Functionalized Cyclopentadienes: Synthesis of 1,3-Bis(1,1-dimethylbut-3-cyclopentadienyl) Systems

Abstract

The synthesis of the new cyclopentadiene C 5 H 4 (CMe 2 CH 2 CHCH 2 ) 2 4 is described. The key step of the reaction sequence is the regioselective formation of the 2-(1,1-dimethylbut-3-enyl)-6,6-dimethylfulvene 2. The substitution pattern of 4 can be confirmed by the structure of the Diels-Alder product obtained from 2 and dimethyl acetylenedicarboxylate. Compound 4 is converted into the corresponding cyclopentadienides 5a,b via metallation with n-BuLi and KH, respectively. Compounds 4 and 5a,b are suitable candidates for the synthesis of functionalized cyclopentadienyl complexes in organometallic chemistry.Cyclopentadiene and its derivatives are the most important ligands in organometallic chemistry. 1 In the last decade numerous functionalized Cp-systems have been synthesized and studied by various research groups. 2 We are interested in Cp-ligands, which offer the possibility for the corresponding metal complexes to undergo further reactions in the side chain(s). In this context, we have focussed our research on multiple w-alkenyl substituted cyclopentadienes. The terminal double bonds of the organometallic complexes can be further functionalized by reactions such as hydrosilylation, hydroboration, or Hecktype reactions. Recently, we published the synthesis of 1,2,3,4-tetraallylcyclopentadiene and of 1,2,3,4,5-pentaallylcyclopentadiene and their use in transition metal chemistry. 3 We report here on the regioselective synthesis of a 1,3-di-w-alkenyl-substituted cyclopentadiene.When 6,6-dimethylfulvene is reacted with excess allylmagnesium bromide in Et 2 O, the Grignard reagent adds to the exocyclic double bond of the fulvene. The exothermic reaction can be monitored by the disappearance of the characteristic yellow color of fulvene. After aqueous work-up and distillation, the butenyl-substituted cyclopentadiene 1 was isolated as a slightly yellowish liquid in 62% yield. Compound 1 has been characterized by elemental analysis, 1 H, 13 C and IR spectroscopy. The IR spectrum of 1 exhibits a band at 1629 cm -1 , which is assigned to the exocyclic double bond. NMR spectroscopy gives evidence for the presence of the three tautomers 1a,b,c. The proton resonances of the three possible tautomers are not clearly resolved or appear at identical chemical shifts. Only for the methyl groups, three singlets can be assigned in the range of 1.21 to 1.22 ppm. Integration of the allylic ring protons allows the identification of the main isomers 1a and 1b, which are substituted in a vinylic position (ratio 1a and 1b:1c > 10:1). The 13 C NMR data confirmed the above observation (for details see Experimental). Scheme 1Treatment of 1 with acetone and pyrrolidine in methanol led to the substituted fulvene 2, which was purified by distillation and isolated as a yellow liquid in 72% yield. It is known that increasing steric bulkiness of the first substituent favors the formation of the 2-substituted fulvene, 4 however, the regioselectivity of this reaction could not unequivocally be proven by 1 H 13 C NMR, and COSY ( 1 H, 13 C) spect...

Functionalized Tri‐ and Tetraphosphine Ligands as a General Approach for Controlled Implantation of Phosphorus Donors with a High Local Density in Immobilized Molecular Catalysts

“…The reaction of 6,6-dimethylfulvene with allylmagnesium chloride led to the 1-functionalized cyclopentadiene 2 a' (Scheme 6). [24] Our attempts to position the vinyl group in the b position of the Cp ring by using vinyl-magnesium halides failed. Lithiation of 2 a' with nBuLi quantitatively converted it into 2 b.…”

“…The reaction of 6,6-dimethylfulvene with allylmagnesium chloride led to the 1-functionalized cyclopentadiene 2 a' (Scheme 6). [24] Our attempts to position the vinyl group in the b position of the Cp ring by using vinyl-magnesium halides The monofunctionalized analogues 13-15 of the difunctionalized tetraphosphines 1, 7, and 10 were also synthesized (Scheme 7). Compound 13 was formed from the appropriate Cp lithium salt mixture, with a statistical coproduction of the symmetric compounds L1 and 7.…”

E/ZSwitchable Ring-Closing Metathesis in 1,1′-Bis(but-3-enyl)ferrocenes: Synthesis and Characterization of Axially Chiralansa[6]-Ferrocenes

“…Pentafulvenes have been extensively used as precursors to substituted Cp ligands and related systems . In particular, addition of allylmagnesium bromide to 6,6-dimethylfulvene ( 1a ) provides a convenient route to but-3-enyl-substituted cyclopentadiene 2a . We have extended this methodology to a range of 6,6-disubstituted pentafulvenes 1b – 1f to obtain the but-3-enyl cyclopentadienes 2b – 2f as mixtures of regioisomers.…”