Allenylidene intramolecular cyclization

Scheme 27 Intramolecular cyclization between acetate and allenylidene ligands...

On the basis of these findings, a pathway for this cydoaddition is proposed in Scheme 7.24. The first step is the nucleophilic attack of the carbon atom in the 2-position of 1,3-cyclohexanedione on the Cy atom of the allenylidene complex to give a vinylidene complex, which is transformed into an alkenyl complex by intramolecular nucleophilic attack of the oxygen atom of a hydroxy group of an enol on the C, atom of the vinylidene complex. By the use of Ic with its bulkier alkanethio moiety as a catalyst and at lower temperature, a subsequent intramolecular cyclization may be slow enough to make isolation of the alkylated product possible. 

Thiolate-bridged dirutheniutn complexes catalyze the [3-f3] cycloaddition reaction between propargylic alcohols and cyclic 1,3-dicarbonyl compounds to afford 4,6,7,8-tetrahydrochromen-5-ones or 4//-cyclopenta[b]pyran-5-ones [193] and with 2-naphthols or phenols to afford l//-naphtho[2,l-b]pyrans and 4//-l-benzo-pyrans, respectively [194]. This cycloaddition is considered to proceed by stepwise propargylation and intramolecular cyclization (carbon and oxygen nucleophile additions) reactions, where ruthenium allenylidene and vinylidene complexes are the key intermediates (Scheme 57). Enantioselective mthenium-catalyzed [3-f3] cycloaddition of propargylic alcohols with 2-naphthols has also been described [195]. 

Although the chemistry of pentatetraenylidene complexes [M]=C(=C)3=CR R has not received as much attention as that of aUenylidenes, there is ample experimental evidence to confirm the electrophilic character of the C, Cy and carbons of the cumulenic chain [26-29, 31]. Thus, treatment of tra s-[RuCl(=C=C=C=C=CPh2) (dppe)2][PFg] (132) with alcohols or secondary amines resulted in addition of the nucleophilic solvent across the Cy=Cs double bond to give alkenyl-allenylidenes 138 (Scheme 48) [358]. In chloroform, electrophilic cyclization with one of the Ph groups occurred to give 139. This transformation is actually the parent of the later observed allenylidene to indenylidene intramolecular rearrangement (Scheme 15). 

Only a few cyclization processes of this type have been described up to now. Most of them result from inter- or intramolecular attacks of anionic nucleophiles containing at least two reactive heteroatoms. Thus, sodium dimethyldithiocarbamate reacts with the cationic allenylidene complex [RuTp(=C=C=CPh2)(PPh3)2][PF6] (32) to generate the allenyl-metallacycle 33 (Scheme 2.12), as the result of the nucleophilic addition of one of the sulfur atoms at the carbon and subsequent coordination of the second... 

---