Boron reactions

The reactions of nitrones constitute the absolute majority of metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions. Boron, aluminum, titanium, copper and palladium catalysts have been tested for the inverse electron-demand 1,3-dipolar cycloaddition reaction of nitrones with electron-rich alkenes. Fair enantioselectivities of up to 79% ee were obtained with oxazaborolidinone catalysts. However, the AlMe-3,3 -Ar-BINOL complexes proved to be superior for reactions of both acyclic and cyclic nitrones and more than >99% ee was obtained in some reactions. The Cu(OTf)2-BOX catalyst was efficient for reactions of the glyoxylate-derived nitrones with vinyl ethers and enantioselectivities of up to 93% ee were obtained. 

Boron enolates are often used for aldol reactions. Boron enolates are usually prepared from the corresponding carbonyl compounds, tertiary amine, and a boron source (e.g., dibutylboron triflates). The aldol reactions proceed via a six-membered transition state to give high diastereo-selectivity which depends upon the geometry of the boron enolates. 

Keywords Atom transfer reactions Boron C C bond formation Conjugate addition Radical initiators Radical reaction Tin-free... 

There are some reactions in which boron trifluoride and boron trifluoride-diethyl ether complex are used as fluorinating agents, but they are not so frequently used and widespread. The best-known reaction is the decomposition of fluoroformates. In this type of reaction boron trifluoride or pyridine are essentially required as catalysts for the decomposition process. The formation of fluoroformates is established4 5 and the decomposition proceeds cither by heating in pyridine at higher temperature6 or by addition of boron trifluoride-diethyl ether complex at 0-50°C.7... 

The examples given demonstrate that l,3,5-trichloro-2.4,6-trifluorobenzene can be dispropor-tionated either with or without an additive. Aluminum trifluoride and chromium(III) fluoride act as catalysts and enhance the reaction boron trifluoridc, however, seems to act as an inhibitor. 

Bis(l,5-cyclooctadiene)nickel(0), 35 By Diels-Alder reaction Boron trifluoride etherate, 43... 

Sax(Ref 27) has discussed the toxicity, fire expln hazards of boron hydrides. The max allowable concn(MAC) is O.lp per million in air. These compds are dangerous when exposed to heat or flame or by chem reaction. Boron hydrides evolve hydrogen upon contact with moisture and can propagate a flame rapidly enough to cause expln. The toxicity health hazards are discussed also by Krackow(Ref 20), Rothberg et al(Ref 37) Fein-silver et al(Ref 38)... 

Astrophylline, via ring-rearrangement reactions, 11, 261 Asymmetric addition reactions boron compounds, 9, 215 with zinc reagents, 9, 109... 

Asymmetric 1,4-addition reactions, boron reagents, 10, 388 Asymmetric Alder-ene reactions, characteristics, 10, 579 Asymmetric alkoxycarbonylation, characteristics, 11, 467 Asymmetric alkylations, with alkylzincs, 2, 403 Asymmetric allylation... 

Gonia-ene reaction, for C-C bond formation, 10, 312 Conjugate addition reactions borons to alkenes, 9, 214 in C-C bond formation... 

To determine the diastereoselectivity of the above bora-ene reaction, boronate 193 derived from a-pinene was synthesized. Reaction of a-pinene 192 with Schlosser s base (BunLi + KOBu ) furnishes the allyl carbanion, which upon treatment with triisopropyl borate and subsequent transesterification with pinacol yields a-pinanyl pinacol boronate 193. Bora-ene reaction with this allyl boronate and S02 at — 78 °C in CH2CI2 yields the mixed anhydride 194 as a 2.3 1 mixture of diastereomers upon removal of excess S02. Treatment of this mixture of anhydrides with aryl Grignard led to the formation of two diastereomers of aryl sulfoxides 195 in 3.2 1 ratio (Scheme 33) <2006TL2783>. 

