Boranes, prim

Reductions with pyridine borane Prim, alcohols from aldehydes... 

Ammoniak und prim. Amine spalten Trialkyl-borane nur bei hohen Temp, zu Koh-lenwasserstoffen und Amino-boranen1. Zur Protonolyse von Trialkyl-boranen ist Platin(II)-acetat ebenfalls geeignet2. 

Da bei der Reduktion von Oximen und Oxim-Derivaten mit D i bor an und Aluminium-hydrid reduktive Umlagerungen, besonders im ersteren Fall, praktisch nicht ablaufen, eig-nen sie sich besonders zur Herstellung prim. Amine. Die Reduktion von Oximen mit Di-boran fiihrt selbst in Bis-[2-methoxy-athyl]-ather bei 105-110° nicht immer zur Amin-Stufe7 (s. S. 374). 

Diphenylmethyl bromide allowed to react 5 hrs. at room temp, with triethylamine-borane complex in nitromethane -> diphenylmethane. Y ca. 100%. - The yields depend on the stability of the carbonium ions derived from the halides. Prim, halides can not be reduced by this method. F. e., also in liq. SO2, s. S. Matsumura and N. Tokura, Tetrah. Let. 1968, 4703 with diborane in nitromethane s. Tetrah. Let. 1969, 363. 

Borane-tetrahydrofuranitrimethylsilyl azide Prim, amines from ethylene derivs. under neutral conditions... 

Hydroboration of alkenes was first developed by Brown [26, 27], a Nobel laureate for his contribution to the organic chemistry of boron. The process is based on a multi-step reaction. This consisted of syn-addition of borane to the C=C bond to form IV, followed by peroxide attack and Bayer-Williger-type rearrangement of the intermediate boron-peroxide V-VI, and finally hydrolytic splitting of the boronate ester VII to the prim-alcohol VIII, as outlined in Scheme 6.9. 

The driving force for this process is found in the preferred formation of the prim-alkyl borane IV, due to the preferred addition of the electropositive boron atom to the terminal, more electronegative C-atom of the C=C bond. An opposite charge distribution occurs during addition of H-OH to the double bond, favouring interaction of the heteroatom with the internal C-atom. Relative energies of all species along the reaction coordinate of hydroboration are presented in Fig. 6.2 [28]. 

The calculated reaction coordinate reveals higher stability and lower energy of the transition state alkyl borane on the prim-C-atom than on the scc-C-atom, although the energy content of the product prim-alcohol is higher than that of the sec-alcohol. The whole process is a typical example of a kinetically controlled reaction. 

Prim.-alkyl boranes couple with aryl iodides 319 in the presence of CO, unsyninetric ketones result.320 vinyl boranes react with a wide range of organic halides to give substituted olefins321 or 1,3-dienes of high isomeric purity 322 a fungal pro-hormone has been synthesised in this way.323... 

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