retro-Brook Rearrangement Research Articles

1,2]-Retro-Brook rearrangement induced by electrochemical reduction of silyl enolates

Abstract Electrochemical reduction of the trimethylsilyl enolates of alkyl aryl ketones induces retro-Brook rearrangement to provide 1-aryl-1-trimethylsilylalkan-1-ols. The transformation proceeds through a sequence of 1) single-electron reduction of the silyl enolate, 2) protonation with a phenol, 3) another single-electron reduction to form a siloxy-substituted benzylic anion, and 4) the pivotal retro-Brook rearrangement.

Aluminyl derived ethene functionalization with heteroallenes, leading to an intramolecular ligand rearrangement.

The aluminacyclopropane K[Al(NON)(η-C2H4)] ([NON]2- = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3) reacts with CO2 and iPrNCNiPr to afford ring-expanded products of C-C bond formation. The latter system undergoes a 1,3-silyl retro-Brook rearrangement of the NON-group, to afford the [NNO]2- ligand ([NNO]2- = [N(Dipp)SiMe2N(Dipp)SiMe2O]2-). The mechanism of transformation was examined by density functional theory (DFT).

Unexpected Propargylic Retro-Brook Rearrangements in Alkynes.

Retro-Brook rearrangements refer to the intramolecular migration of a silyl group from oxygen to carbon. In this study, we report a novel propargylic retro-Brook rearrangement observed in terminal alkynes bearing a silyl ether moiety. Retro-Brook rearrangements involving [1,2]-, [1,4]-, and [1,5]-migrations are described, affording propargylsilanes in reasonable yield. The reaction mechanism was investigated experimentally by deuterium quenching and rationalized by density functional theory calculations. The terminal alkyne and the subsequent propargyl/allenyl dianion were shown to be crucial for the reaction favoring the retro-Brook rearrangement product over the Brook rearrangement. The second deprotonation at the propargylic position was determined to be the rate-limiting step. In addition, a gas-phase Brook-type rearrangement of the propargylsilanes was observed under GC-MS conditions. This observation was also further confirmed by DFT calculations.

Concise Approach to Cyclohexyne and 1,2-Cyclohexadiene Precursors.

Silyl triflate precursors to cyclic alkynes and allenes serve as valuable synthetic building blocks. We report a concise and scalable synthetic approach to prepare the silyl triflate precursors to cyclohexyne and 1,2-cyclohexadiene. The strategy involves a retro-Brook rearrangement of an easily accessible cyclohexanone derivative, followed by triflation protocols. This simple, yet controlled, method should enable the further study of strained alkynes and allenes in chemical synthesis.

Retro-Brook Rearrangement of Ferrocene-Derived Silyl Ethers.

An intramolecular Li-Si exchange was observed on various lithiated ferrocenylbenzyl silyl ethers. The thermodynamically more stable C-silylated isomers were isolated in good yields and fully characterized. The reaction mechanism of the [1,4] retro-Brook rearrangement was investigated by DFT calculations. Two distinct reaction routes were proposed and a possible stabilization effect of the ferrocenyl fragment on the C-silylated isomers was described. The diastereoselective rearrangement of the trimethylsilyl group to the ortho position of the ferrocenyl cyclopentadienyl ring was also accomplished and the absolute configuration of the product was determined.

Synthesis with perfect atom economy: generation of furan derivatives by 1,3-dipolar cycloaddition of acetylenedicarboxylates at cyclooctynes.

Cyclooctyne and cycloocten-5-yne undergo, at room temperature, a 1,3-dipolar cycloaddition with dialkyl acetylenedicarboxylates 1a,b to generate furan-derived short-lived intermediates 2, which can be trapped by two additional equivalents of 1a,b or alternatively by methanol, phenol, water or aldehydes to yield polycyclic products 3b–d, orthoesters 4a–c, ketones 5 or epoxides 6a,b, respectively. Treatment of bis(trimethylsilyl) acetylenedicarboxylate (1c) with cyclooctyne leads to the ketone 7 via retro-Brook rearrangement of the dipolar intermediate 2c. In all cases, the products are formed with perfect atom economy.

ChemInform Abstract: The Hexadehydro‐Diels—Alder Reaction: A New Chapter in Aryne Chemistry

AbstractReview: 22 refs.

Silicon-Bridged Biaryls

The authors present a modular approach to silicon-bridged biaryls. Their key step is the palladium-catalyzed intramolecular direct arylation of 2-(arylsilyl)aryl triflates 7, 9 and 11. The approach is applicable to symmetrical, asymmetrical as well as heteroaromatic substrates and usually proceeds in good to excellent yields. The 2-(arylsilyl)aryl triflates were obtained by sequential silylation, retro-Brook rearrangement and triflation.

Ortho-Silylated Derivatives of Tetrakis(2-hydroxyphenyl)ethene: A Sterically Isolated Structural Model for Oxo-Surface Binding Domains

The introduction of sterically isolating ortho-trialkylsilyl, -aryldialkylsilyl, and -diarylalkylsilyl substituents onto the structurally preorganized tetrakis(2-hydroxyphenyl)ethene ligand framework has been accomplished by a 4-fold retro-Brook rearrangement. Installation of the most sterically demanding silyl substituents required the development of an iterative procedure, involving successive double silylation/metalation/migration sequences without the isolation of intermediates. This system was designed to function as a soluble structural model for the planar binding domains of heterogeneous "oxo-surfaces" of silica and alumina supports.

Expedient, high-yielding synthesis of silyl-substituted salen ligands.

