Photochemically generated silicon—carbon double-bonded intermediates : XII. Some properties of silacyclopropenes and silapropadienes produced by photolysis of ethynylpolysilanes

The photolysis of six ethynyl-substituted polysilanes has been investigated in the presence or absence of methanol. Irradiation of 1-ethynyl-2-phenyl- and 1-ethynyl-1-phenyltetramethyldisilane (I and II), 1-ethynyl-1,1-diphenyltrimethyldisilane (III) and 2-ethynylheptamethyltrisilane (V) in the presence of methanol produced methoxysilanes arising from the corresponding reactive silacyclopropene and silapropadiene intermediates. Irradiation of 1-enthynylheptamethyltrisilane (IV) led to the formation of dimethylsilylene, ethynylpentamethyldisilane and 1,1-dimethyl-2-pentamethyldisilanyl-1-silacyclopropene intermediate. The photolysis of tris(trimethylsilyl)ethynylsilane (VI) in the presence of methanol produced 1-trimethylsilyl-1-bis(trimethylsilyl)methoxysilylethene and hexamethyldisilane. Photolysis of II and III in the absence of a trapping agent gave the respective bis(silyl)acetylenes, via a 1,2-hydrogen shift, from the transient silacyclopropenes, in moderate yields, whereas the yields of the rearranged acetylenes from IV and V were low.     

Synthesis of Tris(1,2-dimethoxyethane-O,O')barium Bis(2,2,5,5-tetramethyl-2,5-disilaphospholanide) and the Monomer-Dimer Equilibrium in Toluene Solution

(1,2-Dimethoxyethane-O,O')lithium phosphanide (dme)LiPH(2) reacts with 1,2-bis(chloro-dimethylsilyl)ethane to give 2,2,5,5-tetramethyl-2,5-disilaphospholane, 1, as well as 1,1,4,4-tetramethyl-1,4-bis(2,2,5,5-tetramethyl-2,5-disilaphospholanyl)-1,4-disilabutane, 2 (P(2)Si(6)C(18)H(48), space group P&onemacr;, a = 943.3(2) pm, b = 1278.3(3) pm, c = 1413.3(2) pm, alpha = 72.45(1) degrees, beta = 78.13(1) degrees, gamma = 70.83(1) degrees, d = 1.081 g cm(-)(3), Z = 2, wR2 = 0.1553 at 6548 F(2) values). The reaction of 2,2,5,5-tetramethyl-2,5-disilaphospholane 1 and barium bis[bis(trimethylsilyl)amide] in 1,2-dimethoxyethane yields nearly quantitatively tris(1,2-dimethoxyethane-O,O')barium bis(2,2,5,5-tetramethyl-2,5-disilaphospholanide), 3A, which crystallizes in the monoclinic space group C2/c (BaP(2)Si(4)O(6)C(24)H(62), a = 2152.3(1) pm, b = 1381.5(1) pm, c = 1459.7(1) pm, beta = 113.73(1) degrees, d(calc) = 1.268 g cm(-)(3), Z = 4, wR2 = 0.0989 at 5220 F(2) values). Due to the high coordination number of eight of the barium center, rather long Ba-P distances of 333 pm are observed. With loss of the complexating ether solvent this compound forms a dimer 3B of the type R(dme)Ba(&mgr;-R)(3)Ba(dme)(2) in toluene or benzene solution as can be proven by (31)P{(1)H}-NMR spectroscopy ((2)J(P-P) = 6.7 Hz) and by X-ray structure analysis (Ba(2)P(4)Si(8)O(6)C(48)H(106), space group P2(1)/n, a = 1256.3(2) pm, b = 2000.0(3) pm, c = 2986.9(2) pm, beta = 98.929(9) degrees, d(calc) = 1.257 g cm(-)(3), Z = 4, wR2 = 0.1334 at 11580 F(2) values). The Ba-P bond lengths vary between 318 and 338 pm.     

