Cycloaddition, 2-butyne

The only stannole complex 290 was found by cycloaddition of species 289 to 2-butyne [98ICA(281)53],... 

Thieno[3,4- ] and [3,4-f]pyridine derivatives can be generated from a cycloaddition reaction of oxazinones with l,4-dichloro-2-butyne to afford polyhalogenated pyridine products which cyclize to thienopyridines (Scheme 39) <2001T4203>. In a similar reaction, pyrazinones, 122, containing alkynyl substituents cyclize to form thieno[3,4-3] and [3,4-i ]pyridine derivatives <2002TL799, 2004T429>. 

Radical cations (94 +) of simple alkynes (e.g., butyne) generated by y-irradia-tion in frozen matrices are stable at 77 K. Upon warming, changes in the ESR spectra support an interesting cycloaddition forming cyclobutadiene radical cations... 

A key step in one route to the synthesis of hexamethyl Dewar benzene is the cycloaddition of 2-butyne to tetramethylcyclobutadiene (stabilized by A1 cation). Using the parent compounds (no methyls), develop a Woodward-Hoffmann orbital correlation diagram for the reaction and determine whether the reaction is thermally allowed. 

Lewis acids play a dominant role on the chemoselectivity of the cycloaddition of allylsilanes and 3-butyn-2-ones. AICI3 and EtAlCl2 promote [2+2] cycloaddition predominantly, whereas TiCLt-mediated reaction gives significant amount of the [3+2] adduct288,289. The reaction of 2,2-bis(trifluoromethyl)ethylene-l,l-dicarbonitrile with allylsilane without any Lewis acid catalysts yields the cyclobutane derivative via a nonconcerted fashion290. 

Attempts to isolate the silole monomer 2 or to trap it by reaction with maleic anhydride or perfluoro-2-butyne have failed. This is surprising given the successful trapping of 1-methylsilole5 and 1,1-dimethylsilole10. The authors conclude that silole 2 is not very reactive in Diels-Alder cycloadditions, except toward self-reaction19. 

Intramolecular cycloaddition of a diazo ketone to a cyclopropene. Rhodium) II) acetate is markedly superior to copper or copper(II) sulfate as the catalyst for cyclopropanation of l,4-diacetoxy-2-butyne with /-butyl diazoacetatc. The product (1) was converted by known steps into the diazo ketone 2. In the presence of rhodium(II) acetate, 2 undergoes intramolecular cycloaddition to the cyclopropene double bond to give the highly strained tricyclic pentanone derivative 3 in 30% yield. C oppcr catalysts are less efficient for this conversion. 

All pnictohetarenes with Y = P, As, Sb, and Bi react with dienophiles such as hexafluoro-2-butyne affording hetero-barrelene derivatives (P- at 100 °C, As- at 25 °C, Sb- and Bi-pnictohetarenes at even lower temperatures). The equilibrium low-temperature dimerizations of bismabenzene and stibabenzene (37-38) also involve such cycloadditions. 

Treatment of 2-butyne with ozone leads to unstable primary ozonides that cleave to cr-oxo-carbonyl oxides these could be trapped in the presence of aldehydes or ketones affording cross- -oxo-l, 2,4-trioxolanes. Subsequent cycloadditions between such cr-oxo-ozonides and cyclohexanone oxide, generated in situ from O-methylcyclohex-anone oxime (which affords methyl nitrite as a side-product), yield cr-diozonides 101 (Scheme 30) <1997J(P1)1601>. 

The [2+2] cycloaddition reactions of various 4-dialkylamino-3-butyn-2-ones with substituted phenyl isothiocyanates in refluxing tetrahydrofuran gave access to a series of thietimines 92a-j in poor to satisfactory yields (Table 6) <2001SL361>. As it may be concluded from Table 6, when diethylamine derivatives were replaced by dimethylamine... 

Table 6 [2+2] Cycloadditions of 4-dialkylamino-3-butyn-2-ones with phenyl isothiocyanates...

Aryl isoselenocyanates 4-RC6H4NCSe (R=H, Br, Cl, MeO) (prepared by selenation and dehydration of A-arylformamides) undergo regioselective cycloaddition reactions with 4-diethylamino-3-butyn-2-one in refluxing THF yielding W-arylselenetimines 11 (Scheme 9) <2004H(62)521>. 

The reaction of homochiral allylsilanes with a-enones and a-enals is very valuable for the asymmetric synthesis of multisubstituted cyclopentanes (Equation (40)).161,162 The [3 + 2]-cycloaddition of allylsilanes is applicable to />-quinones163 and />-quinoneimines.164 3-Butyn-2-one undergoes a double cycloaddition with an excess amount of allyltriisopropylsilane to give a bicyclo[3.3.0]octane in good yield (Equation (41)).165... 

Related to ketene cycloadditions are the group of cycloadditions with vinyl cation intermediates. The reaction between 2-butyne 6.120 and chlorine giving the dichlorocyclobutene 6.122 is the Smirnov-Zamkow reaction, and there is a similar reaction between allene 6.123 and hydrogen chloride giving the... 

Few examples of cycloaddition reactions of the type [2 + 2+1] where fluorosubstituted compounds are participating are known. Perfluoro-2-butyne and elemental sulfur react to give tetrakis(trifluoromethyl)thio-phene [84JFC(25)47]. Analogously, a mixture of tetrakis(trifluoromethyl-thio)thiophene, 2,3,4-tris(trifluoromethylthio)-5-trifluoromethylthiophene, and tetrakis(trifluoromethylthio)-l,2-dithiin was obtained from bis(triflu-oromethylthio)acetylene and sulfur at 170°C (85JHC1631) (Scheme 91). 

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

Cycloadditions [40] Perfluoro-2-butyne is a highly reactive dienophUe and many [4 + 2] cycloaddition and 1,3-dipolar addition reactions involving this alkyne have been reported (Table 7.20). Moreover, [2 + 2] additions with hydrocarbon alkenes are possible (Table 7.20). 

Table 7.20 Cycloaddition reactions with perfluoro-2-butyne...

Heptafluoro-2-butene, which is readily available in a laboratory synthesis from hexa-chlorobutadiene, may be used as a synthon for perfluoro-2-butyne in cycloaddition reactions where in situ elimination occurs [322] (Figure 7.92). 

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

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