Brook rearrangement reverse

Brook rearrangement, reverse 44, 514 Bruylants-type reaction 44, 899 1,3-Butadienes... 

These reactions are thought to proceed by initial formation of the lithio propargylic alcohol adduct, which undergoes a reversible Brook rearrangement (Eq. 9.14). The resulting propargyllithium species can equilibrate with the allenyl isomer and subsequent reaction with the alkyl iodide electrophile takes place at the allenic site. An intramolecular version of this alkylation reaction leads to cyclic allenylidene products (Eq. 9.15). 

Rearrangement of a-silyl oxyanions to a-silyloxy carbanions via a reversible process involving a pentacoordinate silicon intermediate is known as the [l,2]-Brook rearrangement, or [l,2]-silyl migration. 

It should be noted that the sequence deprotonation/reverse Brook rearrangement between the triisopropylsilyloxy allene and isopropylmagnesium chloride in THE does not provide the magnesium enolate. 

Sakaguchi, K. Fujita, M. Suzuki, H. Higa-shino, M. Ohfune, Y. Reverse Brook rearrangement of 2-alkynyl trialkylsilyl ether. Synthesis of optically active (l-hydroxy-2-alkynyl)-trialkylsilanes. Tetrahedron Lett. 2000, 43, 6589-6592. 

The two reverse Brook rearrangements shown below follow stereochemically different pathways. Suggest an explanation. 

Acylsilanes have been synthesized by several other approaches including (i) a reverse Brook rearrangement (silyl-Wittig rearrangement) of allyltriinethylsilyl ether followed by oxidation (Figure Si6.5)... 

Interestingly, the stereochemical outcomes of these reactions depend on the substituents. With aryl substituents at the carbon atom, an inversion of stereochemistry was observed (equation 28)78, while with alkyl groups at the carbon atom, the reverse Brook rearrangement occurs with retention of the stereochemistry (equation 29)80. 

Remarkably, 57, the trimethylsilyl analogue of 55, appears to racemise much more slowly. At -50 °C, it undergoes a Brook rearrangement (section 8.2) to give 59 with 99% inversion of stereochemistry. If the anion 58 is held at -40 °C for 20 h, or 25 °C for 13 min, the product is formed with about 10% racemisation, indicating the rearrangement is reversible and that... 

Trimethylsilyloxy)vinyllithium 548 has been prepared by tin-lithium exchange from the vinylstannane 552, which is generated from acetyltri-n-butyltin 551 (Scheme 149)830. This vinyllithium suffers a reverse Brook rearrangement to generate the alkoxide 553 used for the synthesis of acylsilanes831. 

Allenyl trialkylsilyl ethers 771 can be a-deprotonated with f-BuLi in THF at — 78 °C to give the allenyllithiums 77210901091. They underwent reverse Brook rearrangement to afford the silaacrolein enolates 773, which react with aldehydes and ketones to yield the a,/9-unsaturated acyl silanes 774 (Scheme 200). For enolizable aldehydes transmetallation with ZnCl2-TMEDA, and MgBr2 for ketones, provided better yields. 

Anionic migration of an alkylsilyl group from carbon to oxygen to afford silyl ether a-oxycarbanion is known as a Brook rearrangement, and the reverse reaction is called... 

One important aspect of the Cp2Zr-promoted cycliza-tion that is now well-established is the ready reversibility through decarbozirconation, presumably involving an interaction between the empty Zr-orbital and the C/S -Cfi bond of the five-membered zirconacycle and subsequent displacement of ethylene (equation 53). Reaction of 2-azadienes on zirconocene affords via a retro-Brook rearrangement interesting azaoxazirconacycles. ... 

Rearrangements have also been examined. In a study of the stereoselectivity of the reverse Brook rearrangement of... 

Liu, G., Sieburth, S. M. Enantioselective a-Silyl Amino Acid Synthesis by Reverse-Aza-Brook Rearrangement. Org. Lett. 2003, 5,4677-4679. 

Wang, Y., Dolg, M. Theoretical confirmation of the stereoselectivity In the reverse Brook rearrangement. Tetrahedron 1999, 55,12751-12756. 

LInderman, R. J., Ghannam, A. Synthetic utility and mechanistic studies of the aliphatic reverse Brook rearrangement. J. Am. Chem. Soc. 1990,112, 2392-2398. 

The above reverse Brook rearrangement also occurs during treatment of the silyl enol ether of o-bromoacetophenone with BuLi. Quenching the enolate with PhCHO gives an aldol product. ... 

The reverse process of a Brook rearrangement, the O- to C-trialkylsilyl group shift from a silyl ether to a carbanion terminus, is well-known and has been intensively studied [Eq. (22)] [25]. 

Hydroxysilanes from 1,1-siloxystannanes Reverse Brook rearrangement... 

Linderman and Ghannam have utilized EtsSiOTf to trap the alkoxide formed from the addition of stannyl anion to aldehydes (eq 7). Upon treatment with excess butyllithium, these adducts undergo a reverse Brook rearrangement to afford a-hydroxysilanes. 

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