Titanocene(II)-promoted reaction of gem-dihalides possessing a terminal double bond. New intramolecular cyclopropanation

Olefin Metathesis by First-Row Transition Metals

Catalytic olefin metathesis based on the second- and third-row transition metals has become one of the most powerful transformations in modern organic chemistry. The shift to first-row metals to produce fine and commodity chemicals would be an important achievement to complement existing methods with inexpensive and greener alternatives. In addition, those systems can offer unusual reactivity based on the unique electronic structure of the base metals. In this Minireview, we summarize the progress of the development of alkylidenes and metallacycles of first-row transition metals from scandium to nickel capable of performing cycloaddition and cycloreversion steps, crucial reactions in olefin metathesis. In addition, we will discuss systems capable of performing olefin metathesis; however, the nature of active species is not yet known.

Titanium carbenoid-mediated cyclopropanation of allylic alcohols: selectivity and mechanism

A new method for the chemo- and stereoselective conversion of allylic alcohols into the corresponding cyclopropane derivatives has been developed. The cyclopropanation reaction was carried out with an unprecedented titanium carbenoid generated in situ from Nugent's reagent, manganese and methylene diiodide. The reaction involving the participation of an allylic hydroxyl group, proceeded with conservation of the alkene geometry and in a high diastereomeric excess. The scope, limitations and mechanism of this metal-catalysed reaction are discussed.

DENSITY FUNCTIONAL STUDY OF SELECTED MONO-ZINC AND GEM-DIZINC RADICAL CARBENOID CYCLOPROPANATION REACTIONS: OBSERVATION OF AN EFFICIENT RADICAL ZINC CARBENOID CYCLOPROPANATION REACTION AND THE INFLUENCE OF THE LEAVING GROUP ON RING CLOSURE

We report a theoretical study of the cyclopropanation reactions of EtZnCHI, (EtZn)2CH EtZnCHZnI, and EtZnCIZnI radicals with ethylene. The mono-zinc and gem-dizinc radical carbenoids can undergo cyclopropanation reactions with ethylene via a two-step reaction mechanism similar to that previously reported for the CH2I and IZnCH2 radicals. The barrier for the second reaction step (ring closure) was found to be highly dependent on the leaving group of the cyclopropanation reaction. In some cases, the (di)zinc carbenoid radical undergoes cyclopropanation via a low barrier of about 5–7 kcal/mol on the second reaction step and this is lower than the CH2I radical reaction which has a barrier of about 13.5 kcal/mol for the second reaction step. Our results suggest that in some cases, zinc radical carbenoid species have cyclopropanation reaction barriers that can be competitive with their related molecular Simmons-Smith carbenoid species reactions and produce somewhat different cyclopropanated products and leaving groups.

Fragmentation of carbohydrate anomeric alkoxyl radicals: new synthesis of chiral 1-fluoro-1-halo-1-iodoalditols

A new general methodology for the synthesis of 1,1,1-trihaloalditols by starting from 1,5-anhydro-2-deoxy-hex-1-enitol derivatives (glycals) is described. The halogens are introduced sequentially in each of the three different steps of the process. The fluorine is introduced in the first step by electrophilic fluorination of the starting glycal; next, hydroxyhalogenation of the resulting vinyl fluoride allows the addition of any halogen (F, Cl, Br or I) at will, and finally, an iodine atom is inserted through an alkoxyl radical fragmentation reaction. This methodology allows the preparation of diverse types of 1,1,1-trihalogenated compounds (R--CF(2)I, R--CFI(2), R--CFClI and R--CFBrI) under mild conditions compatible with sensitive substituents. In some cases, the diastereomeric mixtures generated from R--CFClI and R--CFBrI can be chromatographically separated, and their configuration determined by X-ray crystallographic analysis. The synthetic usefulness of these compounds has been preliminarily assessed by examining the reactivity of the fluorinated radical generated by rupture of the C--I bond.

Titanium carbene complexes as useful tools in organic synthesis

Various titanium carbene complexes are prepared by the reductive titanation of thioacetals, gem-dihalides, and related organosulfur and organohalogen compounds with the titanocene(II) reagent Cp(2)Ti[P(OEt)(3)](2). Alkylidene-, heteroatom-substituted methylidene-, 2-alkenylidene-, 2-alkynylidene-, and vinylidene-titanocenes thus formed are highly reactive toward organic compounds bearing a multiple bond and are employed for a variety of organic transformations such as carbonyl olefination and olefin metathesis.

Carbonyl olefination utilizing an alkyl halide–titanocene(ii) system

Ketones, aldehydes, esters and lactones are transformed into olefins by treatment with the organotitanium species formed from alkyl halides and titanocene(II).