Tandem alkyne

Scheme 6. Vollhardt s tandem alkyne cyclotrimerization/o-quinodimethane cycloaddition strategy for polycycle synthesis.

In the above-menhoned report [26], it was also shown that complex 20 catalyzed the intermolecular tandem alkyne hydroaminahon/hydrosilylation... 

The Dotz benzannulation reaction, based on the alkyne cycloaddition to chromium carbene complexes, is the most important application of Fischer carbene complexes. Among the various Fischer carbene complexes, alkoxy and aminocarbene complexes of chromium undergo a novel inter- and intramolecular tandem alkyne insertion/ carbene annotation sequence to give 9H-carbazoles and nf/-benzo[fl]carbazoles. 

Trost, B.M., Yang, H. and Wuitschik, G. (2005) A Ru-catalyzed tandem alkyne-enone coupling/Michael addition synthesis of 4-methylene-2,6-ris-tetrahydropyrans. Organic Letters, 7, 4-761-4764. 

Indoles can also be prepared directly from o-chlorophenyl-2-alkyl alkynes and primary amines via a tandem alkyne hydroamination/Buchwald-Hartwig iV-arylation sequence, as illustrated for the conversion of 7 to 8 via intermediate enamine 9. ... 

The dirhodium tetra(triphenylacetate) (Rh2TPA4)-catalyzed decomposition of ethyl 2-diazo-3-phenylpropanoate in the presence of aryl alkynes yields the angularly substituted dihydroazulenes 115. The formation of 115 presumably takes place by a tandem alkyne insertion/Buchner ring expansion pathway via intermediates 112-114. The reactivity of aryl alkynes with 2-diazo-3-phenylpropanoate is altered dramatically by changing the catalyst from dirhodium tetra(triphenylacetate) to dirhodium tetrapivalate (Rh2Piv4) cyclopropenes 116 are formed in 40-75% yields. 

Moreover, spiroketals are produced from tandem hydroalkoxylation of 4-alkynols (Scheme 15) [130]. Starting from diynediols, bis-spiroketals are obtained using Au (I) as catalysts [131]. Furthermore, Barluenga et al. reported the formation of spirocychc compounds in a tandem alkyne hydroalkoxylation [4 -1- 2] cycloaddition reaction [132, 133], together with a tandem intramolecular hydroalkoxylation of a triple bond followed by a Prins-type cyclization [129]. 

Insertion of alkyne Tandem insertion polycyclic Terminal alkyne Coupling... 

Alkynes substituted with one or two trifluoromethyl groups are also highly reactive dienophiles [9] Indeed, hexafluoro-2-butyne is used increasingly as a definitive acetylenic dienophile in "difficult Diels-Alder reactions. It was used, for example, to prepare novel inside-outside bicycloalkanes via its reaction with cir,trnns -l,3-undecadiene [74] (equation 67) and to do a tandem Diels-Alder reaction with a l,l-bis(pyrrole)methane [75] (equation 68) Indeed, its reactions with pyrrole derivatives and furan have been used in the syntheses of 3,4-bis(tri-fluoromethyl)pyrrole [76, 77] (equation 69) and ],4-bis(trifluoromethyl)benzene-2,3-oxide [78] (equation 70), respectively. 

The selectivity for two-alkyne annulation can be increased by involving an intramolecular tethering of the carbene complex to both alkynes. This was accomplished by the synthesis of aryl-diynecarbene complexes 115 and 116 from the triynylcarbene complexes 113 and 114, respectively, and Danishefsky s diene in a Diels-Alder reaction [70a]. The diene adds chemoselectively to the triple bond next to the electrophilic carbene carbon. The thermally induced two-alkyne annulation of the complexes 115 and 116 was performed in benzene and yielded the steroid ring systems 117 and 118 (Scheme 51). This tandem Diels-Alder/two-alkyne annulation, which could also be applied in a one-pot procedure, offers new strategies for steroid synthesis in the class O—>ABCD. 

Keywords Alkenes Alkynes Carbene complexes Ruthenium Tandem reactions... 

Scheme 20 Mechanism for iron-catalyzed tandem coupling/hydroarylation of aldehyde, aromatic alkyne, and anilines...

NHC-Pd(ll) complexes have also been used in tandem reactions involving a Sonogashira couphng and hydroalkoxylation of the resulting alkyne for the synthesis of benzofurans [131] as well as sequential Heck-Sonogashira couphngs [132] (Scheme 6.44). 

The direct, stereoselective conversion of alkynes to A-sulfonylazetidin-2-imines 16 by the initial reaction of copper(l) acetylides with sulfonyl azides, followed, in situ, by the formal [2+2] cycloaddition of a postulated A-sulfonylketenimine intermediate with a range of imines has been described <06AG(E)3157>. The synthesis of A-alkylated 2-substituted azetidin-3-ones 17 based on a tandem nucleophilic substitution followed by intramolecular Michael reaction of primary amines with alkyl 5-bromo-4-oxopent-2-enoates has been... 

The base-catalyzed intramolecular cyclization of appropriately substituted 4-alkyn-1-ols, followed by in situ Claisen rearrangement, has been investigated by Ovaska and coworkers (Scheme 6.85) [179]. The tandem cydization-Claisen rearrangements were best carried out in N,N-dimethylformamide or phenetole as solvent in the pres-... 

A tandem enolate-arylation-allylic cyclisation, in which an essential z-butyldimethylsilyl ether protecting group delays the cyclisation step until the Pd-catalysed arylation is complete, enables 1-vinyl-l//-[2]benzopyrans 54 to be prepared from 2-bromobenzaldehyde (Scheme 32) <00CC1675>. 4-Substituted isochromans 55 are formed from aldehydes by a Pd-catalysed termolecular queuing cascade. The sequence involves cyclisation of an aryl iodide onto a proximate alkyne followed by an allene insertion. Transmetallation with indium then allows addition to the aldehyde (Scheme 33) . 

Tandem hydroacylation-isomerization of 5-alkynals catalyzed by a cationic rhodium(l)/BINAP complex was applied to the short synthesis of dihydrojasmone (Scheme 49).88... 

The vinyl metal intermediate arising from intermolecular nucleophilic addition of an oxygen nucleophile to a metal-alkyne complex has been harnessed for further transformations prior to protonation. An example is the ruthenium-catalyzed benzannulation of 1,5-enedyines that occurs through a tandem sequence involving hydroalk-oxylation, carbometallation, and protonation (Equation (82)).293... 

Tandem Reductive Alkyne Silylation/C-C Bond Formation 809... 

The key features of the catalytic cycle are trapping of the radical generated after cycliza-tion by an a,P-unsaturated carbonyl compound, reduction of the enol radical to give an enolate, and subsequent protonation of the titanocene alkoxide and enolate. The diaster-eoselectivity observed is essentially the same as that achieved in the simple cyclization reaction. An important point is that the tandem reactions can be carried out with alkynes as radical acceptors. The trapping of the formed vinyl radical with unsaturated carbonyl compounds occurs with very high stereoselectivity, as shown in Scheme 12.21. 

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