[72952-73-9] · C8H16Si · 5-Trimethylsilyl-1,3-pentadiene · (MW 140.33)
Alternate Name: 2,4-pentadienyltrimethylsilane.
Physical Data: bp 41 °C/36 mmHg or 105 °C/760 mmHg; n
Solubility: sol hexane, benzene, CH2Cl2, ether, THF.
Analysis of Reagent Purity: by NMR, GC (8.4 min on 3 mm × 3 M 5% Apiezon L on Chromosorb W at 80 °C) or TLC: 5% AgNO3/silica gel (4:1 hexane/benzene).
Preparative Method: may be prepared in 85% yield by the reaction of Pentadienyllithium with Chlorotrimethylsilane in THF at room temperature.1-3 Generally the product is ~95% (E) isomer as judged by IR (strong band at 1000 cm-1) or NMR (trans coupling constant of 15.2 Hz for proton at C-3, 5.92 ppm).
Handling, Storage, and Precautions: stable liquid; the toxicological properties of this reagent are unknown. It should be handled in a well ventilated hood.
2,4-Pentadienyltrimethylsilane is used as a stable (storable) nucleophilic substitute for pentadienyllithium. This compound reacts with Lewis acid (Boron Trifluoride Etherate, Iodotrimethylsilane, Titanium(IV) Chloride) activated electrophiles in the same manner as allylsilanes (eq 1). Generally only ε-substitution is observed (cf. Table 1).1,4,5,7
Other electrophiles which have been used include acetals,4 acid chlorides,5 and heteroatom-based electrophiles. A useful varient of the title compound is the more reactive 2,4-pentadienyltrimethylstannane (Table 1, entries 6 and 8).6,7 Lewis acid-catalyzed conjugate addition reactions have been observed with the tin analog,7,8 but are not observed with 2,4-pentadienyltrimethylsilane due to a competing Diels-Alder reaction (Table 1, entries 5 and 7).
In a typical reaction of 2,4-pentadienyltrimethylsilane with electrophiles, a carbonyl compound was dissolved in CH2Cl2 and cooled to -40 °C and 0.3 equiv TiCl4 (or BF3.OEt2) was added. After 5 min, 0.66 equiv 2,4-pentadienyltrimethylsilane was added and the mixture warmed to 5-25 °C over a 5 min to 2 h period (depending on the reactivity of the electrophile). After quenching with NaHCO3 the product was extracted with ether.
2,4-Pentadienyltrimethylsilane may also be deprotonated adjacent to silicon with Lithium Diisopropylamide, forming the 1-(Me3Si)-pentadienyl anion.2 This anion reacts with many electrophiles to give ε-substituted dienes (eq 2) which are ideal candidates for protodesilyation or intramolecular Diels-Alder reactions.
The reaction of the 1-(Me3Si)-pentadienyl anion with cyclohexanone gives a 59/41 mixture of g- and ε-regioisomers.
Stephen R. Wilson
New York University, NY, USA