Methoxy islands

Figure 28 clearly shows the importance of diffusion within a chemisorbed layer to surface reaction processes (Leibsle and Bowker, in prep.). In this series of STM images the surface methoxy species on Cu(110) is decomposing (evidenced by the loss of total area of methoxy islands), but diffusion is taking place between islands since big islands get bigger at the expense of smaller ones, which eventually disappear. This kind of diffusion phenomenon can be classified as surface mediated Ostwald ripening. 

Fig. 28. Sequential STM images of methoxy islands on a Cu(110) surface showing the loss of methoxy as it decomposes. Between images a anti b the island labelled 1 has increased in size, while that labelled 2 has decreased in c island 2 has gone altogether.

What happens to the methoxy formed by this process is strongly temperature dependent. At low temperature (up to - 340K) it is stable on the surface and forms the beautiful structures shown in fig.2. Since the active oxygen is used in such reactions then the methoxy must (i) not block the active site at its formation or (ii) diffuses away from the active site. Our evidence indicates the latter to be the case since methoxy is present at sites away from the oxygen islands. Above approximately 340 K the methoxy is unstable and decomposes to yield formaldehyde and hydrogen in the gas phase. Above approximately 400 K, the stoichiometry of the reaction changes to... 

Figure 5. Cartoon models of the reaction of methanol with oxygen on Cu(llO). 1 A methanol molecule arrives from the gas phase onto the surface with islands of p(2xl) CuO (the open circles represent oxygen, cross-hatched are Cu). 2,3 Methanol diffuses on the surface in a weakly bound molecular state and reacts with a terminal oxygen atom, which deprotonates the molecule in 4 to form a terminal hydroxy group and a methoxy group. Another molecule can react with this to produce water, which desorbs (5-7). Panel 8 shows decomposition of the methoxy to produce a hydrogen atom (small filled circle) and formaldehyde (large filled circle), which desorbs in panel 9. The active site lost in panel 6 is proposed to be regenerated by the diffusion of the terminal Cu atom away from the island in panel 7.

Although it was not possible to distinguish between the specificity of oxygen states responsible for methoxy and formate formation, they were to be associated with isolated oxygen adatoms and oxygen states present at the periphery of the (2 x 1)0 islands. 

Niphatynes A (1) and B (2) isolated from a Niphates sp. collected off Vitu Levu in the Fijian Islands were the first reported examples of monomeric 3-alkylpyridines from sponges [17]. These two metabolites, which contain the alkyne and methoxy amine functionalities encountered in many of the monomeric 3-alkylpyridines, were reported to be cytotoxic to murine leukemia P388 in vitro (niphatyne A (1) ED50 0.5 [ig/ml). Niphatesines A (3), B (4), C... 

Surface-science studies using copper single-crystal surfaces of (110) and (310) orientation onto which ZnO islands had been deposited indicate that CO and CO2 chemisorption can be used to identify the metal and the oxide sites, respectively. Methanol chemisorption produces both formate and methoxy species. The concentration of formate is enhanced by the presence of ZnO-copper interfaces, implicating these species as a reaction intermediate. 

Ellipticine (41) and its 9-methoxy derivative (42) have great interest as antitumor dmgs. These alkaloids are isolated from the leaves of Ochrosia elliptica, a shmb found on scattered islands in the Indian and Pacific Oceans. Plant cell cultures would thus be of interest as an alternative means of production of the antitumor alkaloids. For a review on the genus, its alkaloids, and cell culture, the reader is also refered to Chenieux et al. (716). 

The methoxy-methylenedioxy pattern is also found in nature with the 2,4,5-orientation pattern. The allyl-2,4,5-isomer is called asaricin. It, and its propenyl-isomer, carpacin, are from the Carpano tree which grows in the Solomon Islands. All these plants are used in folk medicine. These two systems, the 2,3,4- and the 2,4,5-orientations, potentially give rise, with ammonia, to MMDA-3a and MMDA-2. 

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