Electrophilic addition hydration

We can extend the general principles of electrophilic addition to acid catalyzed hydration In the first step of the mechanism shown m Figure 6 9 proton transfer to 2 methylpropene forms tert butyl cation This is followed m step 2 by reaction of the car bocation with a molecule of water acting as a nucleophile The aUcyloxomum ion formed m this step is simply the conjugate acid of tert butyl alcohol Deprotonation of the alkyl oxonium ion m step 3 yields the alcohol and regenerates the acid catalyst... 

Hydroboration-oxidation (Sections 6.11-6.13) This two-step sequence achieves hydration of alkenes in a stereospecific syn manner, with a regiose-lectivity opposite to Markovnikov s rule. An organoborane is formed by electrophilic addition of diborane to an alkene. Oxidation of the organoborane intermediate with hydrogen peroxide completes the process. Rearrangements do not occur. 

Acid-catalyzed hydration of isolated double bonds is also uncommon in biological pathways. More frequently, biological hydrations require that the double bond be adjacent to a carbonyl group for reaction to proceed. Fumarate, for instance, is hydrated to give malate as one step in the citric acid cycle of food metabolism. Note that the requirement for an adjacent carbonyl group in the addition of water is the same as that we saw in Section 7.1 for the elimination of water. We ll see the reason for the requirement in Section 19.13, but might note for now that the reaction is not an electrophilic addition but instead occurs... 

In the laboratory, alkenes are often hydrated by the oxymercuration procedure. When an alkene is treated with mercury(II) acetate Hg(02CCH3)2, usually abbreviated Hg(OAc)2l in aqueous tetrahydrofuran (THF) solvent, electrophilic addition of Hg2+ to the double bond rapidly occurs. The intermediate orgnnomercury compound is then treated with sodium borohydride, NaBH4, and an alcohol is produced. For example ... 

Hydration of an alkene—the addition of water—is carried out by either of two procedures, depending on the product desired. Oxymercuration involves electrophilic addition of Hg2+ to an alkene, followed by trapping of the cation intermediate with water and subsequent treatment with NaBH4. Hydroboration involves addition of borane (BH3) followed by oxidation of the intermediate organoborane with alkaline H202- The two hydration methods are complementary oxymercuration gives the product of Markovnikov addition, whereas hydroboration/oxidation gives the product with non-Markovnikov syn stereochemistry. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Once again, a large amount of diverse evidence indicates the intermediacy of a vinyl cation in electrophilic additions to arylacetylenes. As in the case of the hydration of alkynyl ethers and thioethers, the vinyl cation formed is especially stable because of resonance interaction and charge delocalization with the adjacent rr center of the aromatic system. 

Acids that have weakly nucleophilic anions, e.g. HS04e from dilute aqueous H2S04, are chosen as catalysts, so that their anions will offer little competition to H20 any R0S03H formed will in any case be hydrolysed to ROH under the conditions of the reaction. Rearrangement of the carbocationic intermediate may take place, and electrophilic addition of it to as yet unprotonated alkene is also known (p. 185). The reaction is used on the large scale to convert cracked petroleum alkene fractions to alcohols by vapour phase hydration with steam over heterogeneous acid catalysts. Also under acid catalysis, ROH may be added to alkenes to yield ethers, and RCOzH to yield esters. 

It is concluded that the selectivities of electrophilic additions are not directly related to the reactivities but to the transition-state positions. Extensive comparison with similar data on the bromination and hydration of other ethylenic compounds bearing a conjugated group shows that this unexpected reactivity-selectivity behaviour can arise from an imbalance between polar and resonance effects (Ruasse, 1985). Increasing resonance in the ground state would make the transition state earlier and attenuate the kinetic selectivity more strongly than it enhances the reactivity. Hydration and halogenation probably respond differently to this imbalance. 

Fig. 11.11. Mechanism postulated for hydrolysis of organic isocyanates involving electrophilic addition of H20 with general base catalysis. The product of hydration is a carbamic acid that spontaneously decomposes to the primary amine with loss of C02 [117].

Predicting the outcome of electrophilic additions to alkynes from an extension of alkene reactivity usually works well, and can be applied to halogenations and hydrations. Hydration of an alkyne has a subtle twist, however the product is a ketone This can still be rationalized quite readily, though. 

A general type of chemical reaction between two compounds, A and B, such that there is a net reduction in bond multiplicity (e.g., addition of a compound across a carbon-carbon double bond such that the product has lost this 77-bond). An example is the hydration of a double bond, such as that observed in the conversion of fumarate to malate by fumarase. Addition reactions can also occur with strained ring structures that, in some respects, resemble double bonds (e.g., cyclopropyl derivatives or certain epoxides). A special case of a hydro-alkenyl addition is the conversion of 2,3-oxidosqualene to dammara-dienol or in the conversion of squalene to lanosterol. Reactions in which new moieties are linked to adjacent atoms (as is the case in the hydration of fumarate) are often referred to as 1,2-addition reactions. If the atoms that contain newly linked moieties are not adjacent (as is often the case with conjugated reactants), then the reaction is often referred to as a l,n-addition reaction in which n is the numbered atom distant from 1 (e.g., 1,4-addition reaction). In general, addition reactions can take place via electrophilic addition, nucleophilic addition, free-radical addition, or via simultaneous or pericycUc addition. 

This is ordinary electrophilic addition, with rate-determining protonation as the first step.164 Certain other alkynes have also been hydrated to ketones with strong acids in the absence of mercuric salts.165 Simple alkynes can also be converted to ketones by heating with formic acid, without a catalyst.166... 

This chapter represents an update to the previous two editions, published in 19771 and 19892, and covers the literature of the period 1989-1994 with some references to 1995 papers. It deals mainly with electrophilic additions across the C=C, C=Si and Si=Si bonds and includes both theoretical (ab initio calculations, orbital approach, molecular modelling etc.) and experimental aspects. Particular attention is paid to mechanistic studies, facial selectivity and neighbouring group participation. Synthetic utilization of electrophilic addition is discussed only if including substantial mechanistic insight purely synthetic work is not covered. Aside from the classical reactions, such as hydration, bromination etc., newly included material comprises aziridination (Section VI), attack at C=C bond by an electron-deficient carbon (Section VII) and those electrophilic reactions which utilize a transition or non-transition metal as the electrophile (Section VIII). 

II. PROTON AS AN ELECTROPHILE A. Hydration, Addition of ROH and Related Reactions... 

Yukawa-Tsuno correlations for electrophilic addition of alkenes 322 Hydration of arylalkenes 322 Bromination of arylalkenes 326... 

Electrophilic addition reactions are some of the most important processes (21) involving a cationic transition state or intermediate. The acid-catalysed hydration of arylalkenes such as styrene [32] is typical of such processes... 

This equation describes satisfactorily the rates of 30 alkenes covering a reactivity range of 8 log units. Whereas (22) does not imply any interaction terms, i.e. the p-values, p and pp, are constant throughout the whole reactivity range, a more detailed analysis shows slight variations in p values for the hydration (Table 10). The different behaviour of these two electrophilic additions, both of which go through carbocations has been interpreted in terms of differences in transition state positions. 

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