Alkynes reaction with arenes

Reaction with C H 1.6.2.1.2 Reaction with C5H5CN 1.6.2.1.2 Reaction with H2 to produce (CH20H)2 1.6.6.1 Reaction with alkenes 1.6.6.1 Reaction with alkynes 1.6.6.1 Reaction with arenes 1.6.6.1... 

Surprisingly, the organometallic catalyst shows good stability under the reaction conditions (CFjCOOH/CH Cl ). In the absence of 107 (Fig. 2.18), Pd(OAc)j under the same conditions catalyses the same reaction with reduced activity (ca. 50% conversion in 24 h) and different chemoselectivity. Arenes, substituted by electron-withdrawing snbstitnents react slower. Both internal and terminal alkynes undergo the reaction, however, the former require more forcing conditions [90],... 

Another recent development in the field of palladium-catalyzed reactions with alkynes is a novel multicomponent approach devised by the Lee group. Starting from a-bromovinyl arenes and propargyl bromides, the assembly ofeight-membered car-bocycles can be realized via a cross-coupling/[4+4] cycloaddition reaction. The authors also presented the combination of a cross-coupling and homo [4+2], hetero [4+2], hetero [4+4] or [4+4+1] annulation leading to various cyclic products [147]. 

Dinuclear palladium complexes catalyze m-hydroarylation of alkynes with arenes.56 The reaction of 3-hexyne with benzene in the presence of a dinulear palladium complex Pd2R2(M-OH)(//-dpfam) [dpfam = j/V,Ar -bis[2-(diphenyl-phosphino)phenyl]formamidinate, R=/>-Tol] and tri(/z-butyl)borane at 100 °C for 4h affords ( )-3-phenyl-3-hexene quantitatively (Equation (53)). The hydroarylation of 3-hexyne with monosubstituted benzenes ( )-3-aryl-3-hexenes with a 2 1 ratio of the meta- and ra -isomers. This regioselectivity is different from that of the hydroarylation of diphenylacetylene catalyzed by Rh4(GO)12.57... 

Success was obtained with Ru3(CO)i2 as catalyst precursor [6], but the most efficient catalysts were found in the RuCl2(arene)(phosphine) series. These complexes are known to produce ruthenium vinylidene spedes upon reaction with terminal alkynes under stoichiometric conditions, and thus are able to generate potential catalysts active for anti-Markovnikov addition [7]. Similar results were obtained by using Ru(r]" -cyclooctadiene)(ri -cyclooctatriene)/PR3 as catalyst precursor [8]. (Z)-Dienylcarba-mates were also regio- and stereo-selectively prepared from conjugated enynes and secondary aliphatic amines (diethylamine, piperidine, morpholine, pyrrolidine) but, in this case, RuCl2(arene) (phosphine) complexes were not very efficient and the best catalyst precursor was Ru(methallyl)2(diphenylphosphinoethane) [9] (Scheme 10.1). 

Reactions with Protic, ionic, Poiar Reagents. The reactions of radical anions with proton donors include the reduction of arenes, the well-known Birch reduction, as well as alkynes by alkali metals in liquid ammonia. Both reactions have synthetic utility and belong to the few radical ion reactions included in elementary textbooks. 

Metallacyclopentadienes undergo a range of synthetically versatile reactions which proceed with extrusion of the metal atom and attendant ligands. Thus, reactions with alkenes and alkynes afford cyclohexa-1,3-dienes and arenes (Scheme 6), and thiophenes, selena-cyclopentadienes, pyrroles and cyclopentadienones (indenones, fluorenones) can be obtained by treatment with sulfur, selenium, nitroso compounds and CO, respectively. The best studied substrates for such reactions are cobaltacyclopentadienes of the type (24a), which have been converted into a wide variety of arenes, cyclohexadienes and five-membered heterocycles, many of which would be very difficult to obtain by conventional organic procedures (74TL4549, 77JOM(139)169, 80JCS(P2)1344). 

Ozonium Ion (H03+). Ozone is a resonance hybrid of canonical structures 50a-50d.135 Ozone does in fact act as a 1,3-dipole—that is, either as an electrophile or a nucleophile. The electrophilic nature of ozone has been recognized for a long time in its reactions with alkenes, alkynes, arenes, amines, phosphines,... 

Hexa(—)menthyldistannane, preparation, 3, 856 Hexamethylbenzenes, with titanium, 4, 246 Hexamethylcyclotrisiloxane, in polymerization, 3, 654 Hexamethyldigermane, terminal alkyne reactions, 10, 747 1,1,2,2,3,3-Hexamethylindane, metallation, 9, 15—16 Hexanuclear arenes, in hexaruthenium carbido clusters,... 

Excitation of the alkene or alkyne followed by reaction with ground-state arene (Sec. Ill)... 

