Cyclohexynes

A derivative of cyclopentyne has been trapped in a matrix. Although cycloheptyne and cyclohexyne have not been isolated at room temperatures, Pt(0) complexes of these compounds have been prepared and are stable." The smallest cyclic allene" so far isolated is l-/err-butyl-l,2-cyclooctadiene 107." The parent 1,2-cyclooctadiene has not been isolated. It has been shown to exist transiently, but rapidly dimerizes." " The presence of the rert-butyl group apparently prevents this. The transient existence of 1,2-cycloheptadiene has also been shown," and both 1,2-cyclooctadiene and 1,2-cycloheptadiene have been isolated in platinum complexes." 1,2-Cyclohexadiene has been trapped at low temperatures, and its structure has been proved by spectral smdies." Cyclic allenes in general are less strained than their acetylenic isomers." The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air." ... 

Scheme 176 represents the opposite situation, with stable phenyl azide used as a probe to trap very reactive and short living alkynes. Thus, diazirine 1064 generates cyclohexyne 1066 that is too reactive to be isolated and characterized. However, when phenyl azide is added to the reaction mixture, it traps species 1066 in situ to give triazole 1068 in 84% yield. Similarly, even more strained norbornyne 1067, generated from diazirine 1065, is trapped by phenyl azide to afford triazole 1069 in 22% yield <2006AGE309>. 

Generally, cyclohexyne is an unstable molecule because of its ring strain. However, it can be stabilized by coordination to transition metals.35 The reduction of 1,2-dibromocyclohexene by sodium/mercury in the presence of a nickel-bromide complex afforded the Ni-alkyne complex 66 as a thermally stable and isolable compound (Scheme 22).36 Complex 66 smoothly reacted with C02 under atmospheric pressure to give nickelacycle 67 in good yield. Dimethyl acetylenedicarboxylate was inserted into the vinyl-nickel bond in 67 to give the seven-membered oxanickelacycle 68. 

Benzynes are highly strained molecules, which are recognized as useful intermediates in organic synthesis.44 They can be isolated by coordination to transition metals.45 Similar to the reaction of the cyclohexyne species 66, Ni-benzyne complex such as 85 reacted with C02 to give the corresponding five-membered oxanickelacyle complex 86 (Scheme 31).46... 

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],... 

Caubere et al. [63, 64] treated 32a with sodium amide-sodium tert-butoxide (NaNH2-NaOtBu) in tetrahydrofuran (THF) in the presence of secondary amines and obtained enamines. Analogously, the corresponding thioenol ethers were formed from 32a and sodium amide-sodium thiolate in the presence or absence of NaOtBu. It was shown, however, that cyclohexyne rather than 6 is the decisive intermediate en route to the enamines as well as the thioenol ethers [63b, 64], As already mentioned above, the enol ether 41 arises inter alia from 32b and KOtBu in DMSO. The best yield (47%) was obtained in refluxing THF (Scheme 6.11) [60],... 

Guitian and colleagues126 performed some Diels-Alder reactions between in situ generated cyclohexyne and several a-pyrones. The reactions were performed at 100° C which resulted in immediate loss of carbon dioxide from the primary cycloadducts. Reaction yields were generally above 80%. The reaction between 184 and cyclohexyne, derived from 185, to give 186 has been depicted in equation 51. 

Note that not all cyclopropenones eliminate CO photochemically. Thus, the first cyclohexynes had to be generated by pyrolysis of the precursor cyclopro-penone. °°... 

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... 

Unexpectedly, the methoxo complex shows no tendency to decompose to a hydride by / -elimination, although it does react immediately with water to give the hydroxo complex. In contrast, cyclohexyne bis(triphenylphosphine)platinum(0) reacts with methanol to give a hydrido complex, presumably via an undetected intermediate methoxide (see Equation 3) ... 

Alkynyl zincates (59), derived from 5-hexynyl tosylates, undergo a re-type endo cyclization to fonn cyclohexynes (60).58 This reaction takes place in competition with exo cyclization, leading to the formation of l-(cyclopentyhdene)alkylzincs (61). 

The 7r-type endo-cyclization of metal acetylides (425) to form strained cyclohexynes (426) has been observed556 for the first time in the reaction of alkynylzincates derived from 5-hexynyl tosylates. 

Generation of some small-ring cyclic acetylenes has been achieved either by matrix isolation studies or by trapping and oligomerization experiments157-160. However, no example of the isolation of cyclohexyne has been reported. While the six-membered cyclic acetylene 156a was prepared for the first time by the ring-contracted photochemical... 

That 1,2-cyclohexadiene (161, n = 3) is formed in a related system, from the reaction of 1-bromocyclohexene with potassium t-butoxide in dimethyl sulphoxide, is shown by trapping it by the highly reactive 1,3-diphenylisobenzofuran (162). The Diels-Alder adduct (163), differs from that of cyclohexyne (164) which was obtained from 1,2-dibromo-cyclohexene and Mg in the presence of (162). The product (164) did not isomerize to (163) (Wittig and Fritze, 1966). 

Synthesis, reactions, and physical properties of stable mononuclear platinum and zirconium complexes of cyclohexyne reported prior to the late 1980s have been comprehensively covered in earlier reviews.2-8 More recently, reaction of the zirconocene complex of cyclohexyne with trimethyla-luminum and trimethylgallium has been reported to give 247 and 248, respectively [Eq. (36)].57-58 These products are novel because four atoms (carbons Cl and C3, the transition metal, and the main group metal) are covalently bonded to an sp2 hybridized carbon (C2) in a planar tetracoordi-nate fashion. Synthesis of this type of complex, which Erker describes as anti-van t Hoff/LeBel, does not require the strained cycloalkyne ring zirconocene complexes of acyclic alkynes react similarly.57-58... 

Reactions of 250 with four electrophiles are recorded in Scheme 31. In general, the products are more stable than those from complexes with monodentate ligands. Reaction of 249 with CS2, 249 or 250 with alkynes, and 249-251 with ethylene gives products in which the C6H8 has been lost but its fate has not been determined attempts to trap free cyclohexyne failed.93 Loss of the organic ligand appears to occur more readily in nickel complexes than in those with platinum. 

The properties of the complex of homoadamantyne, which is formally a cycloheptyne, more closely resemble platinum complexes of cyclohexyne than those of the seven-membered ring alkyne. The bend-back angle (50.6°) (Table VIII) is significantly larger than that of the corresponding cycloheptyne complex (41.3°) and nearly as large as that of the cyclohexyne analogue... 

The triple-bond stretching frequency (1735 cm 1) is also closer to that of the cyclohexyne complex (1721 cm 1) than to that of the corresponding cycloheptyne (1770 cm-1 ).2... 

It is also possible to prove the occurrence of cycloheptyne (IS) and cyclohexyne (20) in solution. Here, as in the isolation of very reactive cycloalkynes, it is essential to generate the intermediate in a fast reaction at low temperatures in the absence of a reactive reagent which could add to the cycloalkyne. Again, an oxidative decomposition with lead tetraacetate, here of the corresponding 1-aminotriazoles (22) resp (27), is used for the generation of (16) and (20)14). 

Since for most angle strained cycloalkynes, particularly for those which are not isolable, structural data is lacking, calculations have been carried out. The structural parameters and strain energies of cyclononyne (6)152 154,160>, cyelooctyne (14) 152,154,106,144), cycloheptyne (16) 152,154), cyclohexyne (20)1S4) and cyclopentyne (18) 154) were calculated. Some values for strain energies and C—C s C bond angles are given in Tables 3 and 4. 

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