03:31本頁(yè)面
【正文】 bonyl, ester and nitrile, and a lower tendency towards homocoupling but nevertheless in some cases homocoupling occurs in a few percent sufficiently small to facilitate purification so overall offering much improved results however, zinc reagents are still moisture sensitive which is a practical disadvantage particularly on a large industrial scale. the low degree of homocoupling and the low degree of disproportionation (the loss of the leaving group for H) makes aryl zinc reagents very useful in crosscoupling reactions one particular strength of the aryl zinccouplings is the high degree of selectivity between iodo and bromo leaving groups One area of coupling reactions of ano zincs, seemingly not matched by any other metal is that of alkynyl zinc couplings to aryl halides – such a coupling is obviously useful for the introduction of alkynyl units – but more importantly the hydrogenation of the coupled alkyne gives an alkyl chain Crosscouplings involving Organoborons: Suzuki Reaction crosscoupling reactions involving arylboronic acids and aryl halides were first reported in 1981 by Suzuki hence Suzuki Reaction – such couplings were, at first glance, just another methodology similar to magnesiums and zincs but Suzuki Couplings have bee the most monly used arylaryl bond forming reaction. unlike magnesiums and zincs, arylboronic esters and acids are stable to air and moisture and can be isolated and purified many are now merciallyavailable such stability means that the coupling reactions can be performed in aqueous media without special precautions. advantageous (a) normally no homocoupling or so little. (b) borons are not toxic.(c) anoborons are stable to air and moisture, and can be purified, stored and used as and when required. (d) couplings tolerate a wide range of functional groups, esters, nitriles, carbonyls, nitros, alcohols and phenols (e) consistently high yields for a wide range of substrates. (f) very efficient even for sterically hindered couplings. disadvantages (a) some boronic acid species are easily hydrodeboronated. (b) selectivity between iodo and bromo leaving groups is not as good as for zinc couplings. although a stronger base is required, sterically hindered boronic acids couple very efficiently, even to sterically hindered halides note that in many cases it is best to use a milder base to prevent hydrodeboronation loss of boron unit for a proton stronger bases do enhance coupling, but they also enhance hydrodeboronation sometimes a strong base helps, other times a weak base helps!! Selective Crosscouplings involving Organoborons Alkenyl Couplings involving Organoborons alkenyl couplings are little different to the successful arylaryl couplings as they both involve the same type of carbon and conditions can be similar to arylaryl couplings discussed previously. Alkyl Couplings involving Organoborons Couplings involving alkyl species are particularly poor because of ?hydride elimination, which can take place at any time throughout the catalytic cycle straight after oxidative addition if an alkyl halide is used, or straight after transmetallation if an alkyl metal is used.
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