BORANES IN FUNCTIONALIZATION

BORANES IN FUNCTIONALIZATION OF DIENES TO CYCLIC KETONES BICYCLO[3.3.1]NONAN-9-ONE... 

Heteroatoni groups such as boron or silicon can activate or direct synthetic reactions. Use of such activation has become of major importance in organic syntheses. Examples in this volume are BORANES IN FUNCTIONALIZATION OF DIENES TO CYCLIC KETONES BICYCLO[3.3.1]NONAN-9-ONE and BORANES IN FUNCTIONALIZATION OF OLEFINS TO AMINES 3-PINANAMINE. Use of trimethylsilyl or trimethyl-silyloxy groups to activate a 2-butenone or a butadiene are illustrated by the preparations 3-TRIMETHYLSILYL-3-BUTEN-... 

Quenching of the same lithiated species with CO2, followed by reduction of the carboxyUc acid functionality obtained with BH3-THF complex, yielded the next higher analogues 78 to these alcohols [94]. Subsequent treatment of the depro-tonated alcohols with TsCl or MsCl afforded (l )-l-boranato[alkyl(methyl)plios-phino] ethanol-2-tosylates or the mesylate phosphine-boranes in over 90% ee and excellent overall yields. 

Similarly to the P-CHj group, secondary phosphine-boranes react smoothly in the presence of a base (BuLi, NaH) under mild conditions to afford other kinds of functionalized phosphine-boranes in good to high yields, without racemi-zation. Yet the success of deprotonation/treatment with an electrophile process to afford substituted phosphine derivatives without any loss in optical purity may depend on the deprotonation agents employed. Use of butyllithium usually provides the products with high enantiomeric excess in good to high yields [73]. 

Haloboranes will cleave MOM ethers in functionally dense substrates. Trichloro-borane without the nucleophilic assistance of thiol or thioether additives has been used457 and trifluoroborane etherate, in the presence of thiophenol, was used by Corey and co-workers in a synthesis of Ginkolide A45K and by Vedejs and Larsen to cleave a tertiary MOM ether in a synthesis of Fulvine and Cris-patine [Scheme 4.24b]459... 

Alternatively, borane in tetrahydrofuran (BH3/THI ) is a useful reagent for reducing carboxylic acids to primary iilcohols. Reaction of an acid with BH / l HF occurs rapidly at room temperature, and the proceaurc is often preferred to reduction with I iAlH4 because of its relative ease and safety. Borane reacts with carboxylic acids faster than with any other functional group, thereby allowing selective transformations such as tliat shown below on / -nitrophenylacetic acid. If the reduction of p-nitrophcnylacetic acid were done with LiAll-14, both nitro and carboxyl groups would be reduced. 

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],... 

It is now apparent that there are as many diverse structural types in this area as there are in the more established field of organo-transition metal chemistry. There are now many known examples where the borane or hetroatom borane moiety functions as a mono-, di-, tri-, tetra- or penta-hapto ligand. This paper is limited to borane and heteroatom borane groups which function as monohapto ligands. 

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