Email Discussion: 50 Deposition of x-ray data at Cambridge Rzepa,Henry

Synthesis and Structural Hydrogen Bonding of 2-ethynyl Adamantane Systems

Michael B. Keller, Henry S. Rzepa, Andrew J. P. White and David J. Williams

Department of Chemistry, Imperial College, London, SW7 2AY.

There is much current interest in probing the diversity and limits of hydrogen bonding interactions to ¼ systems such as phenyl rings and double and triple bonds. We have previously identified[1] a small number of molecules for which there is crystallographic evidence for relatively strong OH...¼ interactions to double or triple bonds. Of these, the most intriguing is the structure of 2-ethynyl adamantan-2-ol 1, which independently has correctly been identified[1] as containing both a dimeric intermolecular OH...¼ framework, with additional C-H...O interactions.

1
As part of our program[2] of studying ¼-facial interactions, we wished to establish the structural tolerance of such interactions, and in particular whether simple analogues of 1 might also demonstrate this effect. We report here the syntheses of five related ethynyl adamantane derivatives. Although the crystal structure of 1 has been reported, surprisingly, no details in the literature of a high yield synthesis are given. In our hands, the simple coupling of ethyne with adamantanone gave very poor yields, irrespective of the metal counterion used, and we adopted coupling of the trimethylsilyl alkyne followed by desilylation as the preferred route. Click on any of the reaction schemes shown below for active "TGF" files showing the reaction sequences for all the compounds reported here Full experimental details are also available via hyperlinks to each scheme.

Scheme 1

Scheme 2

Scheme 3

Scheme 4

Scheme 5

Results and Discussion.

2-chloroethynyl-Adamantan-2-ol: 2

This compound was prepared to investigate the result of suppressing the terminal C-H...O interaction found in 1. The pattern of hydrogen bonds in this system changes completely from that found in 1, with the OH interactions to the triple bond being replaced by a characteristic cyclic tetrameric structure comprising an 8-membered ring. Clearly, this type of molecule is balanced on a knife edge between forming O-H..O and O-H...¼ interactions, and any slight change in the basicity of the alkyne system may perturb sufficiently to radically alter the hydrogen bonding pattern.

2-ethynyl-2-phenylamino-Adamantane: 3

The phenylamine analogue of 1 exhibits a dimeric NH...¼ interaction very similar to that found in 1, albeit with significantly longer C...H distances of around 2.8A rather than 2.2A. There are however two significant differences in the interactions. A second "ligand" to the triple bond is one of the ortho hydrogens from the intramolecular phenyl group. Conspicuously, the terminal C-H hydrogen bond found in 1 appears entirely absent in 3, with no apparent hydrogen bond forming to the part of the molecule. This might be a result of greater steric hindrance caused by the phenyl group.

1,4-bis(2-Adamanta-2-ol)-butadiyne: 4

This system represents our first attempt to extend the length of the alkyne chain to four atoms. In theory, the electron density on such a C4 unit should promote the basicity of the carbon chain. In practice, the X-ray structure reverts to the cyclic tetrameric arrangement revealed in compound 2, with no apparent coordination to the linear C4 fragment. This may in fact be a consequence of the large bulk of the two "capping" units.

2-ethynyl-2-amino-Adamantane: 5

The amino analogue of 1 exhibits only a terminal C-H...N hydrogen bonding interaction, and surprisingly no ¼- H-N interactions. In this it shows a radical departure from the OH system.

2-butadiynyl-2-Adamantanol: 6

This system shows the closest analogy to 1, with a strong ¼- H-O interaction to the terminal alkyne group rather than the central one. This interaction is also fairly symmetrical.

Conclusions.

A general theory of the circumstances of how and why ¼- hydrogen bonds form will require a larger selection of systems than we have yet prepared. Nevertheless some themes are already emerging. For example, both NH and OH hydrogen bonds can form. Capping the terminal C-H with a variety of functional groups such as Cl, SiMe3 (not reported here), phenyl, and adamantanol itself, appear to suppress ¼ interactions, and initial indications are that extending the alkyne system to four carbons does not promote the ¼ interactions. It also appears that a hard/soft acid-base concept might apply in such systems. Thus ¼...H-C interactions are relatively easy to promote, whereas OH systems revert very easily to O-H...O interactions. Further structural studies of this system will be reported in due course.

References

[1] E.Steinwender, E.T.G.Lutz, J.H. van der Maas and J.A. Kanters, Vibrational Spectroscopy , 1993, 4, 217.

[2] H. S. Rzepa, M. Smith, and M. L. Webb, J. Chem. Soc., Perkin Trans 2, 1994, 703. See also the URL http://www.ch.ic.ac.uk/rzepa/RSC/P2/3_05613l.html