Aluminium tert-butoxide

Aluminium tert.-butoxide. In a 500 ml. round-bottomed flask... 

Aluminium amalgam, 198, 514 Aluminium tert.-butoxide, 887 Aluminium tso-propoxide, 883 reductions with, 882, 883-886 Aluminon, 959, 960... 

A Lewis acid e.g. aluminium tert-butoxide, boron trifluoride, neutral alumina) is considered to coordinate with both the 17-OH and 20-keto functions, bringing them into a cfs-relationship and effectively locking the side chain in one conformation. Product formation is then determined by the relative ease of migration of the Cps)--C(i ) and C(i6>-C(i7) bonds towards the electron-deficient C<20). The structures of the resulting ketones show that the C i3)-C(i ) bond migrates in the i7j -hydroxy compound (i), and the C(i6)-"C(i7) bond in the i7a-hydroxy isomer (2). The reason for this difference has been the subject of much speculation, and is still not clear. The factors which have been considered [202] as affecting the stability of respective transition states include ... 

The ketones most commonly used are acetone and butanone. Aluminium tert-butoxide is usually employed as the base catalyst (Eq. 2.346). 

The Meerwein-Ponndorf-Verley reduction is a reversible reaction and the reverse pathway was reported in detail a couple of years later by Oppenauer who selectively oxidised hydroxyl functions into ketones or aldehydes with the help of acetone or cyclohexanone as proton quencher (oxidant) and aluminium tert-butoxide as catalyst (other common oxidants are acetaldehyde, anisaldehyde, benzaldehyde, benzophenone and cinnamalde-hyde). This mild oxidation method was in many cases used in the synthesis of steroids and terpenoids. 

The Oppenauer oxidation with aluminium alkoxides provides an alternative method for the oxidation of secondary (and less commonly primary) alcohols. The reaction is the reverse of the Meerwein-Pondorff-Verley reduction (see Section 7.3). Typically aluminium triisopropoxide (or aluminium tri-tert-butoxide) is used, which serves to form the aluminium alkoxide of the alcohol. This is then oxidized through a cyclic transition state at the expense of acetone (or cyclohexanone or other carbonyl compound). By use of excess acetone, the equilibrium is forced to the right (6.45). 

Rathke and coworkers found that combining this bulky alkoxide again with trifluoroacetic acid leads to a highly efficient catalytic system for the Oppenauer oxidation. For instance, cyclohexanol could be oxidised, at 0 °C within a couple of minutes, to give cyclohexanone in 88% yield, in the presence of 5 mol% of aluminium tri-tert-butoxide activated with 2.5 mol% of trifluoroacetic acid as shown in Scheme 18.5. However, the control provided by this catalytic system in the MPV/Oppenauer reactions leaves some room for improvement and the aluminium-catalysed formation of side products via concurrent aldol reactions (Scheme 18.2) could be observed when enolisable substrates were used. ... 

In this context it is worth remembering that despite the many advantages offered by the standard Oppenauer oxidation such as the utilisation of inexpensive and nontoxic reagents like aluminium tri-tert-butoxide and aluminium tri-isopropoxide and a high functional group compatibility, this reaction has still not reached the level of the MPV reduction, as far as the... 

TEjTIP and TB - titanium ethoxide, isopropoxide and n-butoxide, respectively AIP and ATB -aluminium isoptDpoxide and tert-butoxide, respectively. 

---