Cyclohexyne addition

The complex Cp2Zr((j-r 1 n2-C6HR)(p-CH3)GaMe2 112 reacts readily with norbornene at room temperature to give trimethylgallium and the metallacyclic product 121. The norbor-nene/(r 2-cyclohexyne)ZrCp2 addition product was tentatively identified on the basis of its spectroscopic data [194],... 

Similar 1,2-migration/cyclization is observed in the reaction of 5-hexynyl tosylate 92a with zincates (equation 44)61. In addition to the exo-cyclization leading to (alkyli-dene)cyclopentane 93a, endo-cyclization occurs competitively to give cyclohexene 94a, which is a major product in the reaction with PthZnl.i. In the reaction of the phenyl-substituted derivative 92b, two endo-cyclization products 94b and 94 (1 1) were obtained as well as exo-cyclization product 93b. A unified mechanism in which the -electrons of the alkynylzincate 95 participate in exo- and endo-cyclization (Scheme 8) has been proposed to rationalize these observations. The intermediacy of cyclohexyne 96 was verified... 

Dibromocyclohexene (8) yields 11 and 12 on treatment with magnesium . This result can be best explained by assuming the formation of cyclohexyne (9) and dimerization thereof to give cyclobutadiene (10). Addition of 9 to 10 and dimerization of 10 should give 11 and 12, respectively. However, the formation of 11 and 12 cannot... 

The relative stabilities of cycloheptyne and cyclohexyne generated from 1-amino-4,5-cycloalkeno-l,2,3-triazoles (21) and lead tetraacetate were investigated. After the nitrogen evolution, which occurred instantaneously on addition of lead tetraacetate, had ceased, the trapping reagent was added after different time intervals to the reaction mixture, and yields of adduct (23) were determined. The results... 

Competitive addition of phenyllithium and lithium piperidide to cyclooctyne, cycloheptyne, cyclohexyne and cyclopentyne has been studied in order to gain information on the relative stabilities of cyclic acetylenes . 

Comparison of Schemes 50 and 51 shows that elimination-addition by means of a cyclohexyne is certainly the mechanism leading to enamines from 130,131... 

Organomagnesiums add to allenes , but additions to simple alkynes are not yet demonstrated. Some products, however, are ascribed to the addition of allylic reagents to the highly strained, short-lived cyclohexyne formed in situ by the reaction of 1,2-dibromocyclohexene and Mg metal ... 

An alternative route to alkyne complexes avoids the intermediacy of the metallocenes and allows formation of complexes of unstable alkynes, such as benzyne and cyclohexyne. Treatment of Cp2ZrMeCl with an aryl or vinyl lithium gives the expected aryl (or vinyl) complex 5.250 (Scheme 5.71). Mild heating then causes loss of methane and formation of the -q -alkyne complex 5.251, which can also be drawn as its metallacyclopropane resonance structure 5.252. These complexes become sufficiently stable to be isolated if an additional ligand, such as trimethylphosphine, is added, giving the adduct 5.253. Analysis of the structures... 

Bottini s group has investigated the reactions of 1-halogenocyclohexenes with potassium t-butoxide. From 1-chloro-, 1-bromo-, or 1-iodo-cyclohexene and Bu OK in either Bu OH or DMSO a yield of 4—12% of 1-t-butoxy-cyclohexene is obtained this product was rapidly converted into cyclohexanone on treatment with acid. Labelling experiments lead to the conclusion that 1-t-butoxycyclohexene is formed with concurrent operation of elimination addition mechanisms involving cyclohexa-1,2-diene and cyclohexyne. A number of C 2 hydrocarbons are also formed, arising from cyclohexa-1,2-diene via dimerization and 1,2- and 1,4-cycloaddition of cyclohexa-1,3-diene. 

An elimination-addition mechanism has also been invoked for the nucleophilic substitution of cyclohexenyliodonium salts with acetate ion. For example, the reaction of either the 4- or 5-substituted cyclohex-l-enyl(phenyl)iodonium tetra-fluoroborate (31 or 32 respectively) with tetrabutylainmonium acetate in aprotic solvents gives the ipso and cine acetate substitution products (33 and 34 and vice-versa, respectively) in almost the same ratio (Scheme 23). These results were consistent with an elimination-addition mechanism involving 4-substituted cyclohexyne 35 as a common intermediate. The presence of a cyclohexyne intermediate was confirmed by deuterium labelling and trapping studies leading to [4-l-2]-cycloaddition products. 

Cyclohex-l-enyl(phenyl)iodonium tetrafluoroborate reacts with nucleophilic d Pt°(PPh3)2 to form the cr-cyclohexenylplatinum(II) complex 101, characterised by X-ray crystallography, but in the presence of a base forms the cyclohexyne complex 102 (Scheme 45). Since 101 is not converted to 102 on addition of base, it is concluded that 102 is formed by trapping of transient cyclohexyne formed by deprotonation of the iodonium reagent by the base. 

The intermediacy of cyclohexyne or cyclopentyne in the decomposition of, respectively, 2-fluoro- or 2-chloro-l-cyclohexenyllithium and 2-bromo-l-cyclopentenyllithium is hypothetical. The adducts obtained upon treatment with phenyllithium and lithium benzenethiolate may result from an addition/elimination rather than from an elimination/addition sequence. The reaction order, a kinetic gold standard, is not the same in DEE and THF. Moreover, whatever the number, it is treacherous as long as the aggregation state of the organolithiums is unknown. 

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