Asymmetric Allyl- and Crotylboration with the Robust, Versatile, and Recyclable 10-TMS-9-borabicyclo[3.3.2]decanes

Burgos, Carlos; Canales, Eda; Matos, Karl; Soderquist, John A.

doi:10.1021/ja043612i

Cited by 151 publications

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References 62 publications

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“…[1] Prepared through adaptations of known organoborane transformations, these rigid and robust trialkyl borane systems are exceptionally stable and selective. Moreover, the related 10-Ph-9-BBD reagents, which are quite effective for the related addition reactions to ketones and ketimines, can also be prepared readily.…”

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confidence: 99%

“…This relative insensitivity of the enantioselectivity of the process to the reaction temperature is a signature feature of the BBD reagents. [1] The g-silyl group in the allenyl borane introduces a second stereogenic center in the homopropargylic alcohols 7. In related homoallylic systems, this b-silyl substitution has been shown to provide useful functionality for further synthetic conversions.…”

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confidence: 99%

“…The pre-transition-state complex A represents a convenient way to view the origin of the observed selectivity for this and related processes. [1][2][3] An interesting extension of the chemistry of these new organoboranes is illustrated by the conversion of 2 d into the allenyl borane (R)-1 d and the reactivity of this compound, which is both an allenyl and an allyl borane. The treatment of (R)-1 d with PhCHO resulted in the exclusive formation of the spectacular chiral 1,2,3-trienyl alcohol 10 (Scheme 4).…”

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Asