Alkylidynes cycloadditions

The reversible [2+2] cycloaddition of metal alkylidyne or Fischer-type metal carbyne complexes remains the only general methodology for the synthesis of metallacyclobutadiene complexes. Recent literature revolves principally around the heavier group 6 metals and the investigation of intermediates in catalytic alkyne metathesis (Scheme 25 Equation 45) <1996CHEC-II(lb)887> (W <2005OM4684>, Mo <2003JOM56>). 

Alkynes enter into many cycloaddition reactions and by analogy so do some alkylidyne complexes. Selected examples are shown in Figure 5.43 however, those involving alkynes are reserved for discussion in the following section (alkyne metathesis). 

Molybdenum and tungsten carbyne (alkylidyne) complexes frequently undergo 2+2 cycloaddition reactions with alkynes to give the corresponding metallacyclobutadiene... 

Hydrolysis of the vinyl groups from the metal center affords olefins. Weiss, Schubert, and Schrock investigated the reaction of the alkylidyne complex W(CCMe3)Cl3(dme) with cyclohexyl isocyanate (208). Two isocyanate molecules are incorporated into the complex as shown in Eq. (212). The reaction was postulated to proceed via a cycloaddition adduct of isocyanate to the metal-carbon triple bond and cleavage of the four-mem-bered ring into metal imido and ketenyl species. Subsequent insertion of a second isocyanate into the metal-ketenyl bond would then give the observed product. 

By analogy to olefin metathesis, alkyne metathesis occurs between a complex containing a metal-carbon triple bond - a metal carbyne (or alkylidyne) - and an alkyne substrate [66]. As illustrated in Fig. 4.21, this mechanism parallels the Chauvin mechanism for olefin metathesis after alkyne coordination to the metal center, [2 - - 2] cycloaddition between the metal carbyne and the alkyne yields a metallacyclobutadiene, which rearranges and fragments productively to afford a new carbyne and a new alkyne (Fig. 4.21) [54]. 

A similar initial cycloaddition step is proposed in the reaction of the molybdenum alkylidynes TpMo( = CC4H3S-2)(CO)2 with the heterocumulenic mesityl isoseleno-cyanate (SeCNC6H2Me3-2,4,6) to provide a mixture of products including the... 

Scheme 38. Proposed metal-alkylidyne/carbon disulfide cycloaddition step (L = CO, PPh3).

In Section 24.12, we introduced alkene (olefin) metathesis, i.e. metal-catalysed reactions in which C=C bonds are redistributed. The importance of alkene and alkyne metathesis was recognized by the award of the 2005 Nobel Prize in Chemistry to Yves Chauvin, Robert H. Grubbs and Richard R. Schrock for the development of the metathesis method in organic synthesis . Examples of alkene metathesis are shown in Figure 27.3. The Chauvin mechanism for metal-catalysed alkene metathesis involves a metal alkyli-dene species and a series of [2 + 2]-cycloadditions and cycloreversions (Figure 27.4). Scheme 27.6 shows the mechanism for alkyne metathesis which involves a high oxidation state metal alkylidyne complex, L M=CR. 

While in alkane metathesis mechanism (Scheme 20, b), the n-decane undergoes o-bond metathesis to generate methane and the W-bis-decyl species which, upon P-H elimination, produces the W-H with a coordinated olefin. Further, the a-hydrogen transfer from the alkyl to alkylidyne forms the hydrido W-bis-carbene [55, 76]. This upon [2-1-2] cycloaddition and cycloreversion gives an internal olefin and hydrido W-bis-carbene. Successive insertion/elimination steps (by chain walking) [77] give the terminal alkene, which reacts to a new W-alkylidene. The CH activation of the pendant W-hydride with -decane followed by p-H elimination provides 1-decene. A second metathesis between 1-decene and newly formed W-alkylidene followed by hydrogenolysis produces the alkane. 

The metallo-square hauvin s mechanism is still operating, as demonstrated by the isolation of metallacyclobutadiene complexes formed by cycloaddition of alkynes to alkylidyne complexes. The metallacyclobutadiene complexes themselves can also serve as alkyne metathesis catalysts confirming their intermediacy in the catalytic reactions starting from the alkylidyne complexes. Interestingly, they do not react readily with alkenes, rendering alkyne metathesis selective in the presence of olefmic bonds. ... 

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