A series of (isonitrile)tungsten carbonyl complexes of type W(CO) m A C H T U N G T R E N N U N G (CNR) n has been synthesized and evaluated as hydrostannation catalysts. The results are compared with those obtained by the previously reported tri(tert-butylisonitrile)molybdenum tricarbonyl catalyst, Mo(CO) 3 A C H T U N G T R E N N U N G (CN-t-Bu) 3 (= MoBI 3 ). The yields and selectivities strongly depend on the isonitriles used, and with certain substrates better results are obtained compared to the molybdenum catalyst. No side products are observed in hydrostannations under microwave irradiation or when the reactions are carried out under an atmosphere of carbon monoxide. Based on these findings, a mechanistic rational is given, explaining the different pathways responsible for the formation of hydrostannation or distannation products.
Propargylic sulfones undergo regioselective hydrostannation in the presence of Mo(CO) 3 (CNt-Bu) 3 (MoBl 3 ), giving rise to stannylated allyl sulfones. These are interesting building blocks, because in the first step the vinylstannane subunit can be modified via Stille coupling, and in the second step the sulfone can be subjected to a Julia-Lythgoe olefination. In principle, modifications at the double bond should also be possible after the Stille coupling.
Professor Theophil Eicher zum 75. Geburtstag gewidmetMetallorganische Substrate sind dank ihrer Vielseitigkeit und ihres breiten Einsatzgebietes zu unverzichtbaren Ausgangsverbindungen in der modernen Chemie geworden, insbesondere für Kreuzkupplungsreaktionen.[1] Sowohl bei der Synthese als auch bei der Verwendung der hierbei benötigten metallorganischen Substrate wurden in den letzten 40 Jahren enorme Fortschritte erzielt. Neben einfach metallierten Verbindungen sind vor allem auch dimetallierte Substrate interessant, insbesondere wenn es gelingt, die unterschiedlichen metallorganischen Zentren selektiv zur Reaktion zu bringen.Von Mitchell et al. stammen hierzu interessante Arbeiten über die Palladium-katalysierte Distannylierung und Silylstannylierung von Alkinen und Allenen.[2] Von besonderem Interesse ist die katalytische Dimetallierung der Allene, [3] da die entstehenden Produkte sowohl eine vinylische als auch eine allylische Metallgruppe enthalten. Beide metallorganischen Funktionalitäten lassen sich gezielt und selektiv umsetzen, was die Produkte der katalytischen Dimetallierung von Allenen zu wertvollen, hoch funktionalisierten Bausteinen für die organische Synthese macht. [4][5][6] Neben Diborierungen [6] und der Addition von Stannylgermanen [7] und Silylboranen [5, 8] an Allene und Alkine [9] wurde auch die direkte katalytische Disilylierung von Allenen beschrieben, die jedoch deutlich höhere Reaktionstemperaturen erfordert.
Mo(CO) 3 (CNt-Bu) 3 (MoBI 3 ) was found to be a suitable catalyst for the regioselective hydrostannation of several types of alkynes, especially propargyl alcohol derivatives, affording preferentially the a-stannylated products. If propargylic acetates are used, the stannylated allylic acetates produced are suitable substrates for Pd-catalyzed allylic alkylations. Allenyl carbinols also undergo regioselective hydrostannation in the presence of MoBI 3 , because the allenyl carbinols are more reactive than alkynes, and therefore milder reaction conditions are possible. Allylstannanes are formed preferentially, which can easily be converted into allyl iodides or vinyl epoxides.
Propargylic ethers and acetates can be converted into distannylated alkenes in the presence of Bu3SnH and a Pd catalyst. The reaction proceeds via a stannylated allyl alcohol derivative, which undergoes elimination and subsequent dimetalation. If a molybdenum catalyst is used in the hydrostannylation step, and the stannylated intermediates can be reacted with other dimetallic compounds.
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