Stereospecific Single Crystal Reactions of Metal Carboxylates

In order to gain insight into the reactions of acetylenes in solids, we hypothesized that the structures of suitable reactant phases could be predicted and engineered from a detailed knowledge of the structural literature. We thus hypothesized that the structures of metal propynoates might be similar to those of analogous acetates. Inspection of the crystal structure of bis(acetato)tetraaquanickel shows it to have sets of centrosymmetrically-related acetates, with short contacts of 4.0 Å between the methyl groups.

Synthesis and structure determination of bis(propynoato)tetraaquazinc shows it to have a similar arrangement:

...and a crystal structure containing infinite chains of short CC ··· CC contacts:

Accordingly, when this material is irradiated with X- or -rays, a solid-state oligomerization to the analogous zinc polypropynoate occurs.

Extension of the ideas and results of studies on the solid-state acetylene chemistry has been most rewarding. Irradiation of solid sodium trans-2-butenoate 1 with Co -rays leads to conversion of 1 to one of eight possible diastereoisomers of the trisodium salt of 2,4-dimethylhept-6-ene-1,3,5-tricarboxylic acid (2b) in 75-80% yield, based on converted 1.

An X-ray structure determination of the acid allowed assignment of the four chiral centers. This work represented the first (to our knowledge) efficient solid-state synthesis of a small molecule using ionizing radiation. The reaction is all the more remarkable since it is stereospecific, and the product cannot be prepared at all in solution, by thermolysis or by UV irradiation of crystalline 1.

Most recently we have examined the crystal chemistry of calcium crotonate. When the solid material is irradiated with -rays, a unique cyclodimerization reaction occurs:

Note that the product, the calcium salt of nepetic acid, is one of four possible diastereoisomers, as shown below, and that it is formed in 75% yield, based upon conversion (reactant may be recrystallized and recycled).

A possible mechanism for the reaction :

The structure of the reactant phase shows two possible "incipient" rings worthy of consideration. In one set, the Cmethyl ··· C distance is 3.93 Å, while the C ··· C distance is 4.52 Å. In the other set, the Cmethyl ··· C distance is 4.29 Å, while the C ··· C distance is 3.82 Å. Note that the crystal structure of the reactant predicts cis-stereochemistry for the carboxylates, opposite to what is actually observed. Thus, it appears unlikely that the first bond formed would be between the two -carbon atoms.

This material is based upon work supported by the National Science Foundation under Grant No. DMR-0089257. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation (NSF).

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