Coupling reactions of aryl and heteroaryl halides with a trimethylsilylethynylpyrazine

By the coupling reactions of trimethylsilylacetylene and 2-chloro-3,6-diisobutylpyrazine, 3,6-diisobutyl-2-trimethylsilylethynylpyrazine or 1,2-bis(3,6-diisobutylpyrazin-2-yl)acetylene was obtained, depending on the solvent used. The former substance coupled with various aryl and heteroaryl halides to give 1-aryl-2-pyrazin-ylacetylenes.     

Synthesis and properties of the first representatives of terminal acetylenes in 3-imidazoline-1-oxyl 3-oxide series

Method for the synthesis of m-and p-isomers of 4-[2-(ethynylphenyl)vinyl]-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl 3-oxides by the cross-coupling of 4-[2-(3-iodophenyl)vinyl]-and 4-[2-(4-iodophenyl)vinyl]-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl 3-oxides with (trimethylsilyl)acetylene followed by desilylation was elaborated. The reactions at the CH-fragment of the ethynyl group were performed. The Mannich reaction proceeds with the loss of a spin label, whereas the oxidative homocoupling, with its retention. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2051–2054, October, 2007.     

Reaction of Selenostannanes with 1-Alkynes in the Presence of SnCl4

(Organylseleno)triethylstannanes RSeSnEt₃ (R = Me, Ph) react with 1-hexyne and phenylacetylene in the presence of SnCl₄ to give 1,2-bis(organylseleno)-1-organylethenes.     

Indirect electrochemical oxidation of piperidin-4-ones mediated by sodium halide-base system

Indirect electrochemical oxidation of 1-N-subsituted piperidin-4-ones in methanol in an undivided cell in the presence of sodium iodide/sodium methoxide system leads to the corresponding α-hydroxyketals in 50-80% substance yield (50-65% current yield). 2,2,6,6-Tetramethylpiperidin-4-one under the same conditions forms a mixture of methyl 2,2,5,5-tetramethyl-3-pyrrolidinecarboxylate and methyl 2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole-3-carboxylate in 70% substance yield (60-70% current yield) via electrochemically induced Favorskii rearrangement.     

Synthesis of bromo-, boryl-, and stannyl-functionalized 1,2-bis(trimethylsilyl)benzenes via Diels-Alder or C-H activation reactions

1,2-Bis(trimethylsilyl)benzenes are key starting materials for the synthesis of benzyne precursors, Lewis acid catalysts, and certain luminophores. We have developed efficient, high-yield routes to functionalized 4-R-1,2-bis(trimethylsilyl)benzenes, starting from either 1,2-bis(trimethylsilyl)acetylene/5-bromopyran-2-one (2) or 1,2-bis(trimethylsilyl)benzene (1)/bis(pinacolato)diborane. In the first reaction, 5 (R = Br) is obtained through a cobalt-catalyzed Diels-Alder cycloaddition. The second reaction proceeds via iridium-mediated C-H activation and provides 8 (R = Bpin). Besides its use as a Suzuki reagent, compound 8 can be converted into 5 with CuBr(2) in i-PrOH/MeOH/H(2)O. Lithium-bromine exchange on 5, followed by the addition of Me(3)SnCl, gives 10 (R = SnMe(3)), which we have applied for Stille coupling reactions. A Pd-catalyzed C-C coupling reaction between 5 and 8 leads to the corresponding tetrasilylbiphenyl derivative. The bromo derivative 5 cleanly undergoes Suzuki reactions with electron-rich as well as electron-poor phenylboronic acids.     

The effect of the nature of the substituent at the nitrogen atom on transformations of 3-bromo-2,2,6,6-tetramethyl-4-piperidinone and its 1-hydroxy and 1-oxyl derivatives under conditions of the Favorsky rearrangement

3-Bromo-2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl reacts with NH₄OH to give 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl, a product of the Favorsky rearrangement. 3-Bromo-2,2,6,6-tetramethyl-4-piperidinone is transformed under these conditions into a bicyclic amino ketone, while its 1-hydroxy derivative affords acyclic nitrosoenone. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1189–1191, June, 1997.     

