Alkynol complexes

Alkynols complexed to cobalt can be oxidized to alkynals without decomplexation. Propargyl aldehydes are protected from polymerization upon complexation with Co2(CO)6. These aldehydes smoothly undergo Wittig-type reactions. Carbonyl-ene reactions have been demonstrated (Scheme 194). Complexation to cobalt protected the enyne in complex (132) from Michael-type reactions (Scheme 195). Alkenyl-substituted complexes undergo [3 + 2]cycloadditions with nitrile A-oxides (Scheme 196). 

In agreement with the tendency shown by 133 to release a phosphine ligand, the treatment of this compound with 1 equiv of l,l-diphenyl-2-propyn-l-ol in pentane at room temperature leads to the 71-alkynol complex Os(ti5-C5H5)C1 ti2-HCsC(OH)Ph2 (P Pr3) (150). In toluene at room temperature, this compound is stable. However, at 85 °C, it evolves into the allenylidene derivative Os(ti5-C5H5)Cl(C=C=CPh2)(P Pr3) (151), which has a very remarkable nucleophilic character [58]. 

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

The reactivity of OsHCl(CO)(P Pr3)2 toward alkynols depends on the substituents at the C(OH) carbon atom of the alkynol (Scheme 14).47 The reaction with 2-propyn-1 -ol initially affords the alkenyl compound 6s(CI I=CI ICII2OI I)Cl(CO) (P Pr3)2 in 85% yield, as a result of the anti-addition of the Os—H bond to the carbon-carbon triple bond of the alkynol. In chloroform-df solution this complex decomposes to a mixture of products, containing the derivatives OsCl2(CHCH=CH2) (CO)(P Pr3)2 and 6s(CHCHCH6)Cl(CO)(P Pr3)2 (Eq. 5). 

The formation of the complexes shown in Scheme 14 and Eq. (5) has been rationalized according to Scheme 15. Thus, it has been proposed that the insertion of the Os—H bond of OsHCl(CO)(P Pr3)2 into the carbon-carbon triple bond of the alkynol initially gives five-coordinate (E )-alkenyl intermediates, which subsequently isomerize into the Os CH=CHC(OH)R1R2 derivatives. The key to this isomerization is probably the fact that the five-coordinated ( )-alkenyl intermediates are 16-electron species, while the Os CH=CHC(OH)R1R2 derivatives are... 

In addition to the W and Mo carbonyl complexes that have most commonly been used for the cycloisomerization of alkynols, an Rh-based catalyst system has recently been developed which uses substantially lower catalyst loadings (1.5-2.5 mol%) than have typically been required for the W and Mo systems (10-50 mol%).369 Among the various ligands studied, P(/>-F-C6H4)3 proved to be particularly effective. Interestingly, this ligand has also been found to be optimal for an Ru system that catalyzes the same type of cycloisomerization (Equation (104)).370,371... 

Substituting the benzene ring with a double bond, Pd-catalyzed intramolecular alkoxylation of alkyne 122 also proceeded via an alkenyl palladium complex to form furan 123 instead of a benzofurans [99, 100]. In addition, 3-hydroxyalkylbenzo[fc]furans was prepared by Bishop et al via a Pd-catalyzed heteroannulation of silyl-protected alkynols with 2-iodophenol in a fashion akin to the Larock indole synthesis, [101]. 

Alkynes react with the bulky germanium hydride (MejSdjGeH to selectively yield (Z)-alkenes (Equation (105)).67 The hydrogermylation of alkynols or alkynes can be catalyzed by a rhodium complex (Equation (106), Table 18) and some of the intermediates were identified (Scheme 16).132 Similar rhodium species react with alkynes to yield alkenyl complexes,133 and other transition metal complexes have been employed as hydrogermylation catalysts including those containing palladium.134,135... 

To summarize this part, various palladium complexes efficiently catalyze the lactonization of alkynols. Many mechanistic studies remain to be carried out to have a clear understanding of the mechanism in order to anticipate the reactivity of such substrates. Nevertheless, the implication of palladium-hydride intermediates should take a large place in this catalysis. 

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). 

The involvement of transition-metal allenylidene complexes in homogeneous catalysis was reported for the first time by B. M. Trost and co-workers in 1992 (Scheme 35) [293-295]. The catalytic reactions allowed the preparation of a wide variety of tetrahydropyranyl and furanyl p,y-unsaturated ketones starting from hydroxy-functionalized alkynols and allylic alcohols, the key step in the catalytic... 

Finally, it should be mentioned that the reaction of the dianion derived from alkynol HC=CC(OH)f-Bu2 with [Fe(CO)4(NMe3)] led to a complex mixture of mono- and polynuclear products, from which the iron(0)-allenylidene complex 42 (Fig. 7) could be isolated [55]. 

The majority of synthetic approaches for allenylidene complexes use propargylic alcohols HC = CCR R (OH) as sources of the allenylidene C3 skeleton. In 1982, Selegue first introduced this synthetic strategy for the high yield preparation of the ruthenium(II) complex [RuCp(=C=C=CPh2)(PMe3)2][PF6] [8]. The alkynol is converted smoothly into the allenylidene unit via elimination of water (Equation 2.1). 

Alkynol Cycloisomerization Promoted by Croup 6 Metal Complexes... 

Another focus of this chapter is the alkynol cycloisomerization mediated by Group 6 metal complexes. Experimental and theoretical studies showed that both exo- and endo- cycloisomerization are feasible. The cycloisomerization involves not only alkyne-to-vinylidene tautomerization but alo proton transfer steps. Therefore, the theoretical studies demonstrated that the solvent effect played a crucial role in determining the regioselectivity of cycloisomerization products. [2 + 2] cycloaddition of the metal vinylidene C=C bond in a ruthenium complex with the C=C bond of a vinyl group, together with the implication in metathesis reactions, was discussed. In addition, [2 + 2] cycloaddition of titanocene vinylidene with different unsaturated molecules was also briefly discussed. 

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-... 

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