An efficient method for preparing unsymmetrically multisubstituted siloles is described. The reaction of 1-hydrosilyl-4-silyl-1,3-enynes with diisobutylaluminum hydride (DIBAL-H) gave multisubstituted siloles in good to high yields. This method could be applied to the synthesis of benzosiloles using 2-hydrosilyl-1-(silylethynyl)benzenes as substrates. The silole formation was also promoted even by a substoichiometric amount of DIBAL-H. The reaction provides a straightforward method to prepare siloles and benzosiloles.

•An extract from Salvia hydrangea is synthesized in 11 steps with 18% overall yield.•The DIBAL-H-promoted benzocyclization of dienynes is utilized as the key step.•This study provides a novel example of natural product synthesis using DIBAL-H.The diisobutylaluminum hydride (DIBAL-H)-promoted benzocyclization, recently developed by this group, was adopted for the synthesis of a natural product containing a 9,10-dihydrophenanthrene skeleton to demonstrate its synthetic utility. One of the extracts from the roots of Salvia hydrangea DC. ex Bentham (Lamiaceae), a 20-norabietane derivative, was selected as the target molecule. The key step forming the 9,10-dihydrophenanthrene skeleton was achieved by the DIBAL-H-promoted cyclization of a silylated 1,3-dien-5-yne easily accessible from a substituted α-tetralone.Download high-res image (68KB)Download full-size image

•DIBAL-H promotes the cyclization of o-(silylethynyl)styrenes to naphthalenes.•A proposed reaction mechanism includes carboalumination and skeletal rearrangement.•DIBAL-H-promoted cyclizations via C–C bond cleavage are also described.•The cyclized products can be converted into polycyclic aromatic hydrocarbons.The reaction of α,β-disubstituted (E)-o-(trimethylsilylethynyl)styrenes with a substoichiometric amount of diisobutylaluminum hydride at 100 °C gave 1,2,3-trisubstituted naphthalenes. This benzocyclization is initiated by regioselective hydroalumination of the alkyne moiety, and the resultant alkenylaluminums lead to naphthalenes through intramolecular carboalumination, skeletal rearrangement, and dehydroalumination steps. The silylated products could be efficiently transformed into iodinated polycyclic aromatic hydrocarbons.Download high-res image (89KB)Download full-size image

•DIBAL-H promotes the cyclization of o-(silylethynyl)benzyl ethers to 2-silylindenes.•Diethylaluminum chloride assists the DIBAL-H-promoted cyclization.•The DIBAL-H-promoted cyclization is applicable to the synthesis of benzo[b]siloles.The reaction of o-[2-(trimethylsilyl)ethynyl]benzyl methyl ethers with diisobutylaluminum hydride (DIBAL-H) gave 2-(trimethylsilyl)indenes in good yields. This cyclization proceeds by regio- and stereoselective hydroalumination of the alkyne moiety followed by intramolecular nucleophilic substitution. o-[2-(Trimethylsilyl)ethynyl]phenylsilyl methyl ethers also underwent the DIBAL-H promoted-cyclization to be converted into 2-(trimethylsilyl)benzosiloles in good yields. This approach provides straightforward and efficient way to construct benzosiloles from readily available organosilanes.

•In(OTf)3 and Tf2NH cooperatively promote the Conia-ene cyclization.•Less enolizable alk-5-ynyl ketones can be cyclized to cyclopent-1-enyl ketones.•In(OTf)3 promotes the cyclization of alk-5-ynyl ketones to cyclohept-2-enones.Combined use of Tf2NH and In(OTf)3 effectively promotes the cyclization of alk-5-ynyl ketones to cyclopent-1-enyl ketones at 30 °C. Single use of Tf2NH or In(OTf)3 requires heating at 50 °C for efficient cyclization. The In(OTf)3-promoted reaction of certain alk-5-ynyl ketones gives cyclohept-2-enones mainly.

In the presence of catalytic amounts of PtCl2 and AgSbF6, (Z)-alkenylsilanes react with various carbon electrophiles at the γ-position to give allylation products. A plausible mechanism for the Pt-catalyzed allylation involves alkene migration of alkenylsilanes to allylsilanes and subsequent allylation of carbon electrophiles.

Regiocontrolled synthesis of polysubstituted benzenes from silylated 1,3-dien-5-ynes has been achieved by using diisobutylaluminum hydride (DIBAL-H). Hydroalumination of the alkyne moiety with DIBAL-H triggers the aromatic cyclization, which usually proceeds without rearrangement and loss of the existing substituents. The related unusual cyclizations to different types of polysubstituted benzenes are also described.

The reaction of o-(trimethylsilylethynyl)styrenes with diisobutylaluminum hydride (DIBAL-H) provides 2-trimethylsilyl-1H-indenes efficiently. The cyclization mechanism involves regioselective hydroalumination of the alkynyl moiety, geometrical isomerization of the alkenylaluminums formed, and intramolecular carboalumination. With substrates bearing a 2-(trimethylsilyl)ethenyl group (R1 = Me3Si, R2 = R3 = H), bis-silylated benzofulvenes are obtained upon treatment of the reaction mixture with an excess amount of benzaldehyde.

The Pd-catalyzed reaction between β-iodo-β-silylstyrenes and terminal alkynes in i-Pr2NEt gave 1,2,3,5-tetrasubstituted benzenes with complete regioselection. The use of certain silylacetylenes as alkynes enabled efficient synthesis of 1,3,5-trissilyl-2-arylbenzenes, which could be transformed into other multisubstituted benzenes by displacement of the silyl groups.

α-Alkylated (dimethylsilyl)acetonitriles (Me2HSiCR3R4CN) react spontaneously with aldehydes in DMSO to give β-hydroxynitriles in good to high yields. The addition to ketones is effectively promoted by using MgCl2 or CaCl2. (Dimethylsilyl)acetonitrile (Me2HSiCH2CN) shows lower reactivity than the α-alkylated analogues. However, the parent reagent adds efficiently to aldehydes and ketones under catalysis by AcOLi or MgCl2.

The PtCl2-catalyzed hydrosilylation of terminal alkynes with triethylsilane and subsequent alkenylation of aldehydes with the resultant (E)-alkenylsilanes in a one-pot manner are described. By adding p-benzoquinone and LiI, the latter alkenylation step proceeds smoothly to give allyl silyl ethers in moderate to high yields. This one-pot alkenylation is tolerant to a reasonable range of functional groups. PtCl2 plays a dual role as hydrosilylation and alkenylation catalysts.

In the presence of catalytic amounts of PtCl2 and metal iodides, β-substituted vinylsilanes reacted with aldehydes at the β-position to give allyl silyl ethers. The Pt-catalyzed addition to aromatic aldehydes proceeded efficiently in the presence of LiI. The combined use of PtCl2 and MnI2 was found to be effective in addition to aliphatic aldehydes.