Convenient Solution Route To Alkylated Pentalene Ligands: New Metal Monoalkylpentalenyl Complexes

Carbene-induced rearrangement of 8,8-dibromobicyclo[5.1.0]octa-2,4-dienes and subsequent in situ
deprotonation of the dihydropentalenes formed represents a straightforward and versatile solution route
to pentalene ligands for use in organometallic chemistry. We report here the synthesis of 6-alkyl-substituted
8,8-dibromobicyclo[5.1.0]octa-2,4-dienes and their rearrangement to give correspondingly substituted
pentalene ligands. Generation of methyl-substituted dihydropentalenes from 8,8-dibromo-6-methylbicyclo[5.1.0]octa-2,4-diene followed by monodeprotonation with TlOEt gives methylhydropentalenyl salts, Tl(C8H6Me), as a mixture of isomers. Formation of both 1-Me and 3-Me isomers of Re(C8H6Me)(CO)3
from these Tl species shows that monodeprotonation of the dihydropentalenes occurs at the unsubstituted
ring. No evidence for the 2-Me isomer is observed, consistent with the rearrangement mechanism reported
in the literature. Likewise, rearrangement of 8,8-dibromo-6-alkylbicyclo[5.1.0]octa-2,4-dienes (R = Me,
Et, iPr) and subsequent double deprotonation with nBuLi in the presence of DME affords the pentalenyl
salts Li2(C8H5R)·xDME. X-ray crystallographic studies of the dinuclear Mn organometallics anti-[Mn(CO)3]2(C8H5R) synthesized from the Li salts confirm that these alkylated pentalene ligands are substituted
at the 1-position exclusively. The crystal structure of a model hydropentalenyl complex, Re(C8H7)(CO)3,
is also reported.