Carbenes alkynols

REACTIONS OF OsHCI(CO)(P Pr3)2 WITH ALKYNOLS FORMATION OF a,p-UNSATURATED CARBENE COMPLEXES... 

The formation of cyclic Fischer carbene complexes by ruthenium-mediated activation of 3-butyn-l-ol and 4-pent5m-l-ol has been reported before (147,155,158-164). Correspondingly, a reaction of [Ru(bdmpza)Cl(PPh3)2] (24) with these terminal alkynols results... 

Obviously, the first intermediates in the syntheses with terminal alkynols are the vinylidene complexes [Ru(bdmpza)Cl(=C= CH(CH2) +iOH)(PPhg)] (n = 1, 2), which then react further via an intramolecular addition of the alcohol functionality to the a-carbon (Scheme 22), although in none of our experiments we were able to observe or isolate any intermediate vinylidene complexes. The subsequent intramolecular ring closure provides the cyclic carbene complexes with a five-membered ring in case of the reaction with but-3-yn-l-ol and with a six-membered ring in case of pent-4-yn-l-ol. For both products type A and type B isomers 35a-I/35a-II and 35b-I/ 35b-II are observed (Scheme 22, Fig. 22). The molecular structure shows a type A isomer 35b-I with the carbene ligand and the triphenylphosphine ligand in the two trans positions to the pyrazoles and was obtained from an X-ray structure determination (Fig. 25). 

Then, in 1987, Dotz reported an improved procedure for this transformation, for which the use of Et20 as solvent improved the yield of the cyclic carbene complexes considerably [12]. For example, the five-membered Fischer-type carbene complex 29 (n=l) was prepared in 58 66% yield by the reaction of preformed M(CO)5(L) (M = Cr, W, L = Et20) and 3-butynol in Et20 at room temperature. The six-membered cyclic carbene complex could also be prepared by this method. This method has been applied to the preparation of functionalized cyclic Fischer-type carbene complexes from the corresponding alkynols. For example, Dotz et al. reported the preparation of various carbohydrate-functionalized cyclic Fischer-type carbene complexes, one of which is shovm in Scheme 5.9. 

Use of the produced carbene complex intermediate for further carbon-carbon bond formation has been achieved by Barluenga and coworkers [22]. Treatment of the alkynol derivatives 69 with 25mol% W(CO)5(thf) in THF at rt gave tricyclic com-... 

The anionic complexes (50) are obtained from the group VI carbonyl and the dilithium derivatives of the hydroxyalkynes HC=CCR2(OH) subsequent protonation or acylation affords propadienylidene (51) or carbene (52) complexes by complex cyclization and addition reactions (Scheme 3) (28). Attempts to obtain the dimethyl complex by reaction of the chromium alkynolate dianion with COCl2 gave only polymeric material. More stable complexes were obtained with R = aryl (73). 

Intramolecular insertion of carbon monoxide into the metal-carbene bond of the (Ej-isomer of D leads to the t/4-vinyl ketene complex intermediate E. Experimental support for this type of intermediate has been provided by the isolation of Cr( CO) 3-coordinated dienyl ketenes related to 5 (Scheme 4) [15a], and by trapping the vinyl ketene intermediates as vinyl lactone derivatives in the course of the reaction of chromium carbene complexes with 1-alkynols [15b]. 

Besides the conventional methods, the metallo-carbene route to access cyclic compounds has become a versatile tool in sugar chemistry. Synthesis of stavudine 112, an antiviral nucleoside, from an allyl alcohol [101] is realized by a Mo(CO)5-mediated cyclization reaction (O Scheme 26). Molybdenum hexacarbonyl smoothly reacts with the triple bond of 113 to generate the intermediate Mo-carbene, which undergoes a clean cyclorearrangement to yield the furanoid glycal 114. Alkynol isomerization is effected by group-6 transition metal carbonyl complexes [102]. 

Asymmetric synthesis of nucleosides via molybdenum-catalyzed alkynol cycloisomerization coupled with stereoselective glycosylations of deoxyfuranose glycals and 3-amidofuranose glycals. Journal of the American Chemical Society, 118, 6648-6659 (b) McDonald, F.E. and Chatterjee, A.K. (1997) Group VI metal-promoted endo-azacyclizations via alkyne-derived metal vinylidene carbenes. Tetrahedron Letters, 38, 7687-7690 ... 

Dihydropymn formation. Cyclization of 4-alkynols is induced by the tungsten version of a Fischer carbene complex without the need of UV irradiation, its application to a synthetic approach to altromycin-B disaccharide has been demonstrated. ... 

Constructing molecular complexity firom alkynol derivatives A journey firom Fischer carbene complexes to tandem catalysis with gold and other carbophilic Lewis acids 13SL1757. 

We report here on the behaviour of alkynols with 2 or 3 methylene spacers between alcohol and alkyne functionalities, and the differences observed for terminal or internal alkynes in their reaction with the mentioned carbene complexes. 

Occasionally, systems with complexes of ruthenium and cobalt have been reported to catalyze intramolecular cyclopropanation of allylic diazoacetate with high enantiocontrols. For cobalt-salen 39, up to 97% ee was observed (Scheme 30) (128). More recently, methods other than diazo decomposition were applied in intramolecular cyclopropanation. Activated by irradiation, [WlCOle] catalyzed the cyclopropanation of alkynol to give good yield of cyclopropane (Scheme 31) (129). The reaction was proposed via a tungsten—carbene intermediate. 

Vinylidene complexes 8.150, generated in situ by the reverse Fritsch-Buttenberg-Wiechell rearrangement, may also be trapped with heteroatom nucleophiles. Alkynols may be converted to oxygen heterocycles 8.153, or carbene complexes 8.152, according to the conditions used (Scheme 8.42)." The organometallic reagent... 

Treatment of M(C0)5[C(0Me)(CsCPh)] (M = Cr, W) with CpH modifies the carbene by Diels-Alder type addition of the alkyne across CpH. " Carbenes Cr(CO)5(carbene) react with 1-alkynols to form hydroquinone and/or vinyl lactone derivatives via vinyl ketene intermediates. Annulation reactions of Cr carbene complexes with 3-carbomethoxy-5-hexynoate and CO form part of a new regiospecific route relevant to the synthesis of... 

---