- (E)-1-(Phenylsulfonyl)-4-(trimethylsilyl)-1-butene: An Advantageous Synthetic Equivalent for the 1-(1,3-Butadienyl) Anion and the 1,1-(1,3-Butadienyl) Dianion
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The (E)-1-(arylsulfonyl)-4-(trimethylsilyl)-1-butenes 9, 25, and 26 are prepared by CuCl2-promoted and by photolytic additions of their precursor 1-arylsulfonyl chlorides and bromides to 4-(trimethylsilyl)-1-butene (14) and then dehydrohalogenation of the resulting 1-(arylsulfonyl)-2-halo-4-(trimethylsilyl)butanes 15a, 15b, 23a, and 23b with KOH, LDA, or n-BuLi. Silylbutene 14 is obtained from reaction of [(trimethylsilyl)methyl]magnesium chloride (16, X = Cl) and allyl bromide (17) and better by protiodesilylations of (E)- and (Z)-1,4-bis(trimethylsilyl)-2-butenes (20) with sulfuric or trifluoroacetic acids. (Arylsulfonyl)(trimethylsilyl)-1-butenes 9, 25, and 26 are converted efficiently by LDA or n-BuLi at -78°C to 1-(arylsulfonyl)-1-lithio-4-(trimethylsilyl)-1-butenes 10, 27a, and 27b, respectively. Reactions of 27a and 27b with deuterium oxide yield (E)-1-(4-chlorophenylsulfonyl)-1-deuterio-4-(trimethylsilyl)-1-butene (28a, 83%) and (E)-1-deuterio-1-(4-methylphenylsulfonyl)-4-(trimethylsilyl)-1-butene (28b, 89%), respectively. 1-Lithio derivatives 10, 27a, and 27b undergo benzylations by benzyl bromide in THF/HMPA with retention of the positions of their olefinic double bonds to give the (E)-2-(arylsulfonyl)-1-phenyl-5-(trimethylsilyl)-2-pentenes 29a, 29b, and 29c, respectively, in 84-90% yields. Of particular interest is that 29a-c are isomerized to their corresponding 2-(arylsulfonyl)-1-phenyl-5-(trimethylsilyl)-3-pentenes 30a-c, respectively, which then undergo conjugative eliminations of their arylsulfonyl and their trimethylsilyl groups to give (E)-5-phenyl-1,3-pentadiene (33) in 56-63% yields upon reactions with TBAF in THF at 25°C. Further, 27b reacts with 1,3-dichloropropane to form 1-chloro-4-(4-methylphenylsulfonyl)-7-(trimethylsilyl)-4-heptene (35) which is cyclized by n-BuLi to 1-(4-methylphenylsulfonyl)-1-(3-(trimethylsilyl)-1-propenyl)cyclobutane (37, 67%). Elimination of 37 by TBAF then gives allylenecyclobutane (34, n = 3, 84%) simply. This study thus reveals that 9, 25, and 26 have outstanding potential as 1-(1,3-butadienyl) anion (7) and 1,1-(1,3-butadienyl) dianion (8) synthons.
- Meagher, Timothy P.,Shechter, Harold
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p. 4193 - 4198
(2007/10/03)
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- REACTIVITY AND SELECTIVITY IN THE CYCLIZATION OF SILA-5-HEXEN-1-YL CARBON-CENTERED RADICALS
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A trio of sila-5-hexen-1-yl radicals has been prepared from the corresponding halides by reaction with tri-n-butyltin hydride (deuteride).The radicals possessing a dimethylsilyl function α or β to the carbon radical center demonstrated marked reduction in total (but especially exo-trig) cyclization compared to the all-carbon system.The γ-silyl radical behaved, contrariwise, quite comparably to the all-carbon system.The difference in cyclization found in the α-silyl radical was demonstrated to result from both a pronounced decrease in cyclization rate via the expected exo-trig mode and from a significantly enhanced rate of hydrogen abstraction from TBTH.Both the α- and γ-silyl radicals cyclized via the endo-trig mode at rates close to that of the parent 5-hexen-1-yl radical itself.The cyclizations studied were demonstrated to be irreversible.The kinetic control thus shown by the preferred formation of endo cyclized product from the α- and β-silyl radicals is highly unusual and represents the first report of carbon-centered 5-hexen-1-yl type radicals violating the Baldwin-Beckwith rule (exo-trig cyclization preferred by 5-hexen 1-yl radicals).Rationalization of the cyclization behavior of the α- and γ-silyl radicals involves both steric and electronic factors.The behavior of the most unusual case, the β-silyl radical, which has the lowest cyclization propensity and no exo mode product, remains largely unexplained because its hydrogen abstraction rate from TBTH is unavailable as yet.Some speculative considerations involving the preferred radical conformation in this system and its relation to cyclization are given.
- Wilt, James W.
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p. 3979 - 4000
(2007/10/02)
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