Geminal from carbonyl compounds

Geminal Dihydroperoxides. These dihydroperoxides (1, X = OOH, R = H) can be made from many different carbonyl compounds. The stmctural restrictions discussed for hydroxyalkyl hydroperoxides generally do not apply. These peroxides can also be synthesi2ed by perhydrolysis of ketals (143). 

A series of reagents have been developed which are prepared in situ from a geminal dihalide or a dithioacetal [635,730] and a transition metal complex. Titanium-based reagents of this type olefinate a broad range of carbonyl compounds, including carboxylic acid derivatives (Table 3.12), and are a practical alternative to the use of isolated carbene complexes. 

We showed in Figs. 3-2 and 3-3 that the tetrahedral intermediate which is initially formed from the reaction of a nucleophile with a carbonyl compound may further react in a number of different ways. In this section, we will consider some reactions which proceed along the pathway indicated in Fig. 3-3. The hydration of ketones is a reaction analogous to the hydrolysis of an ester, with the first step of the reaction involving nucleophilic attack of water on the carbonyl group. The tetrahedral intermediate is trapped by reaction with a proton to yield the hydrated form of the ketone, the geminal diol (Fig. 3-15). Similar reactions occur with alcohols as nucleophiles to yield, initially, hemiacetals. 

The exact mechanistic pathway is not known. However, it is believed that the T-U olefinatlon proceeds via geminal-dichromium intermediates that are nucleophilic and attack the carbonyl compound. The ( )-alkene is formed from the P-oxychromium species. 

The majority of diaziridine syntheses proceed by intramolecular electrophilic amination in a geminal compound 84/ ° an aminallike °° compound from an amine, a carbonyl compound, and an electrophilic aminating agent like chloramine or hydroxylamine-O-sulfonic acid. The different ways of diaziridine synthesis differ mainly in the timing of the build-up of this geminal intermediate 84, from which the diaziridine 85 is isolated. 

With hydrogen sulfide in the absence of hydrogen, however, aldehydes and ketones give geminal dithiols, the pressures required varying from 35 to 8500 atm according to the nature of the carbonyl compound 392 these dithiols can also be obtained without use of pressure and at room temperature in dimethylformamide as solvent containing a basic catalyst such as morpholine or butylamine.393... 

Trialkylaluminums, 21 By geminal alkylation of carbonyl groups Dichlorodimethyltitanium, 216 From reduction of alkyl halides, alkyl sulfonates and similar compounds From acetates or other esters Triphenylsilane, 334 From alcohols... 

With the exception of a few special cases of mixed halogen compounds, the ease of reduction of carbon-halogen bonds follows the expected order, I>Br>CI>F, and becomes more facile as the number of halogen atoms on a given carbon atom increases. Accessible halfwave potentials range from-2.23 V (see) (C—Cl) to -0.3 V (see) (Br2C—Br)15. The isolated C—F bond cannot be reduced electrochemically below the solvent cut-off limit. However, when the C—F functionality is adjacent to an aromatic or carbonyl moiety, or is geminal, reduction often does occur. 

Yamazaki et al. (221) have postulated that the tryptoquivalines may be biogenetically derived from four amino acids tryptophan, anthranilic acid, valine and alanine. Deoxynortryptoquivalone is thought to be the first compound formed in the biosynthesis of the tryptoquivaline series. Oxidation of the secondary amine in this compound to a hydroxylamine would result in nortryptoquivalone. Oxidative loss of the side-chain would lead to FTE or FTJ. Alternatively, reduction of the side-chain carbonyl group would afford nortryptoquivaline. The geminal dimethyl group at C-15 may result from incorporation of a Cj-unit into deoxynortryptoquivalone or from the direct participation of methylalanine rather than alanine in the initial step of the biosynthesis. 

Except for the rare cases of spontaneous loss of water from geminal diols (a carbonyl forming elimination reaction [Figure 8.4 and Equation 8.37]), and similar situations where one of the -OH groups of the geminal diol is replaced by SH or NH, the simple loss of water from an otherwise stable compound fails. 

---