[72952-73-9]  · C8H16Si  · 5-Trimethylsilyl-1,3-pentadiene  · (MW 140.33)

(useful pentadienylation reagent;1 an allyl silane;7 a precursor for 1-trimethylsilyldienes and intramolecular Diels-Alder substrates2)

Alternate Name: 2,4-pentadienyltrimethylsilane.

Physical Data: bp 41 °C/36 mmHg or 105 °C/760 mmHg; n20D 1.4570.

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.

General Discussion.

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.

1. Seyferth, D.; Pornet, J. JOC 1980, 45, 1722.
2. Oppolzer, W.; Burford, S. C.; Marazza, F. HCA 1980, 63, 555.
3. Seyferth, D.; Pornet, J.; Weinstein, R. M. OM 1982, 1, 1651.
4. Sakurai, H.; Sasaki, K.; Hosomi, A. TL 1981, 745.
5. Hosomi, A.; Saito, M.; Sakurai, H. TL 1980, 3783.
6. Jones, M.; Kitching, W. JOM 1983, 247, C5.
7. Naruta, Y.; Nagai, N.; Arita, Y.; Maruyama, K. CL 1983, 1683.
8. Nishigaichi, Y.; Fujimoto, M.; Takuwa, A. JCS(P1) 1992, 2581.

Stephen R. Wilson

New York University, NY, USA

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