2-Lithioindenyllithium: An Easily Accessible Intermediate for the One-Pot Synthesis of Single- and Double-Functionalised 2-Indene Derivatives

Abstract

We found that double-silicon-bridged bisindenyl complexes of early transition metals of group 4 are able to produce polyethylenes with high molecular weights and narrow molecularweight distributions having high a-olefin incorporation. The bottleneck for the synthesis of these complexes is the ligand synthesis. The corresponding ligands could be obtained via a three-step procedure with low (15%) overall yields. We found that the use of the new 2-lithioindenyllithium enhanced the overall yield to 50% in a one-pot procedure.Double-silicon-bridged bisindenyl derivatives were recently described by us (Scheme 1). 2 We found that reaction of 4a with 0.5 equivalent of dichlorodimethylsilane resulted in a mixture of 5 (70%) 3 and 6 (30%). The main drawback of this route is the low yield (15%) of the single-bridged 2-indenyl intermediate 4a, probably caused by proton abstraction by 2 from 3 or 4a. To avoid this exchange reaction, the starting materials and the reaction products should not contain acidic protons. Therefore, the 3-position should be protected, preferably with a metal like magnesium or lithium, since this gives the possibility of further reaction or facile deprotection via H-quench reactions. Given that several lithiated organometallic compounds are accessible via the bromoderivatives, such as lithioferrocene starting from bromoferrocene, 4 we set out to prepare 7 from the known 2-bromoindenyllithium (8) 5 (Scheme 2 and Scheme 3). (7) Mechanism of formation of 2-lithioindenyllithium (7): 2-Lithioindenyllithium (7) can be prepared from 2-bromoindene (1) with the aid of two equivalents of n-butyllithium in diethyl ether at -100 °C, followed by warming to room temperature (Scheme 4). The GC-MS spectrum of a sample quenched with D 2 O, taken at -40 °C, contained a peak with the retention time of 1. The corresponding MS spectrum contains a molecular ion with m/e = 195/197, which indicates the formation of [D]1 (8a), resulting from a D-quench of 8. Although hardly any L i L i 7 B r L i 8 ? Scheme 2 Formation of 2-lithioindenyllithium

Indolizidine and quinolizidine alkaloids

“…94 By contrast, the complex formed from (−)-sparteine and lithiated 2-bromoindene was monomeric, probably for steric reasons; the interaction between the metal and the indenyl ligand was essentially ionic. 95 In view of the importance of (−)-sparteine-alkyllithium complexes in enantioselective synthesis, crystal structures of such complexes are of especial interest. The monomeric complex 339 obtained from the sparteine-assisted lithiation of the corresponding thiocarbamate with n-butyllithium was found to have (S)-absolute configuration both at the tetragonally coordinated lithium cation and at the deprotonated carbon site, from which the pro-S hydrogen of the precursor had been removed.…”

Recent Advances in the Synthesis of Indanes and Indenes

“…The subsequent quenching with D 3 O + allowed an easy synthesis of deuterated indenes 382 (Scheme ) . 2‐Lithioindenyllithium 384 , obtained by dilithiation of 2‐bromoindene 383 , was also used for the preparation of deuterated indenes and silicon‐bridged bisindenyl derivatives, as exemplified in Scheme for the preparation of 385 …”

“… Synthesis of silicon‐bridged bisindenyl derivative 385 by reaction of 2‐lithioindenyllithium 384 with dimethyldichlorosilane …”

Lewis Base Assisted Brønsted Base Catalysis: Direct Asymmetric Allylic Alkylation of Indenes

“…Unfortunately, although both starting materials were consumed, a mixture of complex products was obtained (Table 1, Entry 1). Nevertheless, it was pleasing to observe that a cleaner reaction occured when 2-bromoindene [13] (1b) was applied. The desired allylic alkylation product 3a was isolated in moderate yield, although a few unidentified minor products were also detectable (Table 1, Entry 2).…”