Diols, geminal

Step 3 Proton transfer from the conjugate acid of the geminal diol to a water molecule... 

The December 2000 issue of the Journal of Chemical Education (pp 1644-1648) contains an article entitled Carbinolamines and Geminal Diols in Aqueous Environmental Organic Chemistry... 

Geminal diol (Section 17 6) The hydrate R2C(OH)2 of an aldehyde or a ketone... 

The pinacol rearrangement is frequently observed when geminal diols react with acid. The stmcture of the products from unsymmetrical diols can be predicted on the basis of ease of carbocation formation. For example, l,l-diphenyl-2-metltyl-l,2-propanediol rearranges to... 

Effects of Structure on Equilibrium Aldehydes and ketones react with water in a rapid equilibrium. The product is a geminal diol. 

Aldehyde or ketone Water Geminal diol (hydrate) ... 

Mechanism of Base-Catalyzed Hydration The base-catalyzed mechanism (Figure 17.5) is a two-step process in which the first step is rate-detennining. In step 1, the nucleophilic hydroxide ion attacks the carbonyl group, forming a bond to carbon. An alkoxide ion is the product of step 1. This alkoxide ion abstracts a proton from water in step 2, yielding the geminal diol. The second step, like all other proton transfers between oxygen that we have seen, is fast. 

Aldehydes and ketones react with water to yield 1,1-diols, or geminal (gem) diols. The hydration reaction is reversible, and a gem diol can eliminate water to regenerate an aldehyde or ketone. 

The bishydroxylation of peripheral C —C double bonds of porphyrins, e.g. 6, with hydrogen peroxide under acidic conditions or with osmium(VlII) oxide yields the corresponding diols, e.g. 10, which on pinacol rearrangement are transformed into geminally dialkylated chlorins, e.g. 11.9,97... 

However, there was evidence for an oxidative mechanism in the opening of the 4,5-epoxy group, such that the first step was cytochrome P450 catalyzed hydroxylation of C(4) to produce an unstable geminal diol epoxide. Such a product would immediately rearrange to a 5-hydroxy-4-oxo derivative, which would, in turn, be stereoselectively reduced by an oxidore-ductase to the trans-4,5-diol. 

Diastereomeric 1,3-diols 5 could be easily identified by the equivalence and nonequivalence of the geminal methyl carbon resonances in the meso- and <7,/-form, respectively415. 

The catalytic mechanism proposed by Veerapandian et al. [31] based on the x-ray structure of a difluoroketone (geminal-diol) inhibitor bound to endothiapepsin. A water molecule tightly bound to the aspartates in the native enzyme is proposed to nucleophilically attack the scissile-bond carbonyl. The resulting geminal-diol intermediate is stabilised by hydrogen bonds with the negatively charged carboxyl of aspartate 32. 

The Validity of the J Function. For aqueous sodium hydroxide solutions the validity of the J- function, describing the basicity of a solution by its ability to add hydroxide ions to a carbonyl group to form an anion of the geminal diol, has been proved earlier (19). For all substituted benzaldehydes studied in water-ethanol or water-dimethylsulfoxide (DMSO), the value of log (CArCH(OH)o-/CArCHo) determined from absorbance measurements was found to be a linear function of the J- function with a slope varying between 0.95 and 1.05 in the region of J values where measurements were possible. The Can-... 

The relatively modest increase in the value of the J- function upon increasing the ethanol content of a sodium hydroxide solution from 90 to 98 vol % indicates that either the nucleophilic reactivity of the ethoxide ion under these conditions does not differ substantially from that of the hydroxide ion (while solvation of the hydroxide ion and the geminal diol anion is similar to solvation of the ethoxide ion and the hemiacetal anion) or that compensation of effects takes place. 

If it is assumed, in a first approximation, that the activity coefficient ratios /ha/ /a- and /ArCHo//ArCH(OH)o do not differ very much, the difference between H and J for a given solution would be expressed by H- — J = —log flH2o-However a calculation of the difference by the latter formula, using the only available information (33) about the activity of water in water-DMSO mixtures, leads to values much smaller than those found in practice. This would indicate that the ratio /ha//a-> for the H indicator acids, is considerably larger than the ratio of /ArCHo//ArCH(OH)o- for the benzaldehydes used as J indicators. Probably this difference in activity coefficient ratios is caused by a larger extent of charge delocalization in the A anion compared to the geminal diol anion, which causes an extra stabilization of A- (manifested by a decrease in f ) by dispersion interaction with DMSO. 

Alcohol dehydrogenases also oxidize aldehydes, probably most often as the geminal diol forms, according to Eq. 15-8. No ATP is formed. The same enzymes can catalyze the dismutation of aldehydes, with equal numbers of aldehyde molecules going to carboxylic acid and to the alcohol.96-98... 

The isomerization reactions. At least 10 reactions of the type described by Eq. 16-34 are known397 (Table 16-1). They can be subdivided into three groups. First, X = OH or NH2 in Eq. 16-34 isomerization gives a geminal-diol or aminoalcohol that can eliminate H20 or NH3 to give an aldehyde. All of these enzymes, which are called hydro-lyases or ammonia-lyases, specifically require K+ as well as the vitamin B12 coenzyme. 

Di-2-pyridyl ketone complexes to Zn2+ in an N,N-chelate fashion, while the keto group is present in the form of the geminal diol. In basic solution, this latter group is ionized, leading to A, O-coordination by the ligand.529... 

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