TCNE, cycloaddition with

Examine conformational energy profiles for Z-penta-1,3-diene and E,E-hexa-2,4-diene together with transition-state geometries for cycloadditions with TCNE (Z-penta-1,3-diene+TCNE and E,E-hexa-2,4-diene+TCNE, respectively). Predict the rates of Diels-Alder reactions involving these two dienes, relative to that for cycloaddition of E-penta-1,3-diene with TCNE. 

Diazomethylene)phosphoranes 33 (Scheme 8.10), which represent another type of diazocumulenes (12) are easily obtained by the oxidative ylidation of the corresponding phosphanyl(trimethylsilyl)diazomethane with CCI4. The increased stability of these compounds as compared with diazocumulenes (R2C=C=N2) is probably due to the ylidic character of the P=C bond. These diazo compounds exhibit the expected dipolar reactivity toward electron-deficient alkenes, alkynes, phosphaalkenes, and heterocumulenes (12). Thus, 33 reacts with TCNE to form A -pyrazoline 35 (60). Furthermore, 33 could be converted into the phosphonio-borate-substituted diazo compound 34, which underwent subsequent cycloaddition with electron-deficient alkenes (e.g., 34 36) (61). 

If the C=N function is attached to an electron-withdrawing group, 1,3-dipolar cycloaddition with diazoalkanes occurs leading to 1,2,3-triazoles (5, 276). When diazomethane is used, the initially formed NH-triazole is not isolated due to a rapid subsequent NH deprotonation followed by N-methylation. Consequently, a mixture of the three Wmethyltriazoles is formed when methyl cyanoformate (71) (216) or trichloroacetonitrile (276) (217) is treated with excess diazomethane (Scheme 8.51). Huisgen and co-workers found that methyl diazoacetate reacts with TCNE by a 1,3-dipolar cycloaddition at the C=C bond and not, as published earlier by other authors, at one of the nitrile functions (72). 

Diels-Alder [4 + 2]ir cycloadditions of 1-alkoxycarbonyl-l//-azepines are successful with all but the weakest dienophiles (e.g. maleic anhydride). Early work showed that with tetracyanoethylene (TCNE) cycloaddition at C-2—C-5 takes place readily in benzene solution at room temperature to yield adducts (140) (69JOC2888, 70JHC1249). The structure of the TCNE adduct with 5-bromo-l-ethoxycarbonyl-l//-azepine has been confirmed by X-ray studies (67JCSfBMi2). 

There are a few examples of the participation of the pyrazole ring in Diels-Alder cycloadditions with acyclic olefins. In the reaction of 1-phenyl-5-vinylpyrazole 96 and TCNE, the tetrahydroindazole 234 was obtained. 1-Phenyl- and l-tert-buty 1-4-vinylpyrazoles 90a and 90c react similarly with different dienophiles, but TCNE reacted in a different way and the [2 + 2]-cycloadducts 235 were isolated (86T6683 89M1113). 

The reactions with tertiary amines or phosphines that have no active hydrogen atoms result in platinacyclobutene cations, a rare species for late transition metal (Scheme 39). Substituted carbanions are added to the jj -aUenyl/propargyl platinum complex to yield the neutral substituted- ) -TMM derivatives that undergo huther [3 + 2] cycloaddition with good tt-acids as TCNE or maleic anhydride to produce highly substituted cyclopentanoids (Schemes 40, 41). 

Cycloaddition with the carbonyl group in nonpolar solvents may involve initial formation of an exciplex, but recent evidence indicates that polar solvents preclude the exciplex to biradical pathway [33]. There are a number of carbo-nyl/alkene pairs that are capable of photoelectron transfer due to their redox potentials (e.g., quinones/tetracyanoethylene (TCNE)). The resultant radical ions can bond to give 1,4-biradicals that close to form oxetanes [34]. 

Tetracyanoethylene (TCNE) reacts with thiobenzophenones to yield 2,3-dihydrothiophene, thiophene, and 1,2-dithiin derivatives, thus depending on temperature. Okuma suggested a mechanismfor the formation of 2,3-dihydrothiophenes through [2+2] and [4+2] sequential cycloadditions. 

Cycloaddition of cyclopropylethylenes with tetracyanoethylene (TCNE) takes place by two different routes. In the first (a), electron transfer between the low ionization potential olefins and TCNE occurs with subsequent coupling of the ring-opened cyclopropyl radical with the TCNE radical anion. In the second pathway b), a charge-transfer complex is believed to be involved with cycloaddition to the double bond (Scheme 31). ... 

Since vinyl azides like 34 are electron-rich olefins, [2 + 2] cycloaddition with electron-deficient alkenes such as diphenylketene could lead to azidocyclobutanes. " The stability of the cycloadducts 211, prepared from 34 or 52 and tetracyanoethene (TCNE), allowed characterization in solution but not isolation of these products because rapid ring-expansion regioselectively afforded the dihydropyrroles 212 already at room temperature (Scheme 5.25). "" A similar mechanism via [2 + 2] cycloaddition and quick ring-enlargement may perhaps explain the formation of 213 from 52 and 4-phenyl-l,2,4-triazole-3,5-dione (PTAD). In this " " and other " " cases, however, different interpretations were offered. The 2-azidobuta-l,3-dienes 92a,b underwent [4 + 2] cycloaddition in the... 

The focus of this chapter has so far not been so much on synthesis, but one very nice cascade reaction to dendralenes incorporating DTF donor groups and dicyanomethylene acceptor groups deserves mention. While TCNE can undergo a thermal [2+2] cycloaddition with electron-rich alkynes followed by electrocyclic ring opening [21, 39], TTF can undergo a similar reaction with electron-poor alkynes [40]. These observations were employed to construct a... 

