Radialenes cycloadditions

Diels-Alder cycloadditions of sila- and germa[3]radialenes 541 with MTAD or PTAD provide the corresponding products 542 (Equation 73) <19930M1996, 1994AG723>. Similarly, phospha derivative 543 with MTAD gives 544 (Equation 74). On the other hand, the same reaction of cyclic 545 with MTAD is slower and the formed reaction mixture contains only small amount of a product probably analogous to 544 and compound 546 as the major product (Equation 75) <2000JA12507>. 

Exocyclic double bonds at cyclic systems, which contain cross-conjugated double bonds, cannot be considered as a subgroup of radialenes and shall therefore be treated separately, although many of the structural features are comparable. However, in these systems the exocyclic and endocyclic double bonds are competing with each other as sites for Diels-Alder reactions, cycloadditions and electrophilic attacks. The double bond character of both, as measured by its distance, can provide some evidence for the selec-tivities. If no strain and conjugation are expected, the double bonds should be comparable... 

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. 

That this difference in (4+2)-cycloaddition behavior most likely has steric origins—the methyl groups in 150 or the derived monoadduct preventing an ortho -addition of two equivalents of the dienophile—is supported by the observation that permethyl[6]radialene 95 is inert even towards the extremely reactive dienophile 4-phenyl-1,2,4-triazolinedione68. 

Cyclodimerization of unsymmetrically substituted butatricncs such as 12 give both head-to-head and hcad-to-tail cycloaddition products. The structure of the head-to-head dimer was confirmed by its independent synthesis from the mixed cycloaddition of cumulenes 8 and 10,21 22 These dimerizations proceed by discrete nickel cyclopentanes which was established by the isolation of the 2-bispyridinenickel complex of the l.l,4,4-tetramelhylbuta-l,2,3-triene dimer.23 4-Radialenes with extended conjugation, potential organic conductors and semiconductors, have been prepared by similar methods as illustrated by the examples below.24,25... 

Keywords cumulene derivative, [2+2]cycloaddition, [4]radialene tetracarboxylic acid... 

The structure of 91, a new bisaikylidenesilirane (sila[3]radialene) system, was confirmed by X-ray structure analysis (Figure 3c). Because of the high kinetic stability of 91 due to steric protection, aldehydes such as benzaldehyde as well as ketones did not react. However, [2 + 4] cycloaddition chemistry of 91 with 4-methyl-l,2,4-triazoline-3,5-dione resulted in the formation of a new ring-fused silirene (97) (equation 26). 

The mechanism of the formation of compound 1137 appears to be two sequential [4+2] cycloadditions between the exocyclic diene of compounds 1139 and 1141 and a dienophile (Scheme 223). The 2,3 d ethylenepyrrole required for the Diels-Alder reaction can be generated by the thermal elimination of acetic acid to form compound 1139, which is observed by mass spectroscopy. There are two possible pathway by which diene 1139 can proceed to tricycle 1137. The first is the elimination of a second molecule of acetic acid from diene 1139 to form 5-benzyl-aza[5]radialene 1140, which is also observed by mass spectroscopy. Attempts to improve the yield of compound 1137 by accelerating the elimination of acetic acid by acid or base catalysis failed, resulting in the decomposition of compound 1136 < 20000L73>. 

Concerning the transformation of [4]radialenes, the parent compound (2) has been studied best, despite its high instability in solution at room temperature or in the solid state (see above). We have already mentioned that 2 can be kept indefinitely at — 78 C, but undergoes dimerization and polymerization in solution at 20 C. The dimerization leads to cyclooctadiene derivative 113. Trimethylenecyclobutane behaves analogously, and 5,5,6,6-tetraphenyl[4]radialene 66 reacts in the same manner to give 114 at 60 °C in chloroform solution (equation 6). Since thermal (4 + 4) cycloadditions should not occur in a concerted manner, it has been suggested that this reaction is a stepwise process in which the reacting 1,2-dimethylenecyclobutene unit exhibits 1,4-diradical character. ... 

In the cycloaddition reaction of higher cumulenes different types of cyclodimers are encountered. In [3] cumulenes the cycloaddition can occur across the center bonds to form [4] radialenes 4 or across their end groups to give cyclodimers 5 and 6. 

The cyclodimerization of butatrienes, [3]cumulenes, usually affords tetramethylene-cyclobutanes (radialenes). In contrast, [5]cumulenes afford head-to-tail dimers. In [2+2] cycloaddition reactions of the higher cumulenes, reaction usually occurs across one of the double bonds. In the reaction of hexapentaenes with fluorinated olefins the center C=C bond is involved, while CHS double bond-containing substrates add across the substituted double bonds. 

