Borane generation

Borane generated from sodium borohydride and boron trifluoride etherate in diglyme reduced 7V-arylsubstituted succinimides to JV-arylpyrrolidones in 42-72% yields [7726]. 

The reaction has broad applications and a large number of secondary and especially tertiary amines was prepared in isolated yields ranging from 60% to 84% [1136]. Although the mechanism of this reaction is not clear it is likely that the key step is reduction of the acid by borane, generated in situ from sodium borohydride and the acid, to an aldehyde which reacts with the amine as described in the section on reductive amination (p. 134-136). 

Step E Reduction via Borane Generated in situ Procedure... 

The diagonal lines connecting clusters of equal nuclearity reveal another important relationship - as the number of cluster electron pairs increases in a step-wise fashion the cluster opens up into a net and then a web. As was pointed out in Chapter 1, this is a recurring motif - the more electron rich, the more open the structure. One can continue. Addition of three pairs of electrons to a c/oso-borane generates an even more open shape albeit a less well-accepted nomenclature. The... 

The introduction of the aminoethyl side chain is accomplished by Suzuki cross-coupling of dienyl triflate 10 with the alkyl borane generated by hydroboration of A -vinyl- er butyl carbamate (11) with 9-BBN. ... 

Asymmetric hydroboration of prochiral olefins can be performed either with a chiral borane in the absence of a catalyst or with catecholborane and a chiral rhodium catalyst [549]. The rich chemistry of the boranes generated by these methods opens the way to a variety of enantioenriched molecules bearing different functionalities [583],... 

Antino alcohols. a-Amino acids can be reduced by this system, which is actually a borane generator. 

Sugasawa has explored the use of vinylaminodichloro- and vinyloxyamino-chloro-boranes, generated from imines and ketones respectively, in the aldol reaction. The yields in the reactions were moderate to high (Scheme 75). In an... 

The discovery of boron deltahedra in elemental boron and metal borides and later in polyhedral boranes generated an interest in computational studies on these structures as soon as suitable computational methods became available. The earliest computational work on boron deltahedra was the 1954 study by Longuet-Higgins and Roberts on the Be octahedra found in metal boride studies using the secular determinants obtained from linear combinations of atomic orbitals (LCAO). This work was followed shortly by a study of boron icosahedra which predicted the existence of a stable anionic icosahedral... 

Sodium borobydride reacts with Lewis acids in nonprotic solvents to yield diborane [19287-45-7] 2 6 which can then be used to generate other useful organoboranes such as amine boranes, alkyl boranes, and boron hydride clusters. 

Z)-2-Butenylpotassium is generated from 4.5 mL (50 mmol) of (Z)-2-butene, 2.8 g (25 mmol) or /-BuOK. and 10.8 mL (25 mmol) oT 2.3 M butyllithium in THF for 15 min at —45 JC. This solution is cooled to — 78 C and 30 mmol of a 1 M solution of methoxy(diisopinocampheyl)borane in diethyl elher is added dropwise. The mixture is stirred for 30 min at — 78 °C, then is treated with 4mL (33 mmol) of boron trifluoride-diethyl ctherate complex this removes methoxide from the intermediate ate complex. This solution is immediatelv treated with 35 mmol of an afdchyde. Isolated yields of homoallylic alcohols are 63-79%. 

Several studies of reactions of configurationally unstable a-substituted allylboranes have also been reported19,29. The reactions of dialkyl[( )-l-alkyl-2-butenyl]boranes and aldehydes at — 78 °C provide a mixture of syn- and an/i -diastereomers. reflecting reactions by both the Z-and /f-isomers. When generated and used at — I00°C, however, the ff/m-diastereomer is obtained with >95% diastercoselectivity and >90% selectivity for the /T-olefin isomer by way of a transition state analogous to 429. This result suggests that the allylboron isomerization is slow at —100 JC relative to carbonyl addition. 

Phosphine-borane 63a (75% ee) was obtained by reduction of compound (Sp)-62a using LDBB at -60°C and nucleophilic substitution with iodomethane in 72 % yield. The observed loss of optical purity may be ascribed to stereomutation of the generated tricoordinated phosphorus species. Recrystallization afforded (S)-63a in > 99% ee. On the other hand, severe racemization was observed using the same method with (Rp)-62b. An alternative strategy consisted of deborana-tion of (Rp)-62b using ZSl-methylpyrrolidine, methylation with methyl triflate. 

The dominant factors reversing the conventional ds-hydroboration to the trans-hydroboration are the use of alkyne in excess of catecholborane or pinacolborane and the presence of more than 1 equiv. of EtsN. The P-hydrogen in the ris-product unexpectedly does not derive from the borane reagents because a deuterium label at the terminal carbon selectively migrates to the P-carbon (Scheme 1-5). A vinylidene complex (17) [45] generated by the oxidative addition of the terminal C-H bond to the catalyst is proposed as a key intermediate of the formal trans-hydroboration. 

Catecholborane and pinacolborane are especially useful in hydroborations catalyzed by transition metals.163 Wilkinson s catalyst Rh(PPh3)3Cl is among those used frequently.164 The general mechanism for catalysis is believed to be similar to that for homogeneous hydrogenation and involves oxidative addition of the borane to the metal, generating a metal hydride.165... 

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