Diels radialenes

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. 

Radialenes 86 were obtained when bis(l-diazo-2-oxoalkyl)silanes 84 were decomposed with copper or palladium catalysts60,61. The assumption, that the heterocyclic [3]cumulene 85 is the immediate precursor of 86, is corroborated by its trapping in a Diels-Alder reaction with furan. 

Since the [6]radialenes are triple-diene systems, it comes as no surprise that they have been used in multiple Diels-Alder reactions. In fact, after a first 1 1 addition with 150, leading to 161, has taken place, the reaction could proceed in two fashions—a linear course of addition leading to a para-xylylene 162, and an angular route which produces an ortho-xylylene intermediate 163 (equation 19>I<12 103. 

Silirenes (140, equation 32) could also be involved in the transition-metal catalyzed decomposition of bis(diazoketones) 139 which provides the electron-rich [4]radialenes 14266,67. While the formation of 142 directly from silirene 140 cannot be excluded a priori, it is more reasonable to assume that 140 undergoes twofold ring-expansion to form the cyclic cumulene 141, which then provides 142 by a cyclodimerization reaction. The intermediacy of 141 is corroborated by the isolation of the Diels-Alder product 14366. 

Ten years later, the synthesis of alkyl-substituted thiirene sulfoxide 140 was reported (Scheme 69), making use of a Diels-Alder reaction between thiirano-radialene sulfoxide 138 and the highly reactive dienophilic 4-substituted 1,2,4-triazoline-3,5-diones 139 (TAD) [132]. The fact that other classical reactive di-enophiles, including maleic anhydride and ethyl azodicarboxylate, do not react with sulfinyldiene 138, reveals its low reactivity. 

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>. 

Since the [6]radialenes are triple-diene systems, it comes as no surprise that they have been used in multiple Diels-Alder reactions. In fact, after a first 1 1 addition with 150, leading to 161, has taken place, the reaction could proceed in two fashions—a linear course of addition leading to a ara-xylylene 162, and an angular route which produces an crf/ic-xylylene intermediate 163 (equation 19)102-103 Whereas for the hexamethyl compound 150 only products formed by the linear route have been detected with a sizeable number of dienophiles (X=X inter alia TCNE, maleic anhydride, benzoquinone, 1,4-naphthoquinone, acrolein, methyl acrylate ), the parent system 4 undergoes threefold Diels-Alder addition in a star-shaped manner leading to 164 with dimethyl acetylenedicarboxylate and to 165 with fumaroyl chloride followed by methanolysis (equation 20). ... 

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. 

Section 4.2.3. The parent [5]radialene has been synthesized at last [147]. Key to success was a low-temperature decomplexation of a [5]radialene-bis(Fe(CO)3) complex, that had been prepared from a 2,6-dichloro-3-oxa-[5]dendralene precursor. A 30 mM solution of the hydrocarbon in acetone had a half-life time of around 16 min at -20 °C. According to G4(MP2) calculations for the gas phase, a Diels-Alder reaction, which leads to dimerization/polymerization, is outstandingly facile. 

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