Cycloalkynes, strained reactions

This reaction is useful in conjunction with E2 elimination to prepare alkenyl halides, allenes, and alkynes. Cycloalkenyl halides are easily prepared, but because of angle strain the cycloalkynes and cycloallenes with fewer than eight atoms in the ring cannot be isolated (see Section 12-7) ... 

Strained cycloalkynes can be stabilized by coordination to one or more transition metal centers (198). The unusual vicinal defluorination reaction of perfluoro-l,3-cyclohexadiene with [Co2(CO)8] to give the /i-alkyne complex 45 (see Section III,E) prompted a study of the reactions of OFCOT with cobalt carbonyl precursors. 

More recent developments exploit the energy content of readily accessible cycloalkynes based on phenyl-alkynyl structural motives, albeit not always with fullerene formation in mind. For example, the strained dehydrobenzoannulene 4 [19] could be converted by light, heat (145 °C), or pressure (20000 psi) in a topochemical polymerization reaction typical for butadiynes to a deeply coloured polymer. A similar thermochemical behaviour (strongly exothermic transformation around 200 °C) was observed for compounds 5 and 6 [20]. However, none of the systems 4-6 shows any tendency to produce spherical forms of carbon under the conditions investigated. 

Since the Schrock carbyne is active in the transalkylidynation reaction, the possibility of ring-opening polymerisation of cycloalkynes by acetylene transalkylidynation catalysts (metal alkylidyne complexes) has been evaluated [151]. Unfortunately, cyclooctyne is one of the few relatively stable strained cyclic acetylenes, but it is not strained enough to react selectively with the catalyst and yield a polymer according to the scheme... 

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... 

If a very reactive cycloalkyne is synthesized, the necessary fast reaction at low temperature, must be worked up in a short period. These conditions are met by the oxidative decomposition of bishydrazones with lead tetraacetate in dichloro-methane. By this method the most strained, but still isolable cycloalkyne 3,3,7,7-tetramethylcycloheptyne (10) was prepared8), which on standing at room temperature... 

In most cases reactivity in addition reactions to the triple bond increases with increasing angle deformation. For transient cycloalkynes comparative studies of the reactivity, e.g. competition experiments, are one of the few experimental methods to assess ring strain in these reactive intermediates. 

In the benzoannellated compounds two opposing effects on isolability are to be expected. Annellation of a benzene ring to cyclooctyne should increase ring strain as does introduction of an additional double bond. On the other hand, the size of the benzene ring should, like a bulky substituent, impede oligomerization reactions and thereby kinetically stabilize the angle strained cycloalkyne. 

Angle strained cycloalkynes exhibit enhanced reactivity, particularly in addition reactions to the triple bond, over analogous open chain alkynes. Therefore many reactions can be carried out with angle strained cycloalkynes which are not feasible or at least cannot be carried out under such mild conditions with analogous open chain alkynes. This review concentrates on the reactions typical for angle strained cycloalkynes, while those typical also for normal alkynes are not discussed in detail. 

Although there is ample evidence for nucleophilic additions to benzyne la> and some other unstable angle strained cycloalkyne intermediates 15,27,31,205 207), only a few addition reactions to isolable angle strained cycloalkynes are known which can be classified as nucleophilic. Hydroxylamine and hydrazine add to (31) to yield the corresponding oxime and hydrazone, resp. 208). 

Isolable angle strained cycloalkynes can be hydrogenated in the presence of a catalyst and this reaction has been used to determine the heats of hydrogenation (see Chapter 6). 

More typical for angle strained cycloalkynes is the dehydrogenation of alcohols, amines and thiols208. The reaction of (31) with methanol to yield the alkene (63) and formaldehyde was investigated in detail. On the basis of rate laws and H/D kinetic isotope effects the following reaction mechanism was proposed 208,212>. 

Cycloaddition reactions, particularly the Diels-Alder and 1,3-dipolar cycloaddition, have been used extensively in trapping unstable cycloalkynes. In addition, some typical cycloaddition reactions leading to interesting products are observed only with angle strained cycloalkynes. 

A reaction typical of angle strained cycloalkynes is the addition of CS2 leading to tetrathiafulvalenes 8,7° 85 87 102 65,107). The formation of the tetrathiafulvalenes is explained by the intermediate occurrence of the carbene (70), which has been trapped by reaction with methanol in the case of (10) and (31)85,219>. 

Diels-Alder reactions have been used to a great extent to trap unstable cycloalkynes and to characterize angle strained cycloalkynes, the most popular diene being tetra-... 

In the photoaddition of water (equation 30) or acetic acid (equation 31) to alkynes it seems likely that ionic addition occurs by protonation of the alkyne excited state, particularly in view of the observation that the hydration reaction is speeded up by acid and retarded by base. The sensitized addition of acetic acid to medium-ring cycloalkynes to give enol acetates (equation 32) is strongly reminiscent of the analogous addition to cycloalkenes, which has been shown to go by way of protonation of the highly strained /ra/is-cycloalkene. 

Inspection of molecular models reveals that cyclononyne should be slightly strained, while all smaller cycloalkynes should be strongly strained. In fact up to the present time, cycloheptyne is the smallest cycloalkyne to have been isolated -Since the reactivity of medium-sized cycloalkynes increases with decreasing ring size (see Section III), one may expect the formation of smaller cycloalkynes as highly reactive reaction intermediates. 

The method has found some use in the generation and trapping of strained cyclo-alkynes (equation 72) . Stable cycloalkynes are also obtained, but the rearrangement is accompanied by competing side-reactions, which drastically reduce the yields of the cyclic acetylenes, as illustrated for bromomethylenecyclooctane... 

The intermediacy of strained cycloalkynes and 1,2-cycloalkadienes7) in certain reactions was proven long ago86). Thus reactions of 1-halocycloalkenes often occur by the elimination-addition mechanism pictured in Scheme 4887). 

Dehydrohalogenation with rearrangement. Erickson and Wolinsky have shown that the reaction of a bromomethylenecycloalkane with potassium t-butoxide can be used to generate highly strained cycloalkynes. Thus when m-bromocamphene... 

Unusual decomplexation. A highly strained cycloalkyne cannot be produced from its hexacarbonyldicobalt complex on reaction with CAN. Instead, carbonylation at the two nascent sp-hybridized carbon sites is observed. ... 

Typical reactions of strained cycloalkynes are the [2 + 1] cycloaddition reaction with isonitriles and the [3 + 2] cycloaddition reaction with CS2 (Schemes 8-22 and 8-23) [lb]. As the latter... 

Selenadiazoles 176, which are readily formed by the action of selenium dioxide on semicarbazones of a-methylene ketones , decompose thermally to acetylenes (equation 25)106 the Lalezari reaction, is widely applicable. Recent uses are illustrated by the synthesis of four medium-sized cycloalkadiynes cyclonona-l,5-diyne (177), cyclodeca-1,5-diyne (178), cycloundeca-l,5-diyne (179) and cycloundeca-l,6-diyne (180) (Scheme 5) . Strained cycloalkynes with additional double bonds, e.g. the cyclododeca-3,7,11-triene-l-yne 181, have also been obtained by the Lalezari method. Compound 181 isomerizes spontaneously to the bicyclic hydrocarbon 182 . ... 

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