Allenylidene reactions

The direct comparison of 1 and 2 in a variety of RCM reactions also indicates a presumably close relationship between these catalysts (Table 1) [6]. Both of them give ready access to cycloalkenes of almost any ring size > 5, including medium sized and macrocyclic products. Only in the case of the 10-membered jasmine ketolactone 16 was the yield obtained with 2a lower than that with lc this result may be due to a somewhat shorter lifetime of the cationic species in solution. However, the examples summarized in Table 1 demonstrate that the allenylidene species 2 exhibit a remarkable compatibility with polar functional groups in the substrates, including ethers, esters, amides, sulfonamides, ketones, acetals, glycosides and even free hydroxyl groups. 

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 the case of macrocyclic rings, the situation is better understood. In contrast to earlier statements in the literature [3a], even diene substrates devoid of any conformational pre-disposition towards ring closure turned out to be excellent substrates for macrocylization reactions catalyzed by ruthenium-carbene or -allenylidene complexes. From these investigations [30], however, a set of parameters has been deduced which turned out to be decisive ... 

These reactions are thought to proceed by initial formation of the lithio propargylic alcohol adduct, which undergoes a reversible Brook rearrangement (Eq. 9.14). The resulting propargyllithium species can equilibrate with the allenyl isomer and subsequent reaction with the alkyl iodide electrophile takes place at the allenic site. An intramolecular version of this alkylation reaction leads to cyclic allenylidene products (Eq. 9.15). 

The allenylidene complex formation is indicated by a color change from yellow to purple and can be monitored by the disappearance of the vinylidene p-H NMR resonance. The reaction is completed by heating under reflux for some hours. The neutral allenylidene complexes are rather stable towards oxygen and water. According to the H, and NMR spectra, two isomers of the... 

Phosphines, as nucleophiles, add to many unsaturated substrates giving metal-lated ylides. Scheme 17 collects some representative examples of the addition of phosphines to carbyne complexes, giving (57) [132], to allenylidenes (58) [133], a-alkenyls (59) [134], or a-alkynyls (60) [135]. Moreover, reaction of phosphines with 7i-alkenes [136] and 71-aIkynes (61)-(64) [137-140] have also been reported. It is not possible to explain in depth each reaction, but the variety of resulting products provides an adequate perspective about the synthetic possibihties of this type of reactions. 

Related reactions have also been performed starting directly from M(CO)6 precursors, via decar bony lation (UV irradation) of the corresponding intermediate [M =C(0Li)C=CCR20Li] and subsequent treatment with COCI2 [43, 90, 93]. However, these reactions are not always straightforward and, in some cases, different types of products derived from subsequent cyclization or addition reactions have been obtained. As an example, reaction of the intermediate chromium complex obtained from Cr(CO)6 and [C=CCMe20] with MeCOCl led to the bicyclic dinuclear allenylidene-carbene complex 3 (see Fig. 3) [94]. 

Preparation of the pioneering manganese(I) allenylidene derivatives [MnCp(=C= C=CR2)(C0)2] (5) was accomplished by treatment of the Ti-aUcyne complex [MnCp ( 7 -HC=CC02Me)(C0)2] (4) with excess of an organolithium reagent, followed by deoxygenation with HCl or COCI2 (Scheme 5) [24,42,95]. In essence, this reaction... 

Cp=Cy bond of indenyl-allenylidene complexes 30 which leads to the stereoselective formation of cationic amino-allenylidenes 31. When R = Ph, complexes 31 can be transformed into the secondary derivatives 32 via treatment with LiBHEts and subsequent purification on sUica-gel column. Further insertions of MeC=CNEt2 into 32 allow the preparation of polyunsaturated cumulene chains (related insertion reactions will be discussed in the reactivity section). 

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]. 

The reactivity of Group 6 allenylidenes [M(=C=C=CR R )(CO)5] (M = Cr, W R and R = alkyl, aryl or H) towards nucleophiles is clearly dominated by the additions at the electrophilic a-carbon. In this sense, the most common reaction of these complexes (usually generated in sim) is the addition of alcohols R OH across the C =Cp bond to afford Fischer-type a,p-unsaturated alkoxycarbene... 

Scheme 17 Some reactions of chromium allenylidenes with phosphines...

In this context, it should also be noted that an oxidatively induced C -P coupling has been described in the oxidation reactions of complexes tra i-[RhCl (=C=C=CR2)(P/-Pr3)2] (R = Ph, o-OMeCeH4) with CI2 or PhlClj, affording phos-phonio-allenyl products [RhCl3 C(P/-Pr3)=C=CR2 (Pi-Pr3)]. They are formed by migration of one Pi-Pr3 group from the metal to the allenylidene a-carbon in the six-coordinate Rh(III) intermediates [RhCl3(=C=C=CR2)(Pi-Pr3)2] [205]. 

Related ry -pentatrienyl compounds are also formed in the reactions of [OsClCp (=C=C=CPh2)(P -Pr3)] and [RuClCp (=C=C=CPh2) (P)- -Pr2PCH2C02Me ] with CH2=CHMgBr [197, 279]. In contrast, the insertion of the allenylidene ligand into the Os-C(alkenyl) bond of 69 has been reported to yield the five-membered metallacyclic compound 70 instead of the expected ry -pentatrienyl isomer (Scheme 23) [200]. 

C-C couplings with aUcynes. An unprecedented coupling of this type was found in the reaction of the Ir(I) hydroxo-allenylidenes 71 with excess of HC=CR (R = Ph, C02Me) to afford, under remarkably mild conditions (r.t.), the novel five-coordinate compounds 72 (Scheme 24). The proposed mechanism involves an initial HO /R C=C ligand exchange followed by the oxidative addition of a... 

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