Thermal or Ni(0)-catalyzed [2+2] cyclodimerization reactions of higher [ ]cumulenes give access to radialenes bearing cumulenic ir-systems ( extended radialenes, compare Scheme 4.13, Section 4.2.2, and Chapter 9). However, the success and the regioselectivity of the cycloaddition event depend strongly on the steric demand of the substituents at both ends of the cumulene [7, 8, 51]. Thus, a thermally induced cyclodimerization of a [4]cumulene furnished radialene 77... 

R = Bu, R = H [71]) (Scheme 4.16), whereas with R =R = Me [72] a head-to-head cycloaddition across the cyclopropyhdene=C double bond occurred. Tetra-(ert-butylhexapentaene, by virtue of its bulky end groups, cyclodimerizes across the central C=C bond and yields the symmetrical, thermally highly stable [4]radialene 78 (no solvent, 200"C, 15 min, 90% yield) [73,74]. 

The formation of dodecamethyl[6]radialene (72) by a Ni(0)-mediated [2+2+2] cycloaddition of 2,5-dimethylbuta-2,3,4-triene (70) has already been mentioned (see Section 4.2.2). Much better yields could be obtained, when 3,4-dibromo(or diiodo)-2,5-dimethylhexa-2,4-diene was exposed to a Ni(0) complex. Here, the [3]cumulene 70 is a likely reaction intermediate, and the product distribution ([4]radialene 71, [6]radialene 72, and its isomer 73 see Scheme 4.14) depends strongly on the solvent, with donor solvents favoring the formation of 72. With DMF as the solvent and in s/fw-generated Ni(PPh3)4 as a mediator, a yield of 63 and 50% could be achieved for 72 from the dibromo- and the diiododiene, respectively [65, 66]. 

Only a few facts about the chemical reactivity of the parent [6]radialene (5) are known - certainly because it is not easyto handle - but they reveal its character as a triple 1,3-diene system (catalytic hydrogenation, triple 1,4-addition of Br2, [4+2] cycloaddition reactions [5, 6]). Much more is known about the alkyl-substituted radialenes 113 and 72, in particular due to the detailed investigations of Hopf and coworkers [88]. Because of the presence of three hexa-2,4-diene subunits in the latter radialenes, it is not surprising that isomerization pathways via sigmat-ropic and electrocyclic reactions exist. Thus, in a gas-phase thermolysis of 113, products 125-129 were formed in relative yields that depended on the reaction temperature (e.g., 127 was the major product at 260 C and 129 at 360 °C). The mechanistic scenario includes the isomerization 113 125 by three consecutive 1,5-H shifts, and the sequence 127 128 129 [88]. The permethylated [6]radi-alene (72) is thermally much more stable than 113 the product mixture obtained from its pyrolysis at 350 "C was dominated by benzocyclobutene 130 (an analog of 127), which, however, could be isolated in only 17% yield [88] (Scheme 4.27). 

Cycloaddition (Diels-Alder) reactions have been reported for [6]radia-lene (5) and its hexaalkyl derivatives 113 and 115, but not for the permethylated radialene 72, which was inert even to the reactive dienophiles TCNE and Af-phenyltriazolinedione [67]. The sterically least hindered radialene 5 reacted with acetylenic and olefinic dienophiles in a 1 3 ratio to give triphenylene derivatives such as 139 in low yield (Scheme 4.30) [5, 95]. On the other hand, radi-alenes 113 and 115 gave linear,/)-quinodimethane-type 1 2-adducts, when they were exposed to an excess of various common dienophiles inter alia maleic anhydride, tetracyanoethylene, />-benzoquinone, acrolein, ethyl acrylate, acetylenedi-carboxylic acid) [89, 96, 97]. The 1 1 adduct 140, which was isolated so far only from the reaction with an equimolar amount of TCNE (92% yield) [97], presumably prefers the second cycloaddition step in the linear (para) position (141) over that in the angular (meta) position (142) for steric reasons. 

Only one account of a [4]radialene reacting with more than one molecule of a dienophUe has been described, and, despite involving a hetero-DA reaction, we believe it important enough to warrant inclusion. When Wilke and coworkers reacted octamethyl[4]radialene (181) with IMequiv. of 4-phenyl-1,2,4-triazole-3,5-dione, the expected cycloadduct 182 was obtained (Scheme 12.39) [60]. When 2 M equiv. were employed, a second DA reaction was not observed instead, the [3-1-2] cycloaddition product 183 was isolated (confirmed by X-ray analysis). 

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