Juul L. van der Baan

Intramolecular zinc-ene reactions of alkynes; preparation of 1,5-annulated 4-methylenecyclopentenes

Abstract Intramolecular Type I zinc-ene reaction of 3-(alk-m-ynyl)-2-(methoxymethyl)-2-propenylzinc bromides 2 (m = 4,5,6) gave five-, six- and seven-membered carbometallation product 3, which on Pd(0)-catalyzed cyclization were converted to 1,5-annulated 4-methylenecyclopentenes 4. Preparation of 4-methylenecyclopentenes by intramolecular Type II zinc-ene reactions of 2-(alk-m-ynyloxymethyl)-2-alkenylzinc bromides 6 (m = 2,3) followed by Pd(0)-catalyzed rearrangement of the carbometallation products 7 is not possible. Addition as well as rearrangement are slow or do not take place.

One-pot preparation of 3-methylenepyrrolidines with high diastereomeric excess

Abstract Addition of allylzinc reagents 1 to imines carrying a carbalkoxy group at C-α or C-α′, followed by Pd(0)-catalyzed cyclization permits the one-pot preparation of 3-methylenepyrrolidines with high diastereoselectivity.

3-methylenetetrahydrofurans and 3-methylenepyrrolidines by addition of 2-(bromozincmethyl)-2-alkenyl ethers or 2-(chloromagnesiomethyl)-2-alkenyl ethers to aldehydes, ketones and imines followed by pd(0)-catalyzed cyclization

Abstract Reaction of 2-(bromozincmethyl)-2-alkenyl ethers 1 or 2-(chloromagnesiomethyl)-2-alkenyl ethers 2 with aldehydes, ketones and imines affords the addition products 4 and 8 , which undergo Pd(0)-catalyzed cyclization to the 3-methylenetetrahydrofurans 6 (R 1 = H) and 3-methylenepyrrolidines 10 (R 1 = H). Addition of 1a or 1b to α,β-unsaturated substrates proceeds in the 1,2-mode only. Reaction of the 3-alkyl derivatives 1c,d,e and 2b leads in almost all cases to the branched addition products 4 or 8 . Under the conditions of ring closure, 4 or 8 (M=ZnBr) are unstable and isomerize to the corresponding linear addition products 13 or 17 . With 4 (M = ZnBr), Pd(0)-catalyzed cyclization is faster than isomerization to 13 , thus permitting regiospecific and stereoselective formation of the tetrahydrofurans 6 . In case of 8 (M = ZnBr), however, isomerization to 17 is faster than ring closure and, consequently, Pd(0)-catalyzed cyclization leads to a mixture of cyclization products 10, 19 and 20 . Branched addition products 4 and 8 , derived from magnesium compound 2b , are stable under the conditions of ring closure permitting regiospecific and stereoselective preparation of tetrahydrofurans 6 (R 1 ≠ H) and pyrrolidines 10 (R 1 ≠ H).

Allylmetallation of 1-silylalkynes by 2-(bromozincmethyl)-2-alkenyl ethers followed by Pd(0)-catalyzed cyclization: A one-pot synthesis of 4-methylenecyclopentenes

Abstract Reaction of 2-(bromozincmethyl)-2-alkenyl ethers 1a,b,c,d with 1-(trimethylsilyl)-1-alkynes 2 afforded carbometallation products 3, which were converted by Pd(0)-catalyzed cyclization to 4-methylenecyclopentenes 5. The rates of both the addition and cyclization step depend on the concentration of the organozinc compound and the preparation of 5 is best performed using a 1.4–1.8 M solution of 1. At lower concentrations reaction times must be longer and, when 1c is reacted, the addition is incomplete and gives rise to a mixture of products (3 and 14), which on Pd(0)-treatment leads to a mixture of isomeric methylenecyclopentenes (5, 15 and 16).

Addition of 2-(chloromagnesiomethyl)2-alkenyl ethers to epoxides followed by Pd(0)-catalyzed cyclization: a one-pot synthesis of 3-methylenetetrahydropyrans

Abstract Addition of 2-(chloromagnesiomethyl)-2-propenyl ethers 1a and 1b to epoxides 3 affords ring opening products 6 or 7 which are converted by Pd(0) to 3-methylenetetrahydropyrans 9 . Cyclization of the addition products is best effected by a catalyst system generated in situ from Pd(OAc) 2 and (i-PrO) 3 P.

Asymmetric addition of B-[2-(phenoxymethyl)-2-propenyl]bis(2-isocaranyl)borane to aldehydes and further cyclization to chiral 3-methylenetetrahydrofurans

Allylboration of aldehydes with the functionalized B-allylbis(2-isocaranyl)boranes 3 afforded functionalized homoallyl alcohols 4 with 71–91% e.e.. After deprotonation 4 can be cyclized under Pd(O) catalysis to give chiral 3-methylenetetrahydrofurans 5.

Intramolecular alkoxycobaltation: a novel route to the cobalt–carbon bond in a coenzyme B12 model

CoII(salen) derivatives whose ethanediyl moiety carries an ω-alkenyl side chain react with oxygen and methanol or ethanol to give, via interaction between intermediate CoIII species and the pendant olefin, alkylated products with a β-alkoxy substituted carbon bridge between cobalt and the equatorial ligand; the corresponding intermolecular reaction between CoII(salen) and hex-1-ene does not take place.

A facile conversion of arginine into β-homoarginine dipeptides

Abstract Irradiation of the arginine derived diazomethyl ketone 2 in the presence of amino esters gives the corresponding β-homoarginine dipeptide esters 3c–h in 45–76% yield.

Konjugierte addition von 2-(Bromzinkmethyl)3-phenoxy-1-propen an Methylenmalonsäurederivate; Weitere Umsetzung zu Methylencyclopentanen

Abstract Addition of the title zinc reagent to the CC bond of methylenemalonic acid derivatives followed by aqueous work-up, deprotonation with NaH and Pd(O)-catalyzed cyclization leads to methylenecyclopentanes.

Bridged (alkoxo)CoIII(salen) complexes: synthesis and structure

Abstract The preparation and oxygenation of CoII(salen) complexes 2a–d whose ethanediyl moieties carry an ω-hydroxyalkyl pendant arm is reported. Air oxidation of 2a gives bridged (alkoxo)CoIII(salen) complex 1a with a planar salen ligand. Upon aeration of 2b in MeOH, monomeric bridged complex 1b with a planar salen ligand precipitates, whereas in CH2Cl2 a dimeric bridged complex 1b′ with twisted-salen ligands is obtained and characterized by X-ray crystallography. Formation of a bridged (alkoxo)CoIII(salen) complex upon aeration of 2c is probably followed by β-elimination. The tert-butyl substituted oxidation product of 2d is paramagnetic in non-coordinating and diamagnetic in coordinating solvents, supporting the concept that the salen moiety of (alkoxo)CoIII(salen) complexes can have either a planar or a twisted geometry depending on the solvent and the solid state properties.