Lithium trialkyl borates

Lithiumboranat mit Alkyl-Gruppen als Zweitliganden (z.B. Lithium-trialkyl-hydrido-bo-rate) sind sehr wertvolle, selektive Reduktionsmittel. Sie zersetzen sich an der Luft und werden meist nicht isoliert. Lithium-triathyl-hydrido-borat wird z. B. auf folgende Weise her-gestellt4 (analog erhalt man Lithium-triathyl-detiterido-borat) ... 

Lithium-trialkyl-hydrido-borate mit groBerem effektivem Raumbedarf sindzur stereo-selektiven Reduktion von prochiralen Gruppen gut geeignet. Am einfachsten kann Li-thium-tributyl-(2)-hydrido-borat hergestellt werden5 (Vorschrift S.334). 

Dialkyl-[alken-(l)-yl]-borane werden nach Uberfiihrung mit Alkyl-lithium in die Lithium-trialkyl-[alken-(l)-yI]-borate bereits mit 6 n Natronlauge bei 20° hydrolysiert5. 

Zur Herstellung von axialen Alkoholen aus unbehinderten Cycloalkanonen werden hauptsachlich Lithium-trialkyl-hydrido-borate angewendet2,3 (vgl. a. Tab. 28) ... 

In recent years, a variety of aryl boronic acids are commercially available, albeit in some cases they may be expensive for large scale purposes. During our work in the mid-1990 s boronic acid (II) was not commercially available and so two different protocols were used to prepare this acid. The first approach involved the transmetallation with n-butyl lithium of aryl bromide (I) and trapping the lithio species generated with trialkyl borate followed by an acid quench. Aryl bromide (I) is easily prepared by reaction of o-bromobenzenesulfonyl chloride with 2-propanol in the presence of pyridine as a base. The second approach was a directed metallation of isopropyl ester of benzene sulfonic acid (VII), to generate the same lithio species and reaction with trialkyl borate. The sulfonyl ester is prepared by reaction of 2-propanol with benzenesulfonyl chloride. From a long-term strategy the latter approach is... 

An alternative approach to reduce the levels of impurity (VII) would be to have a "transient" existence of the lithio species, so that it reacts instantaneously with trialkyl borate to form the aryl boronate, prior to being quenched by any extraneous proton source to form (VII). Thus, the preparation of boronic acid (II) was improved by changing the order of the reagents. The slow addition of n-butvl lithium also controls the exotherm of the reaction. There was no reaction observed between n-butyl lithium and triisopropyl borate (to form any butyl boronic acid), nor was there any formation of 2-butyl derivative of (VII) formed by reaction between butyl bromide and the lithio species. The reaction is veiy fast and as soon as the addition of n-butyl lithium is completed the reaction is finished. This indicates a rapid transmetallation and instantaneous boronation of the lithio species. The reaction is very much a... 

The traditional synthesis of organoboron compounds from organic halides is based on the reaction of trialkyl borates with Grignard or lithium reagents. However, the cross-coupling reaction of diboron solves the difficulties associated with the use of Mg/Li compounds. The cross-coupling reaction of diborons... 

CLy -Enals and -enones.17 The TiCl4-catalyzed reaction of lithium trialkyl(l-al-kynyl)borates with methyl vinyl ketone (10, 402) has been extended to a similar reaction with orthoesters, which results in allylic boranes that are oxidized to a,p-enones and -ei als (equation I). 

Boronates (i.e. boronic acids and esters) are usually prepared by one of two methods reaction of organo-lithiums or Grignard reagents with a trialkyl borate, usually tri-Ao-propyl borate, or palladium-catalysed boronation. 

Benzodithiolium perchlorate (218 R = H, X = CIO4) is highly explosive. Its 2-methylthio-derivative (218 R = MeS) condenses with 1-ethylaminonaphthalene to give the stable quinone-methide imine (219). Lithium trialkyl(alkynyl)borates Li R 3BC=CR (R R = n-CsHn or n-C6Hi3) react stereoselectively with the salt (218 R = H, X = Bp4) to produce vinylboranes (220) in high cis itrans ratios, which on hydrolysis and... 

Arylboronic acids are prepared by transmetallation between an or-ganometallic reagent and an appropriate boron reagent. In laboratory-scale preparation, the use of Grignard reagents or lithium reagents and trialkyl borates is the most common from the standpoint of availability and easy preparation (Eqs. 2 and 3). 

Natrium-trialkyl-hydrido-borate haben als Reduktionsmittel eine geringere Be-deutung als die entsprechenden Lithium-Derivate5 (s.S. 14). 

Die Kalium-trialkyl-hydrido-boratezersetzen sich an der Luft und werden daher ohne Isolierung eingesetzt. Kalium-triathyl-hydrido-borat ist z.B. aus Kaliumhydrid und Triathyl-boran erhaltlich1 und hat ein ahnliches Reduktionsvermogen wie Lithium-tri-athyl-hydrido-borat (s.S. 14). 

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