Carbo-mer

In organic chemistry, a carbo-mer (often carbo-mer or carbomer) is an expanded molecule obtained by insertion of C2 units into a given molecule.[1] Carbo-mers differ from their templates in size but not in symmetry when each C–C single bond is replaced by an alkyne bond C-C≡C-C, each C=C double bond is replaced by an allene bond C=C=C=C, and each C≡C triple bond is replaced by C≡C-C≡C. The size of the carbo-mer continues to increase when more C2 units are inserted, so carbo-mers are also called carbon-molecules, where "n" is the number of acetylene or allene groups in an n-expansion unit. This concept, devised by Rémi Chauvin in 1995, is aimed at introducing new chemical properties for existing chemical motifs.

This article is about the exotic expanded molecules. For the commercial water-absorbing polymers, see Carbomer.

Two distinct expansions of benzene can be called carbo-benzene (C18H6):

carbo-benzene

One (above right) expands each C-H bond to C-C≡C-H, making hexaethynylbenzene, a substituted benzene derivative.[2]

One (above left) expands each C=C and C≡C bond of the benzene core, making 1,2,4,5,7,8,10,11,13,14,16,17-dodecadehydro[18]annulene. An analog of this molecule, with the hydrogen atoms replaced by phenyl groups, 3,6,9,12,15,18-hexaphenyl-1,2,4,5,7,8,10,11,13,14,16,17-dodecadehydro[18]annulene, is stable.[3] Its proton NMR spectrum shows that the phenyl protons are shifted downfield compared to a proton position in benzene itself (chemical shift position for the ortho proton is 9.49 ppm), suggesting the presence of a diamagnetic ring current and thus aromaticity. The final step in its organic synthesis is reaction of the triol with stannous chloride and hydrochloric acid in diethyl ether:

carbo-benzene synthesis

With both core and periphery expanded, the total carbo-mer of benzene (C30H6) only exists in silico (computer simulation).[4]

Total carbo-mer of benzene

Calculations predict a planar D6h structure with bond lengths similar to the other two carbobenzenes. Its non-planar isomer is called "hexaethynyl-carbo-[6]trannulene" - a pun on the all-cis annulenes - and resembles a cyclohexane ring. This hypothetical molecule is predicted to be more energetic by 65 kcal/mol.

References
  1. Carbomers. I. A general concept of expanded molecules Remi Chauvin Tetrahedron Letters Volume 36, Issue 3, 16 January 1995, Pages 397-400 doi:10.1016/0040-4039(94)02275-G
  2. Hexaethynylbenzene Rainer Diercks, James C. Armstrong, Roland Boese, K. Peter C. Vollhardt. Hexaethynylbenzene. Angewandte Chemie International. 1986; 25(3):268-269. doi:10.1002/anie.198602681
  3. Synthesis of the first 3,6,9,15,18,18-hexa-substituted-1,2,4,5,7,8,10,11,13,14,16,17-dodecadehydro[18]annulenes with D6h-symmetry Yoshiyuki Kuwatani, Naoto Watanabe and Ikuo Ueda Tetrahedron Letters Volume 36, Issue 1, 2 January 1995, Pages 119-122 doi:10.1016/0040-4039(94)02181-A
  4. Total carbo-Mer of Benzene, Its carbo-Trannulene Form, and the Zigzag Nanotube Thereof Christine Lepetit, Chunhai Zou, and Remi Chauvin J. Org. Chem.; 2006; 71(17) pp 6317 - 6324; (Article) doi:10.1021/jo052551j
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.