III. ORTHO PHOTOCYCLOADDITION VIA EXCITATION OF THE ALKENE OR ALKYNE FOLLOWED BY REACTION WITH GROUND-STATE ARENE... 

In our attempt to extend the coupling reaction of arenes with alkenes to the coupling with alkynes, as shown in Scheme 4, it was found that the reaction of arenes with ethyl propiolate in TFA (trifluoroacetic acid) gave addition products instead of a coupling product [3]. This addition reaction has been extended to various alkynes and various arenes and also to intramolecular reactions for synthesis of heterocycles such as coumarins, quinolines, and thiocoumarins. 

Scheme 4. Reactions of arenes with alkynes in the presence of a Pd catalyst.

While the meta photocycloaddition occurs at the n,n singlet state of the arene [4], different cases must be distinguished for the ortho cycloaddition depending on the structure of the substrates [5] (a) excitation of a ground state charge-transfer complex (b) excitation of the alkene (or alkyne) reaction partner (c) excitation of the arene partner and reaction at the singlet state and (d) excitation of the arene followed by intersystem crossing and reaction with the alkene at the triplet state. 

The most efficient catalyst precursors were found in the RuCl2(arene)(phos-phine) series. These complexes are known to produce ruthenium vinylidene species upon reaction with terminal alkynes under stoichiometric conditions, and thus are able to generate potential catalysts active for anti-Markovnikov addition [8]. Dienylcarbamates could also be selectively prepared from conju-... 

When compounds (7) were heated with alkyne in excess, two types of complexes, both involving alkyne coupling, are formed. A compound with the stoichiometry Co2(CO)4(C4R2CO)2, formed mainly from terminal alkynes having one bulky substituent R, represents derivatives of Co2(CO)g where two CO groups at either metal are replaced by a cyclopentadienone ligand. This compound type represents one of the many instances where alkynes combine with CO in the presence of a transition metal fragment to yield mostly cyclopentadienones, often complexed to the metal this cycloaddition reaction is similar to the Pauson-Khand scheme except for the use of an alkyne in place on an alkene (see also Section 5.1.4 and Scheme 26). The reaction eventually proceeds further to liberate an arene. Thus, from the use of t-BuC=CH, the alkyne trimerization product 1,2,4-tri-f-Bu-benzene was isolated. 

Cationic sandwich complexes of the type CpCo(arene) + were first prepared by hydride abstraction from cyclohexadi-enyl cations (Section 7.1). They are accessible in broader variation from the reaction of CpCoX half-sandwich complexes with arene in the presence of AICI3. Their electrochemical reductions to the corresponding 19-electron monocations and to 20-electron neutral complexes have been studied. The stability of electron-rich sandwich complexes increases with increasing alkyl substitution in either ring despite the more negative redox potential mass spectrometry studies of bond dissociation energies of (arene)Co+ complexes corroborate these results. However, neutral sandwich complexes are not very stable in the polar solvents necessary for the reduction of mono- or dications and have been isolated only from alkyne trimerization with CpCo precursors in nonpolar solvents (Section 5.1.4). 

The NS2+ cation is an important reagent in S-N chemistry, especially in thermally allowed cycloaddition reactions with organic nitriles and alkynes that give quantitative yields of heterocyclic cations (see Scheme 3). The dominant orbital interaction in these cycloadditions is between the LUMO of S2N+ and the HOMO of the alkyne or nitrile (see Figure 2). Cycloaddition reactions also occur with alkenes, and colored charge-transfer complexes are formed with arenes. 

Mechanistically Related Reactions of Arenes and Heteroarenes with Alkynes 18... 

MECHANISTICALLY RELATED REACTIONS OF ARENES AND HETEROARENES WITH ALKYNES... 

When benzylic alcohols are the substrates, arenes behave as nucleophiles, as shown in equation (123) and for the analogs in the 1,4-addition.These reactions have been performed with alcohols that are activated by an arene substituent and another group that could be an olefin, an alkyne, or other arene (equation 140). In the case of dimerization reported by Hashmi (equation 141), primary benzylic alcohols produce dimers of the type in equation (141) that arise from the activation of this moiety, together with the expected cyclization products. In a very recently reported example, aryl-alkyl ethers are obtained by reaction with NaAuCU. ... 

Hexafluoro-2-butyne undergoes 1,4 addition by reaction with Rh(dik)(diolefin) (dik = acac, dpm, dbm diolefin = cod, norbornadiene), with formation of a C—C bond between the alkyne and the methyne carbon atom of the /3-diketonate. Additional trimer-ization of hexafluoro-2-butyne to hexakis(trifluoromethyl)benzene and /j -arene coordination of the latter to rhodium affords 103 (equation 73). 

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