Stereoselective allylation of azirines with allylindium reagents

Allylindium reagents allylated azirines to give allylaziridines in good yields. The delivery of the allyl group was well regulated by the substituents at the C³-carbon of azirines. The cis-allylation with respect to the substituent was realized with azirines bearing a hydroxylmethyl or an acetoxymethyl group due to the chelation with allylindium reagents, whereas the trans-allylation was achieved with azirines substituted by a methyl, phenyl, or ester group owing to the steric repulsion.     

Palladium-catalyzed desulfinylative C-C allylation of Grignard reagents and enolates using allylsulfonyl chlorides and esters

2-Methylprop-2-ene-, prop-2-ene-, 1-methylprop-2-ene-, and (E)-but-2-enesulfonyl chlorides have been used as electrophilic partners in desulfinylative palladium-catalyzed C-C coupling with Grignard reagents and sodium salts of dimethyl malonate and methyl acetoacetate. Neopentyl alk-2-ene sulfonates can also be used as electrophilic partners in desulfinylative allylic arylations and allylic alkylations. The regioselectivity of the allylic arylation and alkylation depends on the nature of the catalyst. With PdCl₂(PhCN)₂, (E)-crotyl derivatives are formed in high regioselectivity using either 1-methylprop-2-ene- or (E)-but-2-enesulfonyl chloride.     

Enantioselective allylation of aldehydes with chiral allyl organolanthanide reagents

The first example of enantioselective allylation of aldehydes with chiral allyl organolan-thanide reagents has been achieved in high chemical yield and moderate optical purity (up to 54% e.e.).     

Facially selective and regioselective carbometalation of cyclopropenes by aryl Grignard reagents

Described is a Cu-catalyzed methodology for adding aryl Grignard reagents to 3-hydroxymethylcyclopropene derivatives with high regio- and diastereoselectivity. The cyclopropylmetals can be trapped with a variety of electrophiles to generate highly substituted cyclopropanes.     

Enantioselective allylation of aldehydes promoted by chiral sulfur reagents

The addition of allylzinc bromide to aldehydes was studied with different chiral sulfur compounds acting as a catalyst. Yield differences and enantiomeric excesses were observed.     

Nucleophilic substitution reactions of 1-sulfonyl substituted cyclopropenes with alkyl lithium reagents

Sulfonyl substituted cyclopropenes react with a variety of alkyl lithium reagents to give disubstituted cyclopropenes in good yield.     

Enantioselective rhodium-catalyzed nucleophilic allylation of cyclic imines with allylboron reagents

Chiral allylrhodium nucleophiles: The highly diastereo- and enantioselective title reaction of a range of cyclic imines with various potassium allyltrifluoroborates most likely proceeds via allylrhodium(I) intermediates, and represents the first rhodium-catalyzed enantioselective nucleophilic allylation of π?electrophiles with allylboron compounds.     

A copper-catalyzed method for the facially selective addition of grignard reagents to cyclopropenes

A Cu-catalyzed method for the addition of Grignard reagents to 1-alkyl-3-hydroxymethylcyclopropenes and their MOM ethers is described. The face of addition is syn relative to the hydroxymethyl and alkoxymethyl groups. Excellent diastereoselectivity is observed for a range of alkyl, alkenyl, and alkynylmagnesium halides. The addition reactions create chiral all-carbon quaternary centers, and the cyclopropylmetals that are generated can be reacted with electrophiles to produce highly functionalized cyclopropanes.     

Preparation of allyltin reagents grafted on solid support: clean and easily recyclable reagents for allylation of aldehydes

The preparation of polymer-supported allyltin reagents was shown to be possible for both unfunctionalized and functionalized allyl units. These reagents were treated with aldehydes in the presence of cerium(III) or indium(III) salts to afford high yields of homoallylic alcohols, practically uncontaminated with organotin residues (less than 5 ppm). Some mechanism aspects are briefly discussed and the potential for regeneration and reuse of these supported reagents is pointed out.     

Cobalt-catalyzed multisubstituted allylation of the chelation-assisted C–H bond of (hetero)arenes with cyclopropenes

Cyclopropenes are highly strained three-membered carbocycles, which offer unique reactivity in organic synthesis. Herein, Cp*Co^III-catalyzed ring-opening isomerization of cyclopropenes to cobalt vinylcarbene has been utilized for the synthesis of multisubstituted allylarenes via directing group-assisted functionalization of C–H bonds of arenes and heteroarenes. Employing this methodology, various substituents can be introduced at all three carbons of the allyl moiety with high selectivity. The important highlights are excellent functional group tolerance, multisubstituted allylation, high selectivity, gram scale synthesis, removable directing group, and synthesis of cyclopenta[b]indoles. In addition, a potential cobaltocycle intermediate was identified and a plausible mechanism is also proposed.

