Allenylidene complexes catalytic reactions

Since the vinylcarbenes la-c and the aryl substituted carbene (pre)catalyst Id, in the first turn of the catalytic cycle, both afford methylidene complex 3 as the propagating species in solution, their application profiles are essentially identical. Differences in the rate of initiation are relevant in polymerization reactions, but are of minor importance for RCM to which this chapter is confined. Moreover, the close relationship between 1 and the ruthenium allenylidene complexes 2 mentioned above suggests that the scope and limitations of these latter catalysts will also be quite similar. Although this aspect merits further investigations, the data compiled in Table 1 clearly support this view. 

The involvement of transition-metal allenylidene complexes in homogeneous catalysis was reported for the first time by B. M. Trost and co-workers in 1992 (Scheme 35) [293-295]. The catalytic reactions allowed the preparation of a wide variety of tetrahydropyranyl and furanyl p,y-unsaturated ketones starting from hydroxy-functionalized alkynols and allylic alcohols, the key step in the catalytic... 

Scheme 35 The first catalytic reaction via allenylidene complexes as key intermediates...

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

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). 

Metal allenylidene complexes (M=C=C=CR2) are organometallic species having a double bond betv een a metal and a carbon, such as metal carbenes (M=CR2), metal vinylidenes (M=C=CR2), and other metal cumulenylidenes like M=C=C= C=CR2 [1]. These metal-carbon double bonds are reactive enough to be employed for many organic transformations, both catalytically and stoichiometrically [1, 2]. Especially, the metathesis of alkenes via metal carbenes may be one ofthe most useful reactions in the field of recent organic synthesis [3], vhile metal vinylidenes are also revealed to be the important species in many organic syntheses such as alkyne polymerization and cycloaromatization [4, 5]. 

Quite recently, some mononuclear ruthenium complexes such as [(p-cymene)RuX-(CO)(PR3)]OTf (X = Cl, OTf, R = Ph, Cy) have been found to work as catalysts for the propargylation of aromatic compounds such as furans, where some ruthenium complexes were isolated as catalytically active species from the stoichiometric reactions of propargylic alcohols (Scheme 7.27) [31]. The produced active species promoted the propargylation of furans vdth propargylic alcohols bearing not only a terminal alkyne moiety but also an internal alkyne moiety, indicating that this propargylation does not proceed via allenylidene complexes as key intermediates. 

Other Catalytic Reactions via Allenylidene Complexes as Key Intermediates... 

Results of catalytic reactions in which allenylidene complexes work as catalytic precursors [51] are not included in this chapter. 

---