The greater part of the modification studies of silica with boron compounds has been directed towards achieving an understanding of the surface structure of silica and silica based adsorbents, utilizing the quantitative reactions with boron containing probe molecules. Because hydroxyl type specificity occurred in some reactions, boron compounds were used to make a distinction between isolated and vicinal surface hydroxyls. Diborane was even utilized as a probe to distinguish surface silanols from hydration water.6,7... 

In these reactions boron trifluoride assisted ring opening occurs to give a carbocation (1 to the silyl group and then rearrangement gives the product silyl enol ether (Figure Si5.16 illustrates the pathway taken by the reaction depicted in Equation Si5.24.)... 

BF3 + NH3 —> BF3NH3. In the ammonia molecule there is a nonbonding pair of electrons that can react in a variety of ways. Later in the book, you will learn about complexes that can form with ammonia. In this reaction, boron trifluoride is a molecule that is usually represented with only 6 electrons around the central boron, making it a perfect mate for the ammonia molecule. As tempted as you might be to try to apply a double-replacement strategy here, remember that they are both molecular substances. This suggests another approach a combination reaction. 

Crotonyl Enolate Aldol Reactions. Boron enolates of the A/-crotonyloxazolidinones have been shown to afford the expected. n-aldol adducts (eq 36). The propensity for selfcondensation during the enolization process is minimized by the use of triethylamine over less kinetically basic amines. 

POMERANZ-FRITSCH REACTION Boron trifluoride-Trifluoroacetic anhydride. 

Suzuki reaction Boronic acid or ester Biaryl 71, 73, 84-86... 

Diamine protonic acids have been used for catalytic asymmetric aldol reaction.Boron triflate derivatives, R2BOTf, have been used for the condensation of ketals and ketone to give (3-alkoxy ketones. 

The first reaction is a Sonogashira coupling between an aryl bromide and an alkyne. There is nc need to have anything but hydrogen on the alkyne for this reaction in contrast to the Stille reaction (RsSn) or Suzuki reaction (boronic acid). 

Thiele reaction Boron trifluoride etherate. Perchloric acid (see 2-Hydroxy-l,4-naphthoquinone). 

Chiral boron catalysts had already been widely used in a variety of reactions before they were applied in Diels-Alder reactions . Boron catalysts were first employed in the Diels-Alder reactions of quinones with electron-rich dienes. Kelly and coworkers found that stoichiometric amounts of a catalyst prepared from BH3, acetic acid and 3,3 -diphenyl-l.r-bi-2-naphthol (344) catalyzed the reaction of l-acetoxy-l,3-butadiene (341) with juglone (342) to afford cycloadduct 343 with 98% ee (equation 96). The reaction was supposed to proceed via a spirocyclic borate complex in which one face of the double bond of juglone was effectively shielded from attack by the diene. 

Later in the book, when we deal with asymmetric enolate reactions, boron enolates will be very important. A simple example20 of an aldol reaction with a boron enolate, prepared from the ester 149 and a boron triflate using an amine as base, shows why. The boron enolate 150 could be prepared with a weak base and reacts with the aldehyde without catalysis to give essentially one diastereoisomer of the aldol 151 in good yield. If the titanium enolate (prepared with TiCI4 and an amine) was used, both the yield and the stereoselectivity were worse. In other circumstances enolates of titanium and other metals are very successful. 

In addition to the very important palladium-catalysed reactions, boronic acids undergo a number of useful reactions that do not require transition-metal catalysis, particularly those involving electrophilic ipso-substitutions by carbon electrophiles. The Petasis reaction involves ip,y(9-replacement of boron under Mannich-like conditions and is successful with electron-rich heterocyclic boronic acids. A variety of quinolines and isoquinolines, activated by ethyl pyrocarbonate, have been used as the Mannich reagent . A Petasis reaction on indole 3-boronic acids under standard conditions was an efficient route to very high de a-indolylglycines. " ... 

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