Organoboranes for synthesis - substitution with retention
The facile reaction of olefins, dienes and acetylenes with various hydroborating agents made a vast array of organoboranes readily available. Organoboranes tolerate many functional groups and are often formed in a stereosnecific manner. The boron atom in these organoboranes can be readily substituted with a variety of functional qroups to give organic compounds under mild conditions such that organoboranes now appear to be among the most versatile internediates available for organic synthesis. Exploration of these substitution reactions revealed that the organoboranes transfer the alkyl group to essentially most of the other elements of synthetic and biological interest, includinci carbon, with complete maintenance of stereochemical integrity. Consequently, the organic groups that are formed stereospecifically by the hydrohoration reaction can be readily incorporated into organic molecules. A recent development preparation of optically pure orqanohorane intermediates makes possible the synthesis of essentially any comnound containina a chiral center in both optical isomers in essentially 100% optical purity. Thus, our research program has taken boranes from an exotic material of little interest to a reagent widely used in organic synthesis, greatly assisting chemists in overcoming synthetic difficulties. INTRODUCTION The remarkably facile addition of diborane in ether solvents to alkenes and alkynes was discovered in 1956 (ref. 1). For the next decade a major portion of the research effort of my students and associates was devoted to systematic studies of the scope and characteristics of hydroboration reaction. These established that hydroboration is essentially quantitative, involves a cia anti—tlarkovnikov addition from the less hindered side of the double bond and can tolerate essentially all functional groups (ref. 2). When we were exploring the hydroboration reaction, many individuals expressed skepticism to me as to the wisdom of devoting so much research effort to this reaction. After all, hydroboration produced organoboranes. At the time we started, only three things were really known about organohoranes: (1) they were oxidized by air; (2) they were stable to later; (3) they formed addition compounds with bases. Besides, relatively little new chemistry of organoboranes had appeared since the original classic publication by Frankland in 1862 (ref. 3). iany individuals took the position that the lack of published material in this area meant that there was little of value there. In this case, it is now clear that this position is not correct. After our exploration of the hydrohoration reaction had proceeded to the place where we felt we understood the reaction and could apply it with confidence to new situations, we began a systematic exploration of the chemistry of organoboranes, with emphasis on reactions of synthetic utility. A systematic study of the reactions of organohoranes revealed their exceptional versatility (ref. 2). Our studies have established that these oroanoboranes transfer the alkyl aroun to essentially most of the other elements of synthetic and hioloqical interest with complete maintenance of stereochemical integrity. Typical transformations are indicated in Fig. 1. It is not possible here to give more than a taste of the rich chemistry. Consequently, this article is restricted to some of the substitution reactions of oroanohoranes that proceed with retention of configuration.