Boronic acids, carbonylation from borates

The addition of allylic boron reagents to carbonyl compounds first leads to homoallylic alcohol derivatives 36 or 37 that contain a covalent B-O bond (Eqs. 46 and 47). These adducts must be cleaved at the end of the reaction to isolate the free alcohol product from the reaction mixture. To cleave the covalent B-0 bond in these intermediates, a hydrolytic or oxidative work-up is required. For additions of allylic boranes, an oxidative work-up of the borinic ester intermediate 36 (R = alkyl) with basic hydrogen peroxide is preferred. For additions of allylic boronate derivatives, a simpler hydrolysis (acidic or basic) or triethanolamine exchange is generally performed as a means to cleave the borate intermediate 37 (Y = O-alkyl). The facility with which the borate ester is hydrolyzed depends primarily on the size of the substituents, but this operation is usually straightforward. For sensitive carbonyl substrates, the choice of allylic derivative, borane or boronate, may thus be dictated by the particular work-up conditions required. 

In this experiment, you will examine the reduction of 9-fluorenone (10) using sodium borohydride to give 9-fluorenol (12), as shown in Equation 17.17. This reaction is mechanistically analogous to the reduction of imines with sodium borohydride (Sec. 17.3) and involves the transfer of hydride ion (H ) from borohydride ion, BH4, to the electrophilic carbonyl carbon with concomitant transfer of the electron-deficient boron atom to the carbonyl oxygen. Theoretically, all four of the hydrogen atoms attached to boron may be transferred in this way to produce the intermediate borate salt 11, which is decomposed upon addition of water and acid to yield 9-fluorenol (12). 

Borane is a Lewis acid that is attacked by electron-rich centers. Thus, when aldehydes or ketones are treated with the BHs THF complex, the borate ester (H2B OR) is rapidly formed, which, upon hydrolysis, gives the corresponding alcohol. The reduction of the carbonyl group is believed to take place by addition of the oxygen atom to the electron-deficient boron atom, followed by irreversible transfer of hydride ion from the now anionic boron to the carbon atom of the (former) carbonyl ... 

Dussault and coworkers have shown that in the ozonolysis of alkenes in the presence of the weak boron Lewis acid triphenoxy borate, a directed addition of the phenoxy group from boron onto the coordinated carbonyl oxide takes place to furnish hydroperoxyacetals (161) under aprotic conditions (Equation 96) [98]. 

---