Ariel Haskel

Designing new catalytic C–C and C–N bond formations promoted by organoactinides

Abstract Organoactinides of the type Cp 2 * AnMe 2 (Cp * =C 5 Me 5 ; An=Th; U) are active catalytic precursors for the oligomerization of terminal alkynes HC≡CR (R=alkyl, aryl, SiMe 3 ). The regioselectivity and the extent of oligomerization depend strongly on the alkyne substituent R, whereas the catalytic reactivity is similar for both organoactinides. Reaction with tert -butylacetylene yields regioselectively the E-2,4-disubstituted 1-buten-3-yne dimer whereas trimethylsilylacetylene is regioselective trimerized to the E,E-1,4,6-tris(trimethylsilyl)-1,3-hexadiene-5-yne, with small amounts (3–5%) of the corresponding E-2,4-disubstituted 1-buten-3-yne dimer. Oligomerization with less bulky alkyl and aryl substituted alkynes produces a mixture of higher oligomers with no regioselectivity. Using the Cp 2 * ThMe 2 catalyst, we have recently developed a strategic method to control the extent and in some cases the regioselectivity of the catalyzed oligomerization of nonbulky terminal alkynes to dimers and/or trimers. The metallocene catalytic precursors ensure the selective synthesis of small oligomers by the addition of specific amines. Catalytic “tailoring” to dimer and trimers can be achieved by using small or bulky amines, respectively. Kinetic and mechanistic data for the controlling experiments argue that the turnover-limiting step involves the acetylide actinide complex formation with the rapid insertion of the alkyne and protonolysis by the amine. The analog Cp 2 * UMe 2 in the presence of primary amines induce the selective C–N bond formation, producing enamines which are tautomerized to the corresponding imines.

TPM: a new protecting group for alkynes

A new alkyne protecting group, trispyrazolylborateplatinum(methyl) (TPM), that can tolerate various reaction conditions and is particularly useful for electron-deficient acetylenes allows for synthetic manipulations of polyfunctional acetylenes. This protecting group withstands a variety of reaction conditions, including basic and acidic media, and the environment required for catalytic hydrogenation and for chromate oxidation reaction. The alkyne product is conveniently released from the protecting complex by the use of carbon monoxide under neutral conditions and ambient temperatures.