Boroxines boranes

Modified MAOs (MMAO) that incorporate Bu and other alkyl groups to improve stability and optimize catalytic activity are commercially available. The use of additional modifiers such as boranes, boroxines, and pentafiuorophenyl derivatives has been investigated in recent years. MAO has also been supported on silica and other materials for the preparation of heterogeneous aUcene polymerization catalysts. These developments are detailed in several reviews. " ... 

Two different approaches can be followed to prepare and use the catalyst. The first is to prepare it in situ by mixing (R)- or (S)-diphenylprolinol (DPP) (4) and a borane complex (Scheme 16.2). This route is advantageous because there is no need to use boronic acids (or boroxines) and to remove water to form the catalyst. Another possible way is to use preformed catalysts, some of which are commercially available from suppliers such as Callery. 

Researchers at Sepracor later disclosed the use of a new class of chiral oxazaborolidines derived from r/. v-aminoindanol in the enantioselective borane reduction of a-haloketones.6,7 The 5-hydrogen oxazaborolidine ligand 10 was prepared in situ from d,v-aminoindanol 1 and BH3 THF.8 Stock solutions of 5-methyl oxazaborolidine 11-16 were obtained by reaction of the corresponding N-alkyl aminoindanol with trimethyl boroxine.6,7 5-Methyl catalyst 11 was found to be more selective (94% ee at 0°C) than the 5-hydrogen catalyst 10 (89% ee at 0°C), and enantioselectivities with 11 increased at lower temperatures (96% ee at -20°C). The catalyst structure was modified by introduction of A-a I kyI substituents. As a general trend, reactivities and selectivities decreased as the steric bulk or the chelating ability of the A -alkyl substituent increased (Scheme 17.4). 

Ebelman and Bouquet prepared the first examples of boric acid esters in 1846 from boron trichloride and alcohols. Literature reviews of this subject are available. B The general class of boric acid esters includes the more common orthoboric acid based trialkoxy- and triaryloxyboranes, B(0R)3 (1), and also the cyclic boroxins, (ROBO)3, which are based on metaboric acid (2). The boranes can be simple trialkoxyboranes, cyclic diol derivatives, or more complex trigonal and tetrahedral derivatives of polyhydric alcohols. Nomenclature is confusing in boric acid ester chemistry. Many trialkoxy- and triaryloxyboranes such as methyl, ethyl, and phenyl are commonly referred to simply as methyl, ethyl, and phenyl borates. The lUPAC boron nomenclature committee has recommended the use of trialkoxy- and triaryloxyboranes for these compounds, but they are referred to in the literature as boric acid esters, trialkoxy and triaryloxy borates, trialkyl and triaryl borates or orthoborates, and boron alkoxides and aryloxides. The lUPAC nomenclature will be used in this review except for relatively common compounds such as methyl borate. Boroxins are also referred to as metaborates and more commonly as boroxines. Boroxin is preferred by the lUPAC nomenclature committee and will be used in this review. 

Since the boroxines are readily available from B203 and trialkyl boranes or trialkoxy boranes, they are in effect reactions of boron oxide with trialkyl alanes. Gallium tribromide and triethylalane give triethylgallane in over 80% yield, but only the first alkyl group on aluminum is involved in alkylation (63, 65) ... 

Trifluorophosphine-borane has also been prepared in the gas phase reaction between boroxine and trifluorophosphine, but the method is somewhat hazardous and probably best carried out with small amounts of material 16). Kinetic studies indicate a first-order dependence on the... 

A general method of obtaining tertiary amine or pyridine complexes of organo-substituted boranes has been developed 172) which avoids the previous necessity of preparing intermediate alkylated diboranes in a separate step 61,66). This relatively new route involves reduction of an appropriately substituted boronate or borinate (or boroxine) with lithium aluminum hydride in the presence of the desired amine 66, 74, 129,173, 174). 

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