Siliciumorganische verbindungen: LXXVIII. Reduktive silylierung von konjugierten trienen zu 1,6-disilyl-dienen

The reductive silylation of 1,3,5-hexatriene (I) with trimethylsilyl chloride and magnesium affords 74% disilylhexadienes, consisting of 8% 1,4-bis(trimethylsilyl)-2,5-hexadiene (II) and 92% 1,6-bis(trimethylsilyl)-2,4-hexadiene (III). The isomers IIIa, IIIb and IIIc can be separated via the Dieis—Alder adducts. Maleic anhydride reacts with IIIa and IIIb to give the bis[(trimethylsilyl)methyl] derivatives of 4-cyclohexene-1,2-dicar?ylic acid anhydride (IVa and IVb), whereas IIIc does not react with maleic anhydride. By a reductive silylation reaction 3-methyl-l,3,5-hexatriene (V) gives the 1,6-bis-silylated 2,4-hexadienes VIa and VIb, which with maleic anhydride give the adducts VIIa and VIIb.     

1, n-Triorganosilyl migrations in the rearrangements of silyl-substituted organolithium compounds

Under the agency of the potent lithiating agent, n-butyllithium in TMEDA, an array of organosilanes was found to undergo 1, n-silyl rearrangements via carbanionic intermediates. Unambiguous 1, 2-, 1, 3- and 1, 4-silyl shifts were uncovered in 1-trimethylsilyl-1, 1, 2-triphenylethane, 1, 1-bis(trimethylsilyl)-1-phenylalkanes and 1, 2-bis(trimethylsilyl)-1, 2-diphenylethane, respectively. Cross-over and competition experiments established that these rearrangements generally are intramolecular and occur with decreasing ease in the order, 1, 2 > 1, 3 > 1, 4. In other compounds, such as 1, 1-bis(trimethylsilyl)-1, 2-diphenylethane, 1, n-bis(trimethylsilyl)benzenes and triphenyl(trimethylsilyl)methane, competing 1, n-silyl shifts occurred. Attack of the organolithium intermediates on solvent and silicon—lithium exchange were significant side reactions in some instances. 1-Trimethylgermyl-1, 1, 2-triphenylgermane underwent no discernible rearrangement but rather gave the product expected from germanium—lithium exchange. By conducting time and competition studies, it was shown that lithiation is the product-determining step in these rearrangements and that dual pathways, namely 1, 3-versus consecutive 1, 2- 1, 4-pathways, are operative in certain rearrangements.     

Stable nitroxyl radicals with triple bonds: 4-acetylenyl-3-imidazoline-3-oxide-1-oxyls

Cross-coupling reaction of 1-hydroxy-2,2,5,5-tetramethyl-4-[2-(p-iodophenyl)vinyl]-3-imidazoline-3-oxide with copper(I) salts of 1-aryl(hetaryl)alkynes leads to the corresponding 2,2,5,5-tetramethyl-4-[2-(p-aryl(hetaryl)ethynylphenyl)vinyl]-3-imidazoline-3-oxide-1-oxyls in high yields.     

Synthesis of 2-indanones via [4 + 1] annulation reactions of (trialkylsilyl)arylketenes

[reaction: see text] (Trialkylsilyl)arylketenes combine with (trimethylsilyl)diazomethane in a new [4 + 1] annulation process leading to 2-indanone derivatives. The (trialkylsilyl)arylketene annulation substrates are available via the photochemical Wolff rearrangement of alpha-silyl-alpha-diazo ketones, which are themselves prepared by silylation of the corresponding diazo ketones. The mechanism of the annulation reaction is proposed to involve the formation of a 2,3-bis(silyl)cyclopropanone, which is in equilibrium with an oxyallylic cation. Electrocyclic closure of this intermediate forms the new cyclopentenone ring.     

