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The hydrolysis of geminal ethers: a kinetic appraisal of orthoesters and ketals

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ABSTRACT

A novel approach to protecting jet fuel against the effects of water contamination is predicated upon the coupling of the rapid hydrolysis reactions of lipophilic cyclic geminal ethers, with the concomitant production of a hydrophilic acyclic hydroxyester with de-icing properties (Fuel Dehydrating Icing Inhibitors - FDII). To this end, a kinetic appraisal of the hydrolysis reactions of representative geminal ethers was undertaken using a convenient surrogate for the fuel–water interface (D2O/CD3CN 1:4). We present here a library of acyclic and five/six-membered cyclic geminal ethers arranged according to their hydroxonium catalytic coefficients for hydrolysis, providing for the first time a framework for the development of FDII. A combination of 1H NMR, labelling and computational studies was used to assess the effects that may govern the observed relative rates of hydrolyses.

No MeSH data available.


Stereoelectronic contributions to hydrolysis; (a) conformationally constrained 1,3-dioxane orthoester; (b) Newman projection of five-membered ZICMED viewed C(4)→C(5).
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Figure 2: Stereoelectronic contributions to hydrolysis; (a) conformationally constrained 1,3-dioxane orthoester; (b) Newman projection of five-membered ZICMED viewed C(4)→C(5).

Mentions: As k1 ≈ kobs in the pH range examined here (Scheme 1), factors associated with the relief of cyclic strain cannot be used to account for the difference observed for five-membered 5 and six-membered 8 (i.e., kH+ = 6.5 and 9.8 M−1 s−1). Further, the hydroxonium catalytic coefficient for the hydrolysis of 5 and acyclic analogue 6 are within experimental error of each other (i.e., kH+ = 6.5 ± 0.2 and 7.0 ± 0.2 M−1 s−1, respectively). It was noted previously that the relative rates of hydrolysis for six-membered 15 and 8 could be explained with the kinetic anomeric effect. Consistent with this, the X-ray crystal structure of an analogous yet conformationally constrained bicyclic orthoester possesses an unusually elongated axial C─O bond (Fig. 2), which undergoes preferential cleavage with Lewis acids [31]. The Cambridge Structural Database (2015) [32] contains a single example of a five-membered 1,3-dioxolane orthoester [33]. Here, the ring adopts a distorted half-chair (C2) arrangement with a dihedral angle θ [O–C(4)–C(5)–O] = 32° [Fig. 2 – ZICMED viewed C(4)→C(5)]. This, along with a rate of hydrolysis similar to an acyclic system suggests that a kinetic anomeric effect does not extend to 1,3-dioxolane orthoesters.


The hydrolysis of geminal ethers: a kinetic appraisal of orthoesters and ketals
Stereoelectronic contributions to hydrolysis; (a) conformationally constrained 1,3-dioxane orthoester; (b) Newman projection of five-membered ZICMED viewed C(4)→C(5).
© Copyright Policy - Beilstein
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4979634&req=5

Figure 2: Stereoelectronic contributions to hydrolysis; (a) conformationally constrained 1,3-dioxane orthoester; (b) Newman projection of five-membered ZICMED viewed C(4)→C(5).
Mentions: As k1 ≈ kobs in the pH range examined here (Scheme 1), factors associated with the relief of cyclic strain cannot be used to account for the difference observed for five-membered 5 and six-membered 8 (i.e., kH+ = 6.5 and 9.8 M−1 s−1). Further, the hydroxonium catalytic coefficient for the hydrolysis of 5 and acyclic analogue 6 are within experimental error of each other (i.e., kH+ = 6.5 ± 0.2 and 7.0 ± 0.2 M−1 s−1, respectively). It was noted previously that the relative rates of hydrolysis for six-membered 15 and 8 could be explained with the kinetic anomeric effect. Consistent with this, the X-ray crystal structure of an analogous yet conformationally constrained bicyclic orthoester possesses an unusually elongated axial C─O bond (Fig. 2), which undergoes preferential cleavage with Lewis acids [31]. The Cambridge Structural Database (2015) [32] contains a single example of a five-membered 1,3-dioxolane orthoester [33]. Here, the ring adopts a distorted half-chair (C2) arrangement with a dihedral angle θ [O–C(4)–C(5)–O] = 32° [Fig. 2 – ZICMED viewed C(4)→C(5)]. This, along with a rate of hydrolysis similar to an acyclic system suggests that a kinetic anomeric effect does not extend to 1,3-dioxolane orthoesters.

View Article: PubMed Central - HTML - PubMed

ABSTRACT

A novel approach to protecting jet fuel against the effects of water contamination is predicated upon the coupling of the rapid hydrolysis reactions of lipophilic cyclic geminal ethers, with the concomitant production of a hydrophilic acyclic hydroxyester with de-icing properties (Fuel Dehydrating Icing Inhibitors - FDII). To this end, a kinetic appraisal of the hydrolysis reactions of representative geminal ethers was undertaken using a convenient surrogate for the fuel–water interface (D2O/CD3CN 1:4). We present here a library of acyclic and five/six-membered cyclic geminal ethers arranged according to their hydroxonium catalytic coefficients for hydrolysis, providing for the first time a framework for the development of FDII. A combination of 1H NMR, labelling and computational studies was used to assess the effects that may govern the observed relative rates of hydrolyses.

No MeSH data available.