Cycloadditions of TCNE with cyclopropylethylenes, in which new bonds to the cyclopropane carbons form, have been reported further. Bicyclo[6,l,0]-nonatriene and the eJco-9-chloro-derivative undergo cycloaddition with TCNE but, though the cyclopropane opens, the stepwise polar additions are across C-3—C-4 to give adducts (424) see also p. The structures of (424) have been proven by 2f-ray crystallography and the trans junction was confirmed in both cases. 

Addition of 1,2,4,5-tetramethoxybenzene (TMB) as a SET quencher completely suppressed the oxygenation but did not affect [3+2]-cycloaddition with TCNE. 

The double bond in A7-methoxycarbonyl-2-azetine (237 Z = COaMe) undergoes acid or photocatalyzed hydration and subsequent ring opening to give the aldehyde (238). In cycloadditions it is inert to TCNE and diphenylisobenzofuran but it does react with dipyridyltetrazine (77CC806). 

A strong acceptor TCNE undergoes [2+2] rather than [4+2] cycloaddition reactions even with dienes. 1,1-Diphenylbutadiene [20] and 2,5-dimethyl-2,4-hexadiene (Scheme 5) [21] afford mainly and exclusively vinyl cyclobutane derivatives, respectively. In the reactions of 2,5-dimethyl-2,4-hexadiene (1) the observed rate constant, is greater for chloroform solvent than for a more polar solvent, acetonitrile (2) the trapping of a zwitterion intermediate by either methanol or p-toluenethiol was unsuccessful (3) radical initiators such as benzyl peroxide, or radical inhibitors like hydroquinone, have no effect on the rate (4) the entropies of activation are of... 

Reactions between much stronger donors and acceptors belong to the electron tranter band. Such olefins do not form cyclobutanes but ion radical pairs or salts of olefins. refrato(dimethylamino)elhylene has an ionization potential as low as Na. The olefin with extraordinary strong electron-donating power is known not to undergo [2+2]cycloaddition reaction, but to give 1 2 complex with TCNE (transfer band in Schane 3) [23]. 

An electron donating butadiene with the methoxy substituents at the 1 and 4 positions was calculated to undergo a concerted [4+2] cycloaddition reaction with TCNE as... 

Benzylideneeyclopropane (156) and diphenylmethylenecyclopropane (157) reacted rapidly with TCNE to afford 159 and 160, respectively, in good yields. Since the reaction rate is highly dependent on the solvent polarity, the cycloadditions of 156 and 157 with TCNE were rationalized as stepwise reactions involving the dipolar ions 158 (Scheme 23) [37],... 

The cycloadditions in entries 1-3 are still believed to occur via a diradical stepwise pathway, as confirmed by obtaining a thermodynamic 78 22 trans/cis mixture of dispirooctanes 536 from frans-dicyanoethylene (533) (entry 3) [13b, 143], The cycloaddition to tetracyanoethylene (131) in the absence of oxygen gives only low yields of the [2 + 2] adduct, due to the simultaneous formation of products 542 and 543 (Scheme 74) [13b]. Still, the formation of the cyclobutanes 537 and 542 is noteworthy, since the reactions of TCNE with phenyl substituted MCPs exclusively afford methylenecyclopentane derivatives [37,144], The reaction is thought to occur via dipolar intermediates 539-541 formed after an initial SET process (Scheme 74) [13b]. The occurrence of intermediates 540 and 541 has been confirmed by trapping experiments [13b]. 

Other sporadic examples of [2 + 2] cycloadditions of olefins on the exo double bond of structurally more complex MCPs, such as methylenecyclo-propenes, allylidene-, and alkenylidenecyclopropanes, have been reported. Thus, dicyclopropylideneethane (2) reacted with TCNE (131) to give the [2 + 2] adduct 164 as a minor product, together with the prevalent [4 + 2] adduct 163 (Scheme 76) [39], The same reaction in a different solvent had been previously reported to furnish exclusively the Diels-Alder product (see Sect. 2.1.2) [5]. 

The nitrogen atoms in ADC compounds are highly electrophilic. Nucleophilic attack on nitrogen is easy, and as with electrophilic acetylenes, such as dimethyl acetylenedicarboxylate, it seems likely that some cycloaddition reactions of ADC compounds with unsymmetrical substrates proceed via a stepwise mechanism. PTAD is a powerful electrophile, although TCNE is more reactive, and chlorosulfonyl isocyanate is more reactive still.58... 

In contrast, with the calicene 230 TCNE is attached to five- and three-membered rings in a more complicated cycloaddition mode giving rise to 496s With a series of other calicenes no cycloaddition, but formation of stable charge-transfer complexes was observed2 93 ... 

Hexamethyl[3]radialene (25) does not undergo Diels-Alder-reactions with the typical electron-poor dienophiles, probably because of the full substitution at the diene termini. With TCNE, however, a violet-blue charge-transfer complex is formed which disappears within 30 min at room temperature to form a 1 1 adduct (82% yield) to which structure 55 was assigned9. Similar observations were made with tris(2-adamantylidene)cyclopropane (34), but in this case cycloaddition product 56 (81% yield) was identified its allenic moiety is clearly indicated by IR and 13C NMR data12. 

The [4 + 2]-cycloaddition reaction of dienylallene 204 with TCNE took place at 70 °C to give the adduct 205 in good yield [173], The dienylallene behaved not as butadiene but vinylallene, partly owing to the thermodynamic stability of the adduct. 

---