Allenylidene deprotonation

A -Methylpyrrole oxidatively adds to [Ru(C=C=C=CH2)(PPli3)2(Cp)][PF6] via its C2 center, the product being the allenylidene species 38 [98JCS(D)467]. Alkyne mesomeric form 39 was postulated to make a significant contribution, which explains well the nature of the deprotonated product 40, obtained from 38 and n-butyllithium. 

In order to overcome all these synthetic drawbacks, an alternative methodology using tetrahydrofuran solvates [M(CO)5(THF)] and deprotonated tris-amino or alkoxo prop-1-ynes was developed by H. Fischer and co-workers (Scheme 3), which allowed the preparation of the bis(amino)- and bis(ethoxy)-allenylidene complexes [M =C=C=C(NMe2)2 (CO)5] (M = Cr, W) [27] and [M(=C=C=C (OEt)2 (CO)5] (M = Cr, W) [28], respectively. Although the latter could not be isolated in pure form, they were conveniently employed as starting materials for further transformations (see below). 

This synthetic strategy proved to be unsuitable for [M(CO)5] (M = Cr, W) metal fragments due to the thermal instability of the corresponding non-donor-substituted allenylidenes [M(=C=C=CR R )(C0)5] (R, R = usually alkyl or aryl groups). An alternative general synthetic procedure using deprotonated functionalized acetylenes has been successfully applied (for example tris-amino or alkoxo prop-1-ynes see Equation 2.2) [4a]. 

Fischer and coworkers have exploited thoroughly the synthetic route based on functionalized acetylides [4, 9]. Thus, by using deprotonated propynoic acid amides (alkynyl metallate) the reaction with M(CO)5(THF)] followedby treatment with [R3O] BF4 affords N/O-substituted allenylidene complexes (Equation 2.3). 

Analogous N/C substituted allenylidenes [M =C=C=CPh(NMeR) (CO)5] (M = Cr, W R= H, Me, Et) are obtained by using C-ethynylimines HC = CC(=NMe)Phinstead of propynoic acid amides. 0/C-, 0/0-, and N/S-substituted allenylidenes are also accessible from ethynyl ketones HC = CC(=0)R, propynoic acid esters HC=CC(=0)0R and propynethioic acid amides HC=CC(=S)NR2, respectively, after sequential deprotonation and the corresponding alkylation (representative examples are shown in Figure 2.2) [4a, 9]. 

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]. 

Cationic allenylidene complexes containing a hydrogen atom in the 6 position, that is, [M]" =C=C=C(R )CHR R, are known to undergo deprotonation processes upon treatment with bases affording neutral 0-enynyl derivatives [M]-C=CC(R )= CR R. Representative examples include deprotonation of [Ru(t] -C9H7)(=C=... 

Protons on the 8 carbon of a ruthenium allenylidene complex are acidic, and deprotonation at this position often occurs to give ene-yne derivatives. Reaction of I with cyclic propargyl alcohols of type 110, for example, did not yield the allenylidene complexes, but rather the ruthenium ene-yne products (111) were isolated [Eq. (97)] (78). Reaction of the cor-... 

Mononuclear allenylidene complexes have been prepared by either (i) reaction of a deprotonated propargyl alcohol with a binary metal carbonyl or (ii) dehydration of the acetylenic alcohol HC=CCR20H with an appro-... 

Scheme 3 Synthesis of Group 6 allenylidenes from deprotonated tris-amino or alkoxo prop-1-ynes...

From a methodological point of view, it should be pointed out the formation of 51, which is a result of the addition of acetone to an allenylidene ligand. Heteroatom-containing cyclic metal-carbene complexes [24] have been conveniently prepared via metal co-haloacyl, carbamoyl, alkoxycarbonyl, or imido intermediates [25], opening of epoxides by deprotonated Fischer-type carbene complexes [26], and activation of homopropargylic alcohols with low-valent d complexes [27], including ruthenium(II) derivatives [28]. In general, the preparation of unsaturated cyclic carbene complexes requires the previous preparation of functional carbenes to react with P-dicarbonyl derivatives, acrylates, and enol ethers [29]. 

Ph=GH2)Gp (PEt3)2][BPh4] and [OsH(G=GGPh=GH2)Gp(PPp3)2][PF6] are also known. They have been obtained by dehydration of the methyl-substituted 3-hydroxyalkynyl ligand in complexes [RuH G=GG(OH)-MePh Cp (PEt3)2][BPh4] and [OsH feCC(OH)MePh Cp(PPr 3)2][PF6], respectively.Deprotonation of the optically active indenyl-ruthenium(ii) allenylidene 138 with KOBu to yield 139 has also been reported (see Equation (9)). 

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