Transition metal catalyzed C-C bond formations belong to the most important reactions in organic synthesis. One particularly interesting reaction is olefin metathesis, a metal-catalyzed exchange of alkylidene moieties between alkenes. Olefin metathesis can induce both cleavage and formation of C-C double bonds. Special functional groups are not necessary. Although this reactionwhich can be catalyzed by numerous transition metals-is used in industry, its potential in organic synthesis was not recognized for many years. The recent abrupt end to this Sleeping-Beauty slumber has several reasons. Novel catalysts can effect the conversion of highly functionalized and sterically demanding olefins under mild reaction conditions and in high yields. Improved understanding of substrate -catalyst interactions has greatly contributed to the recent establishment of olefin metathesis as a synthetic method. In addition to the preparation of polymers with fine-tuned characteristics, the metathesis today also provides new routes to compounds of low molecular weight. The highly developed ring-closing metathesis has been proven to be a key step in the synthesis of a growing number of natural products. At the same time interesting applications can be envisioned for newly developed variants of bimolecular metathesis. Improvements in the selective cross-metathesis of acyclic olefins as well as promising attempts to include alkynes as viable substrates provide for a vivid development of the metathesis chemistry.

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COMMUNICATIONSoxidative addition of 1 to the Pto complex, ["] followed by reductive elimination of diborane (Scheme 4). Formation of D could be suppressed by using [Pt(PPh,),] as a catalyst, that is, in the presence of excess PPh,. 1 A -LnP/ SIR', D Scheme 4. Possible route of formation of a bis(sily1)platinum complexThe silaboration reaction made possible a regioselective C-C bond formation at the boryl-substituted C atom. For example, one-carbon homologation of 2 a and 5, prepared from 1 -octene and 1-octyne, respectively, was carried out (Scheme 5). Treatment of 2a and 5 with chloro(trimethy1silyI)methyllithium gave the corresponding homologation products 6 and 7, respectively, after selective removal of the trimethylsilyl group by tetrabutylammonium fluoride (TBAF) .["] SiMqPh a,b 74% 2a -Hex 0, .o PhMe Si uB a,b 72% Hex 5 -7 Scheme 5. Synthesis of 6 and 7: a) Me,SiCH,Cl, sBuLi, N,N,N',N'-tetramethyl ethylene diamine, THF, -78°C b) TBAF, THF, reflux.Here we have presented preliminary results on the regioselective silaboration of simple alkenes. Optimization of the reaction conditions on the basis of the mechanism shown in Scheme 3 is currently underway. Experimental SectionGeneral procedure for silaboration of alkenes: To a solution of platinum catalyst (0.076 mmol) in dioxane (1 mL) were added silylborane 1 (3.9 mmol) and alkene (5.9 mmol). The mixture was heated at reflux for 2 h. Evaporation of volatile materials under reduced pressure was followed by column chromatography on silica gel to give 2 and 3 in the yields listed in Tables 1 and 2.

Übergangsmetallkatalysierte C‐C‐Verknüpfungen gehören zu den wichtigsten Reaktionen in der organischen Synthese. Eine interessante Reaktion aus dieser Klasse ist die Olefinmetathese, ein metallkatalysierter Austausch von Alkylidengruppen zwischen Olefinen. Die Olefinmetathese ermöglicht die Spaltung und Knüpfung von C‐C‐Doppelbindungen. Besondere funktionelle Gruppen sind dabei nicht erforderlich. Obwohl die durch eine Vielzahl von Übergangsmetallen katalysierte Reaktion industriell genutzt wird, lag ihr Potential für die organische Synthese lange Zeit weitgehend brach. Daß dieser Dornröschenschlaf vor kurzem ein jähes Ende fand, hat mehrere Gründe. So ermöglichen neue Katalysatoren die Umsetzung hochfunktionalisierter und sterisch anspruchsvoller Olefine unter milden Reaktionsbedingungen und in hohen Ausbeuten. Ein verbessertes Verständnis der Substrat‐Katalysator‐Wechselwirkungen hat wesentlich dazu beigetragen, daß sich die Olefinmetathese derzeit als Synthesemethode etabliert. Außer der Herstellung von Polymeren mit maßgeschneiderten Eigenschaften eröffnet die Metathese heute auch neue Zugänge zu komplexen niedermolekularen Verbindungen. Die bereits hochentwickelte Ringschlußmetathese bewährt sich als Schlüsselschritt in der Synthese einer wachsenden Zahl von Naturstoffen. Zugleich zeichnen sich für neuentwickelte bimolekulare Metathesevarianten interessante Anwendungen ab. Fortschritte bei der selektiven gekreuzten Metathese acyclischer Olefine lassen ebenso wie vielversprechende Ansätze zur Einbeziehung von Alkinen eine weiterhin lebhafte Entwicklung der Metathesechemie erwarten.

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