Propargyl complexes reactivity

Early investigations into the chemistry of tr-bound transition metal propargyl complexes concentrated on their reactivity toward charged and uncharged electrophiles. In the former case r/ -bound allene complexes [ML (r --CH2=C=CH2)]" were formed via protonation [Eq. (21)]... 

Compared to the f7 -allyl complexes of Pd(ll) and Pt(ll), the r/ -propargyl analogs were much more prone to accept the central attack of the nucleophile [74]. A metallacyclobutene complex formed then underwent either 1,3-hydrogen shift to give trimethylenemethane complex or further addition of proton to give substituted )7 -allyl complexes (Scheme 8.44). The central carbon of r -propargyl complex of Pt was more reactive to nucleophile than that of Pd [75]. This order of the reactivity at central carbon (Pt > Pd) is the same as that of the stability... 

Electrophilic attack also occurs at the -Y-position of propargyl ligands, and this reaction converts propargyl complexes to allene complexes. An example of this protonation process is shown in Equation 12.61. This reaction has been used to initiate cycloaddition reactions. An example of this type of cycloaddition is the [3-t2] reaction of p-toluenesulfonyl isocyanate with the propargyl complex in Equation 12.62. Propargyl and Ti -allyl ligands are about equally reactive toward such cycloadditions. 

Recently, Aumann et al. reported that rhodium catalysts enhance the reactivity of 3-dialkylamino-substituted Fischer carbene complexes 72 to undergo insertion with enynes 73 and subsequent formation of 4-alkenyl-substituted 5-dialkylamino-2-ethoxycyclopentadienes 75 via the transmetallated carbene intermediate 74 (Scheme 15, Table 2) [73]. It is not obvious whether this transformation is also applicable to complexes of type 72 with substituents other than phenyl in the 3-position. One alkyne 73, with a methoxymethyl group instead of the alkenyl or phenyl, i.e., propargyl methyl ether, was also successfully applied [73]. 

Catalyst Reactivation Using Propargyl Acetate. The Wiped-Film Evaporator/02 reactivation procedure and the Capture of Active Catalyst Using Solid Acidic Support with FI2 Elution procedure (see above) both involve the separation of uncomplexed phosphine from rhodium complex. Since the value of the uncomplexed phosphine is significant, technology that does not require separation of phosphine during catalyst reactivation is desirable. 

The high reactivity of the cobalt-complexed propargylic systems has also allowed for ether oxygens to serve as nucleophiles, which has led to the fashioning of medium-sized ring ethers via ring expansion (Equation (60)), and also... 

Efforts to tune the reactivity of rhodium catalysts by altering structure, solvent, and other factors have been pursued.49,493 50 Although there is (justifiably) much attention given to catalysts which provide /raor-addition processes, it is probably underappreciated that appropriate rhodium complexes, especially cationic phosphine complexes, can be very good and reliable catalysts for the formation of ( )-/3-silane products from a air-addition process. The possibilities and range of substrate tolerance are demonstrated by the two examples in Scheme 9. A very bulky tertiary propargylic alcohol as well as a simple linear alkyne provide excellent access to the CE)-/3-vinylsilane products.4 a 1 In order to achieve clean air-addition, cationic complexes have provided consistent results, since vinylmetal isomerization becomes less competitive for a cationic intermediate. Thus, halide-free systems with... 

This unusual reactivity was subsequently extended to a broad variety of allylic and propargylic amines, in combination with other ruthenium(II) metal fragments, allowing the isolation of aminoaUenylidene complexes 33-35 [47 9] (Fig. 6). In addition, the S3mthesis of the thio-allenylidene [44] and seleno-aUenylidene [50] derivatives 36 from the in situ generated butatrienylidene tra s-[RuCl (=C=C=C=CH2)(L2)2] (L2 = dppm, dppe) and the corresponding aUyl sulfides... 

Highly reactive organic vinylidene and allenylidene species can be stabilized upon coordination to a metal center [1]. In 1979, Bruce et al. [2] reported the first ruthenium vinylidene complex from phenylacetylene and [RuCpCl(PPh3)2] in the presence of NH4PF6. Following this report, various mthenium vinylidene complexes have been isolated and their physical and chemical properties have been extensively elucidated [3]. As the a-carbon of ruthenium vinylidenes and the a and y-carbon of ruthenium allenylidenes are electrophilic in nature [4], the direct formation of ruthenium vinylidene and ruthenium allenylidene species, respectively, from terminal alkynes and propargylic alcohols provides easy access to numerous catalytic reactions since nucleophilic addition at these carbons is a viable route for new catalysis (Scheme 6.1). 

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