Allenylidene substitution reactions

Metal-catalyzed substitution reactions involving propargylic derivatives have not been studied in much detail until recently [311, 312]. In this context, the ability shown by transition-metal allenylidenes to undergo nucleophilic additions at the Cy atom of the cumulenic chain has allowed the development of efficient catalytic processes for the direct substitution of the hydroxyl group in propargylic alcohols [313]. These transformations represent an appealing alternative to the well-known and extensively investigated Nicholas reaction, in which stoichiometric amounts of [Co2(CO)g] are employed [314-317]. 

Some of these complexes have also been used as suitable precursors of related allenylidenes obtained through substitution or insertion reactions (see below). Other synthetic approaches to achieve selectively C/C substituted allenylidenes from reactions of dilithium derivatives Li2[C=CCR20]> obtained by deprotonation of 2-propyn-l-ols, with [M(CO)5(THF)] (M = Cr, W) and subsequent deoxygenation with phosgene are not always straightforward [10]. 

Esteruelas and coworkers reported the stoichiometric Diels-Alder type addition of dienes to the Cp-Cy double bond of allenylidene complexes to give the corresponding substituted vinylidene complexes (Equation 7.7) [33]. The results of this stoichiometric reaction prompted us to investigate the diruthenium complex-catalyzed allenylidene-ene reaction between alkenes and the Cp-Cy double bond of an allenylidene moiety. Results of inter- and intramolecular allenylidene-ene reactions providing novel coupling products between alkynes and alkenes are described in this section [34]. 

After our discovery of the chalcogenolate-bridged diruthenium complex-catalyzed propargylic substitution reactions of propargylic alcohols with a variety of nucleophiles via allenylidene intermediates, as described in the previous sections, some... 

Thiolate-bridged diruthenium complexes such as Cp RuCl(p2-SR)2RuCp Cl catalyze the propargylic substitution reaction of propargylic alcohol derivatives with various carbon-centered nucleophiles [118-120]. Ketones [119] (Eq. 88), aromatic compounds [120] (Eq. 89), or alkenes thus selectively afford the corresponding propargylated products with C-C bond formation. An allenylidene intermediate is proposed in these reactions. They are detailed in the chapter Ruthenium Vinylidenes and Allenylidenes in Catalysis of this volume. 

A number of processes catalyzed by the dithiolate-bridged species have been mentioned already however, the extensive reactivity of alkynes within these systems has led to a number of more recent reports on their use in catalysis. The allenylidene complex 363 (R = Tol, R = Me 864 salt) has been identified as an intermediate in the catalysis of propargylic alcohol substitution reactions with alcohols in high yields and with complete regioselectivities... 

Inter- and intramolecular additions of alkenes and dienes to propargylic alcohols catalyzed by thiolate-bridged diruthenium complexes have been described. The processes, a kind of allenylidene-ene reaction, generate 1,5-enynes and dienynes by reaction of propargylic alcohols with 2-arylpropenes [196] and 1,3-conjugated dienes [197], respectively. The intramolecular version of this reac-ti(Mi has been developed to give diastereo- [196, 198] or enantioselective syn-substituted chromanes (Scheme 58) [199]. Recently, the results of DPT calculations indicated that nucleophilic attack of the olefinic Jt-electrons on a carbocationic... 

Nishiyashi Y, Wakiji I, Hidai M. Novel propargyhc substitution reactions catalyzed by thiolate-bridged diruthenium complexes via allenylidene intermediates. J. Am. Chem. Soc. 2000 122 11019-11020. 

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. 

In accord with the expected trans influence of the 71-acceptor aUenylidene unit [212], substitution of the chloride ligand by different anionic nucleophiles in complexes frans-[MCl(=C=C=CR R )(Pi-Pr3)2] (M = Rh, Ir) is favored, affording new aUenylidene derivatives frans-[MX(=C=C=CR R )(P/-Pr3)2] (X = I, F, OH, N3, etc.) (see reactivity studies below). Of particular interest is the behavior of the Rh(I) species frans-[RhCl(=C=C=CPh2)L2] (L = Pf-Pr3, f-Pr2AsCH2CH20Me) towards NaCsHs since the reactions lead to the clean formation of complexes 40 (Scheme 13), the only half-sandwich-type Group 9 allenylidenes presently known [206, 209]. 

Going one step beyond, the reaction of these n-donor-substituted Group 6 allenylidenes with bifunctional N,N- or W, 5-dinucleophiles opened up a fruitful route for the synthesis of an extensive family of N- or 5-heterocyclic carbenes. Thus, treatment of complex [Cr =C=C=C(NMe2)Ph (CO)5] with benzamidine, guanidine or thioacetamide has been reported to yield the a,(3-unsaturated carbenes 54 (Scheme 16) [62], arising from nitrogen attack at Cy, subsequent HNMe2... 

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