Organoborane reactions

Thus, there is at least evidence that B—C 7r-bonding can influence both the pathway and the rapidity with which organoborane reactions (Eq. 9) take place. 

It is now clear that pure pheromones can be synthesized in quantity. The problem is how to prepare them simply and efficiently. New synthetic methodologies are always welcome to improve the existing syntheses. Organoborane reactions and organotransition metal chemistry contributed much to improve the efficiency of carbon-carbon bond formation, while asymmetric epoxidations and dihydroxylations as well as enzymatic reactions greatly improved the enantiomeric purity of synthetic pheromones. 

With unsymmetrical alkenes, the least substituted alcohol is obtained (Following fig.) and so the organoborane reaction is complementary to the electrophilic addition reaction with aqueous acid. Steric factors appear to play a role in controlling this preference with the boron atom preferring to approach the least sterically hindered site. Electronic factors also play a role as described in the mechanism below ... 

Carbon-boron bonds are generally rather easily oxidized and indeed volatile trialkylboranes such as trimethylborane and triethylborane are spontaneously inflammable when exposed to air. Less volatile or-ganylboranes do not spontaneously inflame but are nevertheless readily oxidized by oxygen and a variety of other reagents. Consequently, it is normally necessary to carry out organoborane reactions in an inert atmosphere. 

The reactions of organoboranes are in general not discussed here, but are distributed throughout Comprehensive Organic Synthesis wherever appropriate reaction types are under consideration. However, protonolysis of organoboranes is treated in Section 3.10.7 because of its necessary role in the reduction of carbon-carbon multiple bonds via hydroboration (Scheme 1). Reviews of other synthetically useful organoborane reactions appear elsewhere. - ... 

Borabicyclo[3.3.1.]nonane (9-BBN) is available by hydroboration of 1,5-cyclooctadiene with one equivalent of BH3," or commercially either as a crystalline dimer or as a THF solution. The reagent is thermally more stable than dicyclohexylborane. It is frequently used as an anchor group in organoborane reactions, allowing an efficient utilization of valuable alkenes. 

A boron-tethered (C-B-O) intramolecular Diels-Alder (IMDA) approach has been used to prepare cyclic alkenyl boronic esters 140 (Scheme 19). Thus, reaction of 2equiv of the dienyl alcohol 138 with 137 in THF, in the presence of molecular sieves, provides the corresponding IMDA precursors 139. The IMDS reaction was then accomplished at 190 °C in a toluene solution, with 5 mol% of 2,6-di-fer7-butyl-4-methylphenol as a free radical inhibitor. Transformation of the carbon-boron bond in 140, using standard organoborane reactions, can then afford a variety of functionalized cyclohexene derivatives <1999JA450>. 

Oxidations of organoboranes involve numerous reagents and several different general ntechanisms, most of which parallel those which occur for other types of organoborane reactions. The mechanisms fall under three broad headings (i) ionic, with a 1,2-shift from boron to a heteroatom (equations 1-3) (ii) radical and (iii) electrocyclic. 

The review problems below present a few additional examples of organoborane reactions. You ll find many more in standard advanced organic texts (see Appendix B). The common feature of all the reactions is that a nucleophile adds to the tricoordinate boron of the organoborane to yield an anionic tetracoordinate boron center, from which the migrating alkyl group takes off. In the chapters to follow, we ll encounter a variety of other platforms, based on other relatively electropositive p-block elements, from which alkyl and aryl groups may migrate. 

Stereoselective addition of B-Br across a terminal acetylene (haloboration) was first reported by Blackborow (66). Suzuki and coworkers examined the intermediate obtained from the haloboration of alkynes with B-bromo and 5-iodo-9-borabicyclo[3.3.1]nonanes for various organoborane reactions. Suzuki has reviewed the applications of haloboration for organic syntheses (67-69). A recent application involving the 1,4-addition of a halovinyl-9-BBN to methyl vinyl ketone (70) for the synthesis of a promising anti-cancer agent, 12,13-desoxyepothilone B due to Danishefsky and coworkers is shown in Figure 19 (77). 

Kabalka, G. W., Delgado, M. C., Sastry, U., Sastry, K. A. R. 1982. Synthesis of carbon-13 labeled carboxylic acids via organoborane reactions. J. Chem. Soc. Chem. Commun. 21 1273-1274. 

Kabalka, G. W., Reed, T.J., Kunda, S. A. 1983. Synthesis of oxygen-17-labeled alcohols via organoborane reactions. Syn. Commun. 13 737-740. 

Hydroboration and B—C Bond Formation Reactions of Organoboranes Reactions of Organoborates Reducing Agents Aluminium and Thallium... 

---