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Cyclopropenylium−Borate Betaines: Opening an Organometallic Route to Neutral Carbocation‐Like Compounds
Author(s) -
VenneDunker Sabine,
Ahlers Wolfgang,
Erker Gerhard,
Fröhlich Roland
Publication year - 2000
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/1099-0682(200008)2000:8<1671::aid-ejic1671>3.0.co;2-v
Subject(s) - chemistry , moiety , carbocation , medicinal chemistry , alkyne , isocyanide , electrophile , stereochemistry , betaine , intramolecular force , nucleophile , covalent bond , ligand (biochemistry) , cationic polymerization , reactivity (psychology) , polymer chemistry , organic chemistry , catalysis , medicine , biochemistry , receptor , alternative medicine , pathology
Treatment of bis(propynyl)zirconocene ( 5 ) with a stoichiometric quantity of B(C 6 F 5 ) 3 leads to C−C coupling of the alkynyl ligands to form the zirconocene−(μ‐hexadiyne)−borate betaine complex 11 . This apparently is in an endothermic equilibrium with a methylenecyclopropene‐type isomer 13 , formed by intramolecular alkyne insertion into the (sp 2 ‐carbon)−zirconium linkage, which is very effectively trapped by added scavenger reagents such as organic nitriles or isonitriles, to yield the stable unsaturated three‐membered ring products 14 or 15 , respectively. Subsequent hydrolysis removes the zirconocene moiety from the product, but the newly formed (sp 2 ‐carbon)−B(C 6 F 5 ) 3 linkage remains intact. The resulting products 16 and 17 formally contain a central methylenecyclopropene framework, but their structural properties point to a substantial π‐delocalization, that indicates a pronounced participation of an enamino‐substituted cyclopropenylium−borate betaine resonance structure. The description of the neutral betaine compounds 16 and 17 as approaching cyclopropenylium cation‐type compounds that have their counteranion covalently bonded to them, is further supported by the observation of low activation barriers of the rotation around the exocyclic C3−C4 bond [Δ G ≠ CCrot (330 K) = 17.5 ± 0.5 kcal mol −1 for 17a ] and around the C5−N linkage [Δ G ≠ CNrot (306 K) = 14.5 ± 0.5 kcal mol −1 for 16a ]. The neutral cyclopropenylium−betaine compound 17a shows a carbocation‐like chemical reactivity, and inserts tert ‐butyl isocyanide to yield the ring‐expanded four‐membered carbocyclic product 21 .