Strained organosilacyclic compounds: synthesis of anti-Bredt olefins and trans-dioxasilacyclooctenes

Prévost, Michel; Woerpel, K. A.

doi:10.1039/c003227a

Cited by 14 publications

(13 citation statements)

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“…[46][47][48] Recent progress has resulted in a number of new routes to heteroatom containing trans-cycloalkenes. Woerpel [49][50][51][52][53][54][55][56][57] and Tomooka [58][59][60][61][62][63] have described a number of methods for preparing oxasila-trans-cycloalkenes as well as the synthesis and chemistry of oxa, aza and sulfa (E,Z)-nonadienes.…”

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Flow Photochemical Syntheses of trans‐Cyclooctenes and trans‐Cycloheptenes Driven by Metal Complexation

Pigga

Fox

2019

Israel Journal of Chemistry

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trans-Cyclooctenes and trans-cycloheptenes have long been the subject of physical organic study, but the broader application had been limited by synthetic accessibility. This account describes the development of a general, flow photochemical method for the preparative synthesis of trans-cycloalkene derivatives. Here, photoisomerization takes place in a closed-loop flow reactor where the reaction mixture is continuously cycled through Ag(I) on silicagel. Selective complexation of the trans-isomer by Ag(I) during flow drives an otherwise unfavorable isomeric ratio toward the transisomer. Analogous photoreactions under batch-conditions are low yielding, and flow chemistry is necessary in order to obtain trans-cycloalkenes in preparatively useful yields. The applications of the method to bioorthogonal chemistry and stereospecific transannulation chemistry are described.The unusual reactivity and well-defined chiral structure of trans-cycloalkenes has made them attractive targets for synthesis for nearly 70 years. [1][2][3] For example, trans-cyclooctene possesses planar chirality and displays a high barrier to racemization (E a = 35.6 kcal/mol), [4] and the most stable "crown" conformer has an alternating sequence of equatorial and axial hydrogens that is akin to chair cyclohexane. [5][6] The double bond of trans-cyclooctene is twisted severely in the crown conformation, [7] and as a consequence the HOMO of trans-cyclooctene is relatively high in energy. [8] trans-Cycloheptene also has a rigid structure with a distorted alkene. [7] The double bonds of medium-ring trans-alkenes are twisted. [7] trans-Cycloalkenes display unusual reactivity in HOMOalkene controlled cycloaddition reactions with dienes, [9] 1,3dipoles [8] and ketenes. [10] Strained trans-cycloalkenes also serve as ligands for transition metals. [11][12][13][14][15][16] This reactivity profile has made trans-cycloalkanes interesting targets for applications in synthesis and biology.The broadest applications of trans-cycloalkenes are in the field of bioorthogonal chemistry. The inverse-electron demand Diels-Alder reaction between tetrazines and strained alkenes has a rich history in physical organic and synthetic chemistry (Figure 1). [17][18][19] In 2008, three groups described the bioorthogonal reactions of tetrazines with strained alkenes. [20][21][22] The variant introduced by our group that used trans-cyclooctene is marked by exceptionally rapid kinetics, with rate constants that can exceed 10 6 M À 1 s À 1 in the fastest cases. [23][24] With the advances including the development of fluorogenic tetrazines [25] and reactions in live cells [26] and animals, [27] the tetrazine ligation has become a widely used tool for applications that span chemical biology, biomedical imaging, and materials science. [28][29][30][31][32][33][34][35][36][37] This account describes the development of general, flow photochemical methods for the preparative synthesis of transcycloalkene derivatives and enabled applications in synthesis and bioorthogonal chemistry. Selectiv...

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Flow Photochemical Syntheses of trans‐Cyclooctenes and trans‐Cycloheptenes Driven by Metal Complexation

Pigga

Fox

2019

Israel Journal of Chemistry

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“…[31,32] Stabilization of a radical by an adjacent alkynyl group supports this mechanism and explains the observed regioselectivity of silaoxetane formation. [27,31,32,36] The relative configurations of the axis of chirality and the point of chirality of the cyclic allene are established in the transition state for aldehyde addition (Figure 2). [34,35] Silylene transfer to the acylic propargyl ethers 14 a,b provided propargyl silanes 15 a,b as single regioisomers [Eq.…”

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confidence: 99%

“…(4)]. [27,36] The cyclic transition-state D accounts for how the point chirality of the starting material is translated to both the point chirality and axial chirality of the product. By contrast, insertions of silylenes into allylic carbon-oxygen bonds are accompanied by allylic transposition.…”

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confidence: 99%

“…By contrast, insertions of silylenes into allylic carbon-oxygen bonds are accompanied by allylic transposition. [27,31,32,36] The relative configurations of the axis of chirality and the point of chirality of the cyclic allene are established in the transition state for aldehyde addition (Figure 2). Coordination of the carbonyl group to the Lewis-acidic silicon atom [37] of the strained four-membered-ring silane forms the adduct C in which the aldehyde is activated for carbon-carbon bond formation.…”

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confidence: 99%

“…[27] This pentacoordinate silicon atom adopts a geometry in which the tert-butyl groups are placed in basal positions to avoid steric interactions with each other. [27,36] The cyclic transition-state D accounts for how the point chirality of the starting material is translated to both the point chirality and axial chirality of the product.…”

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confidence: 99%

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