Cp*Co^III-catalyzed ring-opening isomerization of cyclopropenes to cobalt vinylcarbene has been utilized for the synthesis of multisubstituted allylarenes via directing group-assisted functionalization of C–H bonds of arenes and heteroarenes.     

Catalytic enantioselective 1,2-diboration of 1,3-dienes: versatile reagents for stereoselective allylation

More with boron: The development of catalytic enantioselective 1,2-diboration of 1,3-dienes enables a new strategy for enantioselective carbonyl allylation reactions (see scheme). These reactions occur with outstanding levels of stereoselection and can be applied to both monosubstituted and 1,1-disubstituted dienes. The carbonyl allylation reactions provide enantiomerically enriched functionalized homoallylic alcohol products.     

Copper-catalyzed silylation of cyclopropenes using (trifluoromethyl)trimethylsilane

A variety of cyclopropenes undergo direct silylation using (trifluoromethyl)trimethylsilane in the presence of a copper-bisphosphine catalyst; under these conditions, cyclopropenes that might otherwise undergo ring-opening are silylated efficiently.     

A general catalytic allylation using allyltrimethoxysilane

A general and mild catalytic allylation of carbonyl compounds, applicable to aldehydes, ketones, and imines is developed using allyltrimethoxysilane as the allylating reagent. The reaction proceeds smoothly with 1-10 mol % of CuCl and TBAT in THF at ambient temperature. Mechanism studies indicated that the copper alkoxide, allylfluorodimethoxysilane, and allyltrimethoxysilane are essential to promote the reaction efficiently. Preliminary extension of the reaction to the first catalytic enantioselective allylation of ketones using an allylsilane produced the product with 61% ee from acetophenone, using a CuCl-p-tol-BINAP-TBAT catalyst (15 mol %).     

Silver-catalyzed benzylation and allylation reactions of tertiary and secondary alkyl halides with grignard reagents

Treatment of alkyl halides, including tertiary alkyl bromides, with benzylic or allylic Grignard reagent in the presence of a catalytic amount of silver nitrate in ether yielded the corresponding cross-coupling products in high yields. The coupling reactions of tertiary alkyl halides provide efficient access to quaternary carbon centers.     

Recent advances in the activation of boron and silicon reagents for stereocontrolled allylation reactions

Despite the popularity of boron and silicon allylation reagents in stereocontrolled synthesis, they suffer from a number of inherent limitations that have slowed down their development as synthetic tools for nucleophilic additions to carbonyl compounds and imine derivatives. These limitations are the low reactivity and diastereoselectivity of allyl trialkylsilane reagents, and the lack of catalytic systems for the activation and substoichiometric control of enantioselectivity in the additions of allyl boron reagents. To develop more efficient and general methods for the control of absolute stereochemistry in the resulting homoallylic alcohols, new approaches aimed at solving the problem of activation of allylic boron and silicon reagents are needed. This Minireview describes a number of recent approaches that have been devised to address this problem.     

Decarboxylative allylation using sulfones as surrogates of alkanes

Alpha-sulfonyl functional groups are traceless activating groups that facilitate catalytic decarboxylative allylations in high yield yet can be cleaved to allow the synthesis of simple allylated alkanes. Substrate studies suggest that decarboxylation to form an alpha-sulfonyl anion is rate-limiting. Furthermore, the anion is formed regiospecifically under formally neutral conditions.     

Toward a versatile allylation reagent: practical,enantioselective allylation of acylhydrazones using strained silacycles

A highly practical method for the enantioselective allylation of acylhydrazones has been developed. The previously reported strained silacycle reagent 1 reacts with a wide variety of acylhydrazones to give the hydrazide products with good enantioselectivity (83-89% ee, typically). It has been demonstrated that the products may be isolated without chromatography by recrystallization in >/=98% ee.     

Rhodium-catalyzed hydroformylation of cyclopropenes

The first catalytic diastereo- and enantioselective hydroformylation of cyclopropenes was demonstrated. The reaction proceeds efficiently under very mild conditions and low catalyst loadings providing high yields of cyclopropylcarboxaldehydes. This novel methodology represents a convenient, atom-economic approach toward optically active cyclopropylcarboxaldehydes from readily available prochiral cyclopropenes.