Described is an efficient synthesis of silyl-substituted salen ligands, used for the preparation of enantioselective catalysts. The salicylaldehyde precursors are synthesized from the silyl ethers of 2,6-dibromophenols via a one-pot double lithium halogen exchanges, to induce an intramolecular retro-Brook rearrangement and allow introduction of the aldehyde group. Condensation of the salicylaldehyde products with a chiral diamine affords the silyl-substituted salen ligands in high yields. The use of other electrophiles allows easy access to silyl-substituted phenolic esters, ketones, and boronic acids.

Synthesis and Utility of Fluorinated Acylsilanes

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Synthesis of 2,4,5-Trisubstituted Thiazoles with a 5-(N,N-Dimethylaminomethyl) Substituent

We report synthetic strategies to 2,4,5-trisubstituted thiazoles with a N,N-dimethylaminomethyl residue at C(5). Three different routes to build up these scaffolds are described. Furthermore, we report a retro-Brook rearrangement of a thiazole derivative as well as an unusual cyclization leading to a highly substituted benzothiazole.

Retro-Brook rearrangement induces the formation of an octahedral nickel(benzamidinate) complex; synthesis, structure and catalytic activity

The neutral octahedral nickel complex (PhC(NSiMe 3)NC(Ph) CHSiMe 3)Ni(acac)(TMEDA) ( 7), has been synthesized and characterized including X-ray diffraction analysis. The complex was formed by the reaction of Ni(acac) 2(TMEDA) with the lithium salt of the corresponding β-diketiminate ligand. The formation of the benzamidinate motif from the corresponding β-diketiminate is a consequence of a retro-Brook isomerization that is operative only at the nickel centre. A plausible mechanism for the metal mediated isomerization is proposed. When complex 7 was activated with MAO it showed a good catalytic activity for the addition polymerization of norbornene. Furthermore, this catalytic system has been found to oligomerize ethylene to a mixture of butenes and hexenes with a high turnover frequency, η = 29,300 h −1, when the reaction is performed in dichloromethane.

The Reaction of Butatrienolates with Aldehydes for the Syntheses of ?-Vinylidene Acylsilanes.

For Abstract see ChemInform Abstract in Full Text.

Synthesis of Mono‐ and Difluoroacetyltrialkylsilanes and the Corresponding Enol Silyl Ethers.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of mixed-donor amido/amino/siloxo ligands from symmetrical diamidosilylether ligands via a retro-Brook rearrangement and their chromium(ii) complexes

The dilithio salts of two chelating diamidosilylether ligands undergo a retro-Brook rearrangement from oxygen to the amido-nitrogen, thereby directly yielding a new mixed-donor amido/amino/siloxo ligand. When reacted with chromium(II), siloxo-bridged dimers and a very strong eta(2)-(N,C(ipso)) arylamido interaction are featured.

Can relief of ring-strain in a cyclopropylmethyllithium drive the Brook rearrangement?

α-Cyclopropyl-α-trialkylsilyl alkoxides were formed either by addition of cyclopropyllithiums to acylsilanes or by addition of organolithiums to a cyclopropylformylsilane. [1,2]-Brook rearrangement led to α-silyloxy organolithiums which on warming underwent cyclopropane ring opening and [1,5]-retro-Brook rearrangement to yield γ-silyl ketones. Despite the favourability of the cyclopropane ring opening, the Brook rearrangement still required the presence of an anion stabilising group to proceed. β-Silylketones were similarly formed by Brook–retro-Brook rearrangement on warming acylsilanes with a vinyllithium.

The Reaction of Butatrienolates with Aldehydes for the Syntheses of α-Vinylidene Acylsilanes

Abstract Lithium butatrienolates were prepared in situ by the 1,4-elimination from 2-butynyl trimethylsilyl ethers along with a retro-Brook rearrangement. The addition reaction of the enolates with the aldehydes afforded β-hydroxy-α-vinylidene acylsilanes.

Synthesis of mono- and difluoroacetyltrialkylsilanes and the corresponding enol silyl ethers.

A variety of mono- and difluoroacetylsilanes and the corresponding silyl enol ethers were prepared from trifluoroethanol and chlorotrialkylsilanes in the presence of LDA through retro-Brook rearrangement. Sterically demanding silyl groups, especially those bound to oxygen, resulted in higher yields of difluoroacetylsilanes. The yields of difluoroacetylsilanes were also dramatically affected by the method of the termination of the reaction. Difluorohaloacetylsilanes were prepared from the corresponding difluoroethenyl silyl ethers with electrophilic halogenating reagents in good yields. A gamma-fluorinated beta-diketone 9a was prepared from monofluoroacetyltriisopropylsilane by nucleophilic acylation with methyl trifluoroacetate.

ChemInform Abstract: New Transformations from a 3-Silyloxy 2-Aza-1,3-diene: Consecutive Zr-Mediated Retro-Brook Rearrangement and Reactions with Electrophiles.

Abstract A one-pot procedure for the transformation of the title compound to α-functionalized (silylated) and α,β-unsaturated secondary amides was described. The following steps were involved: a Zr-mediated retro-Brook rearrangement, selective deprotonation with n-BuLi on the organometallic intermediate, and trapping with electrophiles including alkyl and acyl halides and aldehydes. The electrophilic addition step occurred at a stabilized α-silyl carbanion center without affecting the near transition metal residue. In the case of aldehydes, the Peterson alkenation reaction took place on the transition metal complex in a highly stereoselective way.