Benzene alkyne insertion into

Benzyne, which is generated in situ from 2-(trimethylsilyl)phenyl triflate and KF, acts as an alkyne congener in distannation in the presence of palladium-/ r/-alkyl isocyanide complex.157 A variety of substituted benzyne derivatives inserts into the Sn-Sn bond to give l,2-bis(stannyl)benzenes (Equation (59)). The reaction fails to occur in the presence of other palladium catalysts such as Pd(PPh3)4. 

Photolysis of substituted (cyclobutadiene)Fe(CO)3 complexes in the presence of alkynes affords substituted benzenes (Scheme 60). From the substituent patterns of the products, it is clear that this reaction cannot be accounted for by generation of the free hgand. The proposed mechanism involves initial loss of a carbon monoxide ligand and coordination of the alkyne. Insertion of the alkyne predominantly into the... 

Ditertiary phosphane complexes of nickel were found to be effective in the formation of pyrone 108 by cyclocotrimerization of alkynes with carbon dioxide. The formation of the nickelacyclopentadiene 105 from two moles of alkyne and a nickel complex is followed by CO2 insertion into a nickel-carbon bond to give the oxanickelacycloheptadienone 106, which then eliminates 108 with intramolecular C—O coupling. Another route involving [4 + 2] cycloadditions of 105 with CO2 in a Diels - Alder reaction to give 107 cannot be ruled out but is less probable because CO2 does not undergo [4 + 2] cycloaddition with dienes. Addition of another alkyne to 105 results in the formation of a benzene derivative (Scheme 38). ... 

Methylborylene [MeB], is generated from the reduction of methylboron dibromide with CgK in boiling benzene . This borylene intermediate adds to double and triple carbon-carbon bonds, and inserts into C—H bonds of h -CgHgCr(CO)j and ferrocene However, the intermediacy of [MeB] in the reaction of MeBBrj with CgK in the presence of alkynes is questionable since all the NMR evidence points to halobora-tion (see 5.3.2.3.4) of the alkynes in the first step followed by various reduction processes . 

When the reaction of zirconacyclopentadiene 4 with DMAD proceeded in the presence of CuCl at -78 °C, the linear triene 20 was obtained in 78% yield after hydrolysis. When this mixture was wanned to room temperature, benzene derivative 6 was formed as a single product. This clearly indicates that benzene formation involves the insertion reaction of the third alkyne (DMAD) into the metal-carbon bond (path B). As shown in Scheme 11.7, the alkenyl copper moiety added to the carbon-carbon triple bond of DMAD and elimination of Cu metal led to the benzene derivatives 6. Indeed, a copper mirror was observed on the wall of the reaction vessel. However, benzene derivatives were also obtained by using only a catalytic amount of CuCl. In this case, copper metal deposition was obviously not observed. This means that path A cannot be ruled out. 

Iron porphyrins containing vinyl ligands have also been prepared by hydromet-allation of alkynes with Fe(TPP)CI and NaBH4 in toluene/methanol. Reactions with hex-2-yne and hex-3-yne are shown in Scheme 4. with the former giving two isomers. Insertion of an alkyne into an Fe(III) hydride intermediate, Fe(TPP)H, formed from Fe(TPP)Cl with NaBH4, has been proposed for these reactions. " In superficially similar chemistry, Fe(TPP)CI (present in 10 mol%) catalyzes the reduction of alkenes and alkynes with 200 mol% NaBH4 in anaerobic benzene/ethanol. For example, styrene is reduced to 2,3-diphenylbutane and ethylbenzene. Addition of a radical trap decreases the yield of the coupled product, 2,3-diphenylbutane. Both Fe(lll) and Fe(II) alkyls, Fe(TPP)CH(Me)Ph and [Fe(TPP)CH(Me)Ph] , were propo.sed as intermediates, but were not observed directly. ... 

In a similar way as described for the hydroformylation, the rhodium-catalyzed silaformylation can also be used in a domino process. The elementary step is the formation of an alkenyl-rhodium species by insertion of an alkyne into a Rh-Si bond (silylrhodation), which provides the trigger for a carbocyclization, followed by an insertion of CO. Thus, when Matsuda and coworkers [216] treated a solution of the 1,6-enyne 6/2-87 in benzene with the dimethylphenylsilane under CO pressure (36 kg cm"2) in the presence of catalytic amounts of Rh4(CO)12, the cyclopentane derivative 6/2-88 was obtained in 85 % yield. The procedure is not restricted to the formation of carbocycles rather, heterocycles can also be synthesized using 1,6-enynes as 6/2-89 and 6/2-90 with a heteroatom in the tether (Scheme 6/2.19). Interestingly, 6/2-91 did not lead to the domino product neither could 1,7-enynes be used as substrates, while the Thorpe-Ingold effect (geminal substitution) seems important in achieving good yields. 

Ruthenium-borataborepine derivatives 124, which incorporate a seven-membered CsB2 borataborepine heterocycle, have been obtained via the insertion of terminal alkynes into Cp Ru( 1, 3-Me3C3B2lf 2)> where R is Me or CH2SiMe3.143 Other products isolated were the boratabenzene complex 125 and a benzene derivative. 

Regioselective syntheses of 1,3,5-unsymmetrically substituted benzenes (309) are catalyzed by Pd(dba)2/PPh3 mixed alkyne/diyne reactants give mixtures containing homocoupled and mixed products (24 21 from HC CPh + HC=CC= CC Hn). The probable mechanism involves oxidative addition to the Pd(0) center, insertion of the second diyne into the Pd—H bond, reductive coupling and subsequent jr-complexation of this product to Pd(0), followed by Diels-Alder cycloaddition of the third diyne and elimination of product. 

There are now two possible routes to the final product. Reductive elimination would insert the new alkyne into one of the old C-Co bonds and form a seven-membered ring heterocycle. This could close in an electrocyclic reaction to give the new six-membered ring with the cobalt fused on one side and hence the cobalt complex of the new benzene. 

The introduction of an o-methoxy group on the benzene ring of the alkynyl substituent increased the yields of the cyclization products. The reaction proceeds through the nucleophilic attack of the nitrogen atom on electron-deficient alkyne 317, the formation of the alkenylpalladium intermediate 318, the insertion of the alkenes 315 into the C—Pd bond 319, and -hydride elimination. They reported many examples of this type of reactions, but those are summarized in their own review in this issue of Chemical Reviews.168... 

---