Titanocene-catalyzed double silylation of dienes and aryl alkenes with chlorosilanes

Double silylation of 1,3-butadienes with chlorosilanes was found to proceed by using titanocene dichloride as the catalyst in the presence of ⁿBuMgCl, giving rise to 1,4-disilylated 2-butenes in good yields. Aryl substituted alkenes also afforded 1,2-disilylated products under similar conditions.     

Titanocene-catalyzed regioselective carbomagnesation of alkenes and dienes

A new method for regioselective carbomagnesation of alkenes and dienes has been developed by the use of a titanocene catalyst. This reaction proceeds efficiently at 0 degrees C in THF in the presence of Cp(2)TiCl(2) by the combined use of organic halides (R-X; R = alkyl, aryl and vinyl) and n-BuMgCl to afford benzyl, alpha-silylalkyl, or allyl Grignard reagents, which were trapped with various electrophiles. The present reaction involves (i) addition of carbon radicals toward alkenes or dienes in the carbon-carbon bond-forming step and (ii) transmetalation on Ti of benzyl-, alpha-silylalkyl-, or allyltitanocene with n-BuMgCl in the carbon-magnesium bond-forming step. The scope and limitations of this reaction have also been examined.     

Titanocene-catalyzed formation of allylsilanes from allyl ethers and chlorosilanes

A new method for silylation of allyl ethers with chlorosilanes has been developed by the use of Cp₂TiCl₂ as a catalyst. This reaction proceeds efficiently at −20 °C in THF using ⁿBuMgCl. A plausible reaction pathway via allyltitanocene intermediate was proposed.     

Silylation and alkylation of allenes using chlorosilanes and alkyl halides in the presence of palladium catalyst and Grignard reagents

Allenes react with Grignard reagents and chlorosilanes in the presence of a palladium catalyst giving rise to carbosilylated products bearing carbon groups from Grignard reagents at the central carbon and silyl groups at the terminal carbon. When alkyl halides were used instead of chlorosilanes, the corresponding alkylated products were obtained.     

Photochemistry of alkyl halides—VII : Cyclopropanation of alkenes

The previously observed cyclopropaoation of alkenes by irradiation of diiodomethane (I) in their presence has been studied in more detail and found to be a synthetically useful procedure which is significantly less subject to steric effects than the traditional Simmons-Smith method. The results from photocyclopropanation of a variety of alkenes are summarized in Tables 1 and 3–4. In a number of cases the photochemical procedure afforded improved results over the Simmons-Smith method, particularly with sterically congested alkenes. Cycloalkenes showed relative rates of photocyclopropanation as a function of ring size similar to those of the Simmons-Smith method (Table 5). However, the photocyclopropanation reaction exhibited steadily increasing relative rates with increasing substitution about the double bond—in contrast with the Simmons-Smith method (Table 6), in which steric effects offset increasing nucleophilicity of the alkene with increasing substitution. The α-iodocation 2 is suggested as the methylene transfer species. In the presence of lithium bromide cation 2 was trapped to afford bromoiodomethane.     

Cobalt-catalyzed reductive coupling of saturated alkyl halides with activated alkenes

[reaction: see text] An efficient cobalt-catalyzed reductive coupling reaction of alkyl halides with electron-withdrawing alkenes (CH(2)=CR(1)EWG, EWG = electron-withdrawing group) in the presence of water and zinc powder in acetonitrile to give the corresponding Michael-type addition product (RCH(2)CR(1)EWG) was described. The methodology is versatile such that unactivated primary, secondary, and tertiary alkyl bromides and iodides and various conjugated alkenes including acrylates, acrylonitrile, methyl vinyl ketone, and vinyl sulfone all successfully participate in this coupling reaction. For the alkyl halides used in the reaction, the iodides generally gave better yields compared to those of the corresponding bromides. It is a unique method employing CoI(2)dppe, zinc, and alkyl halides, affording conjugate addition products in high yields. Mechanistically, the reaction appears to follow an oxidative addition driven route rather than the previously reported radical route.     

Fe-catalyzed three-component dicarbofunctionalization of unactivated alkenes with alkyl halides and Grignard reagents

A highly chemoselective iron-catalyzed three-component dicarbofunctionalization of unactivated olefins with alkyl halides (iodides and bromides) and sp²-hybridized Grignard reagents is reported. The reaction operates under fast turnover frequency and tolerates a diverse range of sp²-hybridized nucleophiles (electron-rich and electron-deficient (hetero)aryl and alkenyl Grignard reagents), alkyl halides (tertiary alkyl iodides/bromides and perfluorinated bromides), and unactivated olefins bearing diverse functional groups including tethered alkenes, ethers, protected alcohols, aldehydes, and amines to yield the desired 1,2-alkylarylated products with high regiocontrol. Further, we demonstrate that this protocol is amenable for the synthesis of new (hetero)carbocycles including tetrahydrofurans and pyrrolidines via a three-component radical cascade cyclization/arylation that forges three new C–C bonds.

A highly selective iron-catalyzed three-component dicarbofunctionalization of unactivated alkenes with alkyl halides and sp²-hybridized Grignard reagents is reported.     

