1,3-Brook rearrangement

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. 

Adrian G. Brook (1924—) was bom in Toronto, Canada. He is a professor in Lash Miller Chemical Laboratories, University of Toronto, Canada. 

Name Reactions, 4th ed., DOI 10.1007/978-3-642-01053-8 34, Springer-Verlag Berlin Heidelberg 2009 

Example 2, [l,2]-Brook rearrangement followed by a retro-[ 1,5]-Brook rear- 

Name Reactions A Collection of Detailed Mechanisms and Synthetic Applications, DOI 10.1007/978-3-3I9-03979-4 39, Springer International Publishing Switzerland 2014 

Brook had set the basis for developing the reaction that came to bear his name, but clearly many mechanistic details remained to be elucidated. In addition, Brook did not anticipate the potential utility of the rearrangement, in fact concluding the 1959 paper with what turned out to be wholly unwarranted self-deprecation It is doubtful whether the rearrangement reported here will have much synthetic application since in general silyl ethers are much more readily prepared than are the corresponding a-silylcarbinols.  

These careful studies supported the mechanism shown for 8 — 12. The first order kinetics in silylcarbinol were consistent with intramolecular C - O silyl migration. Reaction via 10 as a cyclic transition state, with the conjugate acid of the base remaining nearby, would account for the large negative entropy of activation. The Hammett studies indicated a transition state with buildup of carbanionic character in proceeding toward product. 

Brook addressed this important mechanistic issue early, developing evidence for retention of configuration at silicon in the rearrangement. Based on the precedented assumption of inversion at asymmetric silicon in chloride displacement with either an organolithium or an alcohol, Brook and coworkers showed a stereochemical Walden cycle that implicated retention at silicon in the rearrangement step. Mechanistically, retention at silicon corresponds to a frontside-type displacement, consistent with the mechanism shown above. The displacement might implicate 10 as a pentacoordinate silicon intermediate rather than a transition state. 

Brook was responsible for some of the earliest studies in this area. Initially, he reported retention of configuration at carbon in the rearrangement. However, Brook s later studies on related systems, 

In contemporaneous studies by Kuwajima, the 1-silyloxy allyl anions were generated from 1-trimethylsilyl allylic alcohols such as 24. Treatment with a catalytic amount of base led to silyl enol ether 25 in excellent yield and (Z)-selectivity. The alkene geometry was proposed to arise from chelate 26, a structure consistent with Reich s results. Protonation of anionic intermediate 26 by alcohol 24, would provide product 25 and the alkoxide of 24, poised for Brook rearrangement.  

Base-catalyzed silicon migration from carbon to oxygen.  

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In the well-known Brook rearrangment, silyl groups migrate from oxygen to carbon, but the following example is less obvious and not necessarily predictable ... 

Small groups on silicon favor the formation of the corresponding silyl dienol ether, formed by a Brook rearrangement of 81, and this reaction pathway is minimized by employment of P(OPh)3. 

Bromophenyl trimethylsilyl ether, 41 a-Bromovinyl(triphenyl)silane, 69 Brook rearrangement, 83-4 t-Butoxide-cntalysed condensation, 43 (/ )-4-t-Butyl l-(trimethylsilyloxy)-... 

A recent paper [44] shows that the treatment of silyl thioketones 68 with lithium diethylphosphite proceeds via a thiophiUc attack followed by a thio-phosphate mercaptophosphonate (69 70) carbanionic rearrangement and the migration of the silyl group from the carbon to the sulfur atom leading to the S-silylated sulfanylphosphonate carbanion 71. The last step represents the first example of the thia-Brook rearrangement (Scheme 18). 

Numerous examples of silicon rearrangements [88], for example the Brook rearrangement [89], are covered in pertinent reviews [88, 89]. 

Cyclization of the sulfoxide 1248 with TMSOTf 20/DlPEA affords a 4 1 mixture of the tetrahydroquinolines 1249 and 1250, in 97% yield, and HMDSO 7 [49]. On heating of the sulfoxide 1251 to 80 °C Brook rearrangement then Sila-Pummerer rearrangement-cyclization gives, via 1252, 17% 1253 [50] (Scheme 8.19). 

A novel chromium Brook rearrangement is suggested in the reductive ole-fination as shown in Scheme 26 [63]. 

Leaving the (retro-)aldol addition-initiated threefold anionic domino processes, we are now describing sequences which are initiated by a SN-type transformation. In particular, domino reactions based on SN/1,4-Brook rearrangement/SN reactions are well known. For example, the group of Schaumann obtained functionalized cyclopentanols of type 2-461 by addition of lithiated silyldithioacetals 2-458 to epoxy-homoallyl tosylates 2-459 in 41-75% yield (Scheme 2.106) [248]. 

Nucleophilic addition to an acylsilane followed by a 1,2-Brook rearrangement and final trapping of the resulting carbanion in either an acylation or intramolecular... 

The novel [6+2] annulation approach developed by the Takeda group has also been included in a threefold anionic/pericyclic process (Scheme 2.149) [340]. The reaction leads to functionalized eight-membered rings 2-659 in a highly stereoselective manner, starting from acylsilanes 2-656 and 3-(trimethylsilyl)vinyl-lithium (2-657). After 1,2-addition and 1,2-Brook rearrangement, the cyclobutane 2-... 

Scheme 4.26 Precedent for Brook rearrangement/transmetallation of vinylsilanes bearing allylic alcohols...

As a result of the excellent precedent from the Takeda and Smith groups, we targeted vinylsilane 136 as a key intermediate in a Brook rearrangement/conjugate... 

Scheme 4.27 Synthesis of Brook rearrangement substrate 136 featuring a three-step synthesis of polyfunctional building block 166...

Scheme 4.28 Leighton s precedent for siloxacycle ring opening and Brook rearrangement/C-C bond formation...

Scheme 4.29 Synthesis and Brook rearrangement of model system 173...

Scheme 4.30 Completion of a six-step linear synthesis of strychnine using a Brook rearrange-ment/conjugate addition to generate the Wieland-Gumlich aldehyde...

The spirocompounds 34 (M = Ti or Zr) have been prepared . Studies of the thermolysis of pentacoordinate 1,2-oxasiletanides 35, potential intermediates in both the Peterson reaction and the homo-Brook rearrangement of p-hydroxyalkylsilanes with bases, in the presence of a proton source afforded the olefin, RCH=C(CF3)2 and/or the alcohol, (CF3)2CHOH <99CL1139>. 

In a study designed to test the feasibility of a germanium aza Brook rearrangement, the /V-benzyl derivative 25 of amine 24 was treated with BuLi13. After subsequent addition of water, the hydride 28 was obtained as the sole Ge-containing product (Scheme 11). 

E)-Enol silyl ethers.1 A new highly stereoselective route to (E)-enol silyl ethers involves addition of CH,Li to silyl ketones substituted at the a -position by a SC6H5 group such as 1. The adduct (a) undergoes a Brook rearrangement and... 

Table 9.4 Formation of allenyllithium intermediates by a Brook rearrangement sequence.

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). 

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