A facile synthesis of 1,1-bis(silyl)ethenes

Symmetrical 1,1-bis(silyl)ethenes have been easily prepared via ruthenium complex-catalyzed silylative coupling cyclization of 1,2-bis(dimethylvinylsiloxy)ethane to give 2,2,4,4-tetramethyl-3-methylene-1,5-dioxa-2,4-disilacycloheptane with excellent selectivity and good yield, followed by its reaction with Grignard reagents. The cyclic product can also be effectively transformed into cyclic carbosiloxane, 2,2,4,4,6,6,8,8-octamethyl-3,7-dimethylene-1,5-dioxa-2,4,6,8-tetrasilacyclooctane.     

Chemistry of a 1-silacyclopropene. Thermal production of 1,4-disilacyclohexa-2,5-dienes

Thermolysis of 1,1-dimethyl-2-phenyl-3-trimethylsilyl-1-silacyclopropene in the presence or absence of an alkyne such as diphenylacetylene or ethyl-dimethylsilylphenylacetylene gave a mixture of 1,1,4,4-tetramethyl-2,5-diphenyl-3,6-bis(trimethylsilyl- and 1,1,4,4-tetramethyl-2,6-diphenyl-3,5-bis(trimethylsilyl)-1,4-disilacyclohexa-2,5-diene in high yield. The formation of the 1,4-disilacyclohexa-2,5-diene system can be best rationalized in terms of a mechanism involving direct dimerization of the 1-silacyclopropene.     

Synthesis of Pentafluorophenyl‐ and Pyridinyl‐3 Allenes

The allenes 1,2,3,4,5‐pentafluoro‐6‐(3‐phenylpropa‐1,2‐dienyl)benzene 4 , 3‐(3‐phenylpropa‐1,2‐dienyl)pyridine 11 and 3‐(3‐(pyridine‐3‐yl)propa‐1,2‐dienyl)pyridine 17 and the acetylenes 5 , 12 and 16 were obtained by reduction of the corresponding propargylic acetates 3 , 10 and 15 by Samarium(II) iodide in the presence of Pd(0). Base‐promoted isomerisation of acetylene 12 provided allene 11 in a yield of 80%. 1‐(Pentafluorophenyl)‐3‐phenylprop‐2‐yn‐1‐ol 2 was prepared from phenylacetylene and pentafluorobenzaldehyde. The condensation of nicotinaldehyde with trimethylsilylacetylene gave the 3‐(trimethylsilyl)‐1‐(pyridine‐3‐yl)prop‐2‐yn‐1‐ol 7 . The removal of the silyl group of 7 to acetylene 8 was done in basic conditions. The Pd catalysed condensation of the acetylene 8 with iodobenzene gave 3‐phenyl‐1‐(pyridine‐3‐yl)prop‐2‐yn‐1‐ol 9 . The Pd catalysed condensation of 8 with 3‐bromopyridine gave the 1,3‐dipyridin‐3‐yl‐prop‐2‐yn‐1‐ol 14 . The propargylic alcohols 2 , 9 and 14 were converted to the acetates 3 , 10 and 15 with acetic anhydride‐pyridine.     

First acetylenic derivatives of stable 3-imidazoline nitroxides

The Stephens-Castro reaction of copper(I) salts of 1-aryl(hetaryl)alkynes with 2,2,5,5-tetramethyl-4-[2-(4-iodophenyl)-vinyl]imidazoline-3-oxide-1-ol proved to be a general method for the preparation of 2,2,5,5-tetramethyl-4-[2-(p-aryl(hetaryl)ethynylphenyl)]vinyl-3-imidazoline-3-oxide-1-oxyles.     

