Carbon-oxygen-nitrogen-metal rings

CARBON-NITROGEN-OXYGEN/SULFUR-METAL RINGS... 

Metallaheterocycles with the metal atom capable of supporting a carbon-metal double bond, and with oxygen, nitrogen, sulfur, or selenium as the second heteroatom are the only structures capable of full conjugation. These compounds show furan-like aromaticity with the heteroatom /i-electrons participating. No examples of this type of ring system were uncovered, undoubtedly due to the stability of the metalla-carbon double bonds required for their formation. 

As shown in eq. (3.11), if a hetero atom such as nitrogen, sulfur or oxygen is located at the / -position of the aromatic ring, the lithium metal at first is coordinated by the hetero atom and activated, and then it forms a stable five-membered ring by electrophilic attack at the ortho carbon. The five-membered ring formation reaction as shown in eq. (3.11), proceeds easily not only with lithium but also with the many other metals. This reaction is called orthometalation or cyclometalation [30]. For example, the five-membered ring compounds are produced by reaction of dimethylaminomethybenzene with alkyllithium as shown in eq. (3.12) [31-33]. Similar reactions are known on the compounds of metals such as Si, Sn, Hg, Mn, Cu, Ni, Cr, Co, Pt, Pd, Rh, Ti, V, Sc, Y, La, Nd and Er. [30]. 

In this group of derivatives, we have included all compounds in which attachment of the substituent results in the formation of a cyclopropane ring on the surface of Cgo- The bridging atom may be carbon, nitrogen, a transition metal, or oxygen (epoxyfullerene). Basic structures for these compounds are shown in Fig. 16. The electrochemistry of representative examples of all of these derivatives has been reviewed previously [83], and only general observations are presented below, along with some specific examples of new derivatives with novel electrochemical behavior. 

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