Azidosulfonylation of alkenes, dienes, and enynes

The radical mediated azidosulfonylation of various alkenes and alkynes that are able to undergo a rapid radical rearrangement is reported. For instance, treatment of 1,6-dienes or 1-en-6-ynes with benzenesulfonyl azide affords cyclic azidosulfones. High yields are observed when tertiary alkyl radicals are azidated in the last step of the cascade process. The azidosulfonylation of β-pinene involving ring opening of the bicyclic skeleton is also reported.     

From alkyl halides to alkyltrifluoromethyls using bromine trifluoride

Under the right conditions, bromine trifluoride can be a useful tool for generating new types of reactions and compounds. Thus, tris(methylthio)alkyl derivatives, easily prepared from the corresponding alkyl bromides, were converted to the corresponding RCHBrCF2SMe or RCHBrCF3 compounds. The bromine atom, however, could be easily reduced forming eventually R'CF2SMe or R'CF3. If desired, the bromine atom can serve as an entry for constructing terminal difluoroolefins.     

Reaction pathways of zirconocene-catalyzed silylation of alkenes with chlorosilanes

Reaction pathways as well as stereochemistries and stoichiometries of zirconocene-catalyzed silylation of olefins with chlorosilanes in the presence of ⁿBuMgCl were studied and discussed in detail. Rate determining steps were examined by kinetic studies and labeling experiments.     

Reaction of alkenes and dienes with t-butylmagnesium halides and zirconocene dihalides. A convenient procedure for hydrozirconation and a novel t-butylzirconation of conjugated alkenes

The reaction of nonconjugated monosubstituted alkenes with t-BuMgCl and Cl₂ZrCp₂ at room temperature produces the corresponding monoalkylzirconium derivatives in high yields, while conjugated alkenes undergo either a novel t-butylzirconation or hydrozirconation depending on the reaction conditions.     

Stille cross-couplings of unactivated secondary alkyl halides using monoorganotin reagents

The first catalyst that achieves Stille cross-couplings of secondary (as well as primary) alkyl halides has been developed. The method employs easily handled and inexpensive catalyst components (NiCl2 and 2,2'-bipyridine) and, through the use of monoorganotin reagents, avoids the formation of toxic and difficult-to-remove triorganotin side products.     

Synthesis of ferrocenyl alkenes, dienes, and enynes via samarium diiodide promoted tandem addition and dehydration of ferrocenyl carbonyls with halides

A practical method for preparation of ferrocenyl alkenes, dienes, and enynes from ferrocenyl carbonyls was explored. A one-pot operation using samarium diiodide to promote the condensation reactions of ferrocenecarboxaldehyde, acetylferrocene, benzoylferrocene, and butanoylferrocene with benzyl bromides, allyl bromide, propargyl bromide, and 1-bromo-3-chlorobutane gave the olefinic products in very high yields. The condensation reactions were also achieved by using catalytic amounts of SmI(2) with magnesium to regenerate the divalent samarium reducing agent.     

The Barbier-Grignard-type carbonyl alkylation using unactivated alkyl halides in water

The aqueous Barbier-Grignard-type alkylation of aldehydes with unactivated alkyl iodides and bromides was developed. By using a combination of zinc and cuprous iodide, catalyzed by indium(I) chloride, we successfully added tertiary, secondary, and primary alkyl halides to various aromatic aldehydes in 0.07 M aqueous Na2C2O4. A mechanistic rationale for the success of the reaction has been proposed.     

Nickel-catalyzed carbonylative cycloaddition of allyl halides and alkenes

The carbonylative addition between allyl halides and alkenes is described. The [2+2+1] reaction is catalyzed by Ni(I), and takes place with different strained alkenes under very mild conditions. Changes in solvent and use of different amounts of water in the reaction produce changes on the final products obtained.     

Palladium-catalyzed silylations of hydrosilanes with aryl halides using bulky alkyl phosphine

[Reaction: see text]. A palladium-catalyzed direct trialkylsilyl transfer to aryl halides has been developed. In the presence of Pd(t-Bu3P)2 and K3PO4, electron-rich para- or meta-substitute aryl iodides were coupled efficiently with triethylsilane, triphenylsilane, and dimethylphenylsilane to afford the corresponding silylated products in moderate to good yields.     

A novel and efficient synthesis of alkyl thiocyanates from alkyl halides in water using phase transfer catalysts

1,4-Bis(triphenylphosphonium)-2-butene dichloride(BTPBDC) and 1,4-bis(triphenyl phosphonium)-2-butene dithiocyanate (BTPBDT) were prepared and used as phase-transfer catalysts.Alkyl halides were converted efficiently to the corresponding alkyl thiocyanates under mild reaction conditions in water.No evidence for the formation of isothiocyanates as by-product of the reaction was observed.     

Organic reactions in water: an efficient zinc-mediated stereoselective synthesis of (E)- and (Z)-trisubstituted alkenes using unactivated alkyl halides

[reaction: see text] Treatment of the acetyl derivatives of the Baylis-Hillman adducts 3-hydroxy-2-methylene-alkanoates and 3-hydroxy-2-methylene-alkanenitriles with unactivated alkyl halides in the presence of Zn in saturated aqueous NH(4)Cl solution at room temperature afforded (2E)-2-substituted-alk-2-enoates in the first case and (2Z)-2-substituted-alk-2-enenitriles with high (Z)-selectivity in the second case.