Palladium catalysed regio- and stereoselective synthesis of (E)-4-aryl-1,3-bis(trimethylsilyl)but-3-en-1-ynes

A practical and general synthetic approach to a series of 4-aryl-but-3-en-1-ynes is described. In the presence of palladium complexes a variety of aryl bromides (or iodides) undergo coupling with two equivalents of trimethylsilylacetylene with the formation of (E)-4-aryl-1,3-bis(trimethylsilyl)but-3-en-1-ynes. The protocol is simple, efficient, and affords synthesis of regio- and stereoselectively target products in good to high yields.     

Regioselective haloaromatization of 1,2-bis(ethynyl)benzene via halogen acids and PtCl2. Platinum-catalyzed 6-pi electrocyclization of 1,2-bis(1'-haloethenyl)benzene intermediates

[reaction: see text] Treatment of 1,2-bis(ethynyl)benzene (1) with aqueous HX (X = Br, I) in hot 3-pentanone (100-105 degrees C, 2 h) afforded 1,2-bis(1'-haloethenyl)benzene species 2-Br and 2-I in 98% and 95% yields, respectively. The hydrochlorination of endiyne 1 failed to proceed at elevated temperature but was implemented efficiently by PtCl2 (5 mol %) in hot 3-pentanone (100 degrees C, 2 h) to give 1,2-bis(1'-chloroethenyl)benzene 2-Cl in 80% yield. In the presence of PtCl2 (5 mol %), these halides 2-Cl,2-Br, and 2-I were subsequently converted to 1-halonaphthalenes 3-Cl, 3-Br, and 3-I in the mother solution via sequential 6-pi electrocyclization and dehalogenation reactions. PtCl2 (5 mol %) also effected direct haloaromatization of endiyne 1 with HX (X = Cl, Br, I) and gave 1-halonaphthalenes 3-Cl, 3-Br, and 3-I in 64-71% yields. This investigation reports the scope and the regioselectivity of haloaromatization of various enediynes catalyzed by PtCl2.     

Reductions, Reductive Alkylations, and Intramolecular Cyclizations of Acyl Silanes with Samarium Diiodide or Tributyltin Hydride

A series of acyl silanes including aliphatic-, aromatic-, and bis-acyl silanes, as well as the acyl silanes bearing other substituents such as a bromine atom and alkenyl, succinimide, and carbonyl groups, were prepared, and their reactions with samarium diiodide or tributylstannane were studied. The reactions of acyl silanes occurred in various manners such as reductions, reductive alkylations, intramolecular radical cyclizations, pinacol couplings, aldol reactions, and Tishchenko reactions, depending on the nature of substrates and reaction conditions. Acyl silanes were generally reduced to give the corresponding alpha-silyl alcohols without transfer of silyl groups. Intramolecular radical cyclizations of 5-hexenoyl silanes and 1-silyl-1,5-pentanedione were realized to give alpha-silyl cyclopentanols and 1,2-cyclopentanediol derivatives, respectively. On treatment with samarium diiodide in tetrahydrofuran, 1-(trimethylsilyl)-1,6-hexanedione underwent a pinacol coupling reaction in the presence of t-BuOH, whereas it underwent a Tishchenko reaction in the presence of MeOH. The Tishchenko reaction of 1-silyl-1,5-pentanedione gave a delta-silyl-delta-lactone. On treating with samarium diiodide, 1-(trimethylsilyl)-1,5-hexanedione and 1,5-bis(trimethylsilyl)-1,6-hexanedione, underwent, respectively, intramolecular aldol reactions.     

Orbital symmetry analysis of the reaction of silylenes with acetylenes and the dimerization of 1-silacyclopropenes

The addition of silylenes to acetylenes, and the dimerization of silacyclopropenes, are treated by Orbital Correspondence Analysis in Maximum Symmetry (OCAMS). The products observed in the latter reaction are consistent with an allowed pathway, a b_1g displacement involving rotation of the silacyclopropene molecules relative to one another in the transition state.