Alkynals, cyclization

Since this scheme regenerates the original coordinatively unsaturated Ti+2 centers upon desorption of the aromatic, it could, in principle, represent a catalytic cycle for heterogeneous alkyne cyclization. The present study reports a test of that h3T>othesis—the feasibility of catal5hic cyclotrimerization—on a reduced Ti02 surface in UHV. 

In a reaction similar to the (>-alkoxide elimination reactions seen with zir-conocenes, catalytic Rh(OH)(cod)2 and 2 eq. of arylboronic acids gave cyclic products 165 from enynes 166 (Scheme 35) [100]. In this reaction, transmet-allation of Rh - OR with B - Ph gave Rh - Ph species 167, which inserted into the alkyne, cyclized to 168, and finally underwent [>-alkoxidc elimination to provide Rh-OCH3. This reaction is limited to the formation of five-membered rings, but it can also undergo cascade type reactions of enediynes to give multicyclic products [100]. 

Aside from alcohols, other oxygen nucleophiles have also participated in hydroalkoxylation reactions with alkynes. The most common of these are 1,3-dicarbonyl compounds, whose enol oxygens are readily available to add to alkynes. Cyclization reactions of this type have been carried out under Pd(0) catalysis with various aryl or vinyl iodides or triflates, often in the presence of CO, affording the corresponding furan derivatives (Equation (95)).337-340 A similar approach employing cyclic 1,3-diketones has also been reported to prepare THFs and dihydropyrans under Pd, Pt, or W catalysis.341 Simple l-alkyn-5-ones have also been isomerized to furans under the influence of Hg(OTf)2.342... 

Grigg extended this alkyne cyclization to trapping with stannanes to give 3-exo-dienes [347], alkynes to afford tetracycles [338, 348], and alkenes leading to cyclopropanes [349], an example of which is illustrated. In his studies Grigg and co-workers have found that thallium and silver salts suppress direct capture of these palladium intermediates prior to capture [350]. 

Superheated and supercritical water are used in several applications. Supercritical water is most often used in the destruction of organic wastes, including some chemical warfare agents, as an alternative to incineration (Katritzky et al., 1996 Sherman et al., 1998). Recent reports describe the use of both forms as a solvent and as a reactant in synthetic chemistry (Katritzky et al., 1996 An et al., 1997). Some of the reactions investigated include metal-mediated alkyne cyclizations, Pd-catalyzed al-kene arylations, aldol reactions, the Fischer indole synthesis, and hydrolysis reactions. Waterborne coatings and the destruction of wastes in supercritical water are fully... 

Allyl sulfones offer the possibility to conduct cyclization reactions that are isomerizations, and investigations by Smith and Whitham indicate that this is a very promising technique for the preparation of functionalized products.118 The mechanism consists of reversible addition of a sulfonyl radical to a terminal alkene or alkyne, cyclization, and fragmentation of the resulting -sulfonyl radical. Two examples of these isomerizations are provided in Scheme 26. 

Radical cyclizations to carbon-nitrogen multiple bonds resemble additions to carbon-carbon multiple bonds in that they usually give products of irreversible exo cyclization. To date, the most useful acceptors have been oximes189 and nitriles,190 and one example of each type of cyclization is given in Scheme 45.191 Nitriles are useful because the intermediate imines are readily hydrolyzed by mild acid to ketones. Although this route to ketones is shorter than the two-step sequence of alkyne cyclization/ozo-nolysis, nitriles are slightly poorer acceptors than terminal alkynes, and much poorer acceptors than activated alkynes. Thus, when slow cyclizations are involved, the two-step protocol is preferable. 

Mechanistically, most alkyne cyclizations of this type have been interpreted in terms of the catalyst serving as a Lewis acid, activating the n bond to nucleophilic attack,53-57 and while this pathway is feasible, the authors preferred one involving the intermediacy of a silver carbene.52 Control studies ruled out a pathway involving... 

Evidence based on product mixtures now suggests, at least in the cases of a-halocarbonyl and perhaloalkyl starting marterials, that these reactions are in fact atom transfer radical cyclizations (equation 166)324,325. In them, the palladium catalyst is proposed to have roles both as the radical initiator and as a trap for iodine, similar to the more commonly used hexabutylditin. Intramolecular allyl halide-alkyne cyclizations proceed with trans-addition to the triple bond this is evidence that a still different mechanism may be operating in these cases (equation 167)1,326. 

Fused cycbpentenones. An intramolecular alkene-alkyne cyclization mediated by Co2(CO)8 provides a route to bicyclic [3.3.0]enones (equation I).1 This reaction provides... 

In 1987, Livinghouse and Westling published a general method to prepare 2-acylpyrrolines via acylnitrilium alkyne cyclization 163). In 1992, they presented... 

Reduction of CO and ketones Hydro formylation Fischer-Tropsch synthesis Synthesis of methanol from CO Hydrogenation of amides Alkyne cyclization Hydroxymethylation... 

Intramolecular allene alkyne cyclizations using cobalt complexes are nonspecific and both double bonds of the allene participate in the reaction affording [4.3.0] and [3.3.0]ring systems (Scheme 260). Both inter- and intramolecnlar Pauson - Khand-type reactions have been reported using... 

As is clear from the introductory discussion, most, if not all, of the d-block transition metals are expected to participate in reactions that are related to those discussed here. In addition to the Co-based methodology mentioned earlier, some related reactions of Pd and are known. Also related are the cyclization reactions of metal-carbene complexes containing Cr, Mo, W and other transition metals with alkynes and alkenes and a recently reported Nb- or Ta-promoted diyne-alkyne cyclization reaction, which appears to be closely related to a number of previously developed alkyne cyclotrimerization reactions, such as those catalyzed by Co. Investigations of reactions involving other transition metals may prove to be important especially from the viewpoint of developing asymmetric and catalytic procedures. 

Grigg and co-workers [203] have very recently described an ingenious cascade process. Tliis is initiated by the addition of Bu3SnH to alkyne cyclization and anion capture follow and afford polycyclic products (Scheme 4-52). A palladium-catalyzed hydrogenation of the double bond in ajS-unsaturated A -acyloxy azolidinones was described by Wu et al. [204] the diastereoselectivity was low. 

Oppolzer, W., Robyr, C. Synthesis of ( )-hirsutene by a catalytic allylpalladium-alkyne cyclization/carbonylation cascade. Tetrahedron 1994, 50, 415 24. 

The capnellene and hirsutene marine sesquiterpenes are ideal candidates for radical cyclizations and both have been elaborated via carbonyl-alkyne cyclizations (equations 141-143). Thus treatment of the ketone (117) with the sodium naphthalene radical anion gives the triquinane (118). Subsequent allylic... 

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