Boron addition initiated

Reduction of temperatures of glass melting and pouring on the account of boron additives specially included into an initial solution. This will allow a decrease in the melting temperature up to 850°C and thereby lessen corrosion of refractory, electrodes, water-cooled elements, and sensors of control and measuring instruments. 

RCH=CHCOPh undergo a 1,4-addition of ArB(OEt)2, catalysed by (247b) in the absence of Mg + at 120 °C, affording the products with <98% ee Other reactions involving boron derivatives, which in fact act (partly) as nucleophiles, have been discussed above, in the section on Additions Initiated by Metals and Metal Ions as Electrophiles. 

The second site to be altered was the alkyl substituent on the boron center. Initially, it was proposed that the nature of this group should not have a major impact on the level of enantioselection of the cyclopropanation reaction. It is believed that this group simply adopts the pseudoequatorial position of the envelope conformation of the five-membered ring. Several different dioxaborolane ligands were prepared by the same method as that reported earlier. Four novel dioxaborolane additives were prepared with R = Me, Ph, 2-naphthyl and 2,4,6-trimethylphenyl. The enantioselectivities observed for the cyclopropanation reaction are shown in Table I. In all the cases, the enantioselectivities were in the same range as that obtained with R = Bu, except for the 2,4,6-trimethylphenyl substituent. This information suggests that a sterically encumbered substituent on boron may partially prevent the postulated association between the zinc alkoxide and the boron center. In that case, the non-boron-assisted pathway can eventually become competitive. 

The mechanism of initiation in cationic polymerization using Friedel-Crafts acids appeared to be clarified by the discovery that most Friedel-Crafts acids, particularly haUdes of boron, titanium, and tin, require an additional cation source to initiate polymerization. Evidence has been accumulating, however, that in many systems Friedel-Crafts acids alone are able to initiate cationic polymerization. The polymerization of isobutylene for instance can be initiated, reportedly even in the absence of an added initiator, by AlBr or AlCl (19), TiCl ( )- Three fundamentally different... 

Methane sulfonic acid, trifluoroacetic acid, hydrogen iodide, and other Brmnsted acids can faciUtate 3 -acetoxy displacement (87,173). Displacement yields can also be enhanced by the addition of inorganic salts such as potassium thiocyanate and potassium iodide (174). Because initial displacement of the acetoxy by the added salt does not appear to occur, the role of these added salts is not clear. Under nonaqueous conditions, boron trifluoride complexes of ethers, alcohols, and acids also faciUtate displacement (87,175). 

In reaction with an alkene, initially a three-membered ring Lewis acid/Lewis base-complex 5 is formed, where the carbon-carbon double bond donates r-electron density into the empty p-orbital of the boron center. This step resembles the formation of a bromonium ion in the electrophilic addition of bromine to an alkene ... 

Indole and dimethyl acetylenedicarboxylate yield 2-(indol-3-yl)-2,3-dihydro-l//-l-ben-zazepine (mp 240-242 C) by addition of indole to the initially formed l//-l-benzazepinc,21 whereas 1,3-dimethylindole (10, R = H) fails to react with the diester under a variety of conditions.145 However, in the presence of boron trifluoride-diethyl ether complex at room tem-... 

Boron or yellow phosphorus explode violently on grinding with lead dioxide, while red phosphorus ignites [1]. Mixtures with sulfur ignite on grinding or addition of sulfuric acid [2], An initiating mixture of silicon and lead dioxide (2 1) attains a temperature around 1100°C after ignition by a small flame [3],... 

Addition takes place through the initial formation of a n complex, which changes into a cyclic four-center transition state with the boron atom adding to the less hindered carbon atom. The dashed bonds in the transition state represent bonds... 

A recent total synthesis of tubulysin U and V makes use of a one-pot, three-component reaction to form 2-acyloxymethylthiazoles <06AG(E)7235>. Treatment of isonitrile 25, Boc-protected Z-homovaline aldehyde 26, and thioacetic acid with boron trifluoride etherate gives a 3 1 mixture of two diastereomers 30. The reaction pathway involves transacylation of the initial adduct 27 to give thioamide 28. This amide is in equilibrium with its mercaptoimine tautomer 29, which undergoes intramolecular Michael addition followed by elimination of dimethylamine to afford thiazole 30. The major diastereomer serves as an intermediate in the synthesis of tubulysin U and V. 

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

Upon initial addition of boron trifluoride etherate, a slightly endothermic reaction takes place. 

The unusual amino acid (S)-2-amino-(Z)-3,5-hexadienoic acid (269), which is a component of the toxic y-glutamyl dipeptide isolated from the defensive glands of the Colorado beetle [209], has been synthesized along Scheme 17, after two initial attempts had proved unsuccessful due to the instability of 269 towards various oxidation conditions [210]. Scheme 17 relies on the hydrolysis of an ortho ester to generate the required carboxylic acid. Thus, the L-serine aldehyde equivalent 270 was treated with ( )-l-trimethylsilyl-l-propene-3-boronate to give the addition product 271. Reaction of 271 with KH gave the stereochemically pure (Z)-diene 272. Mild acid treatment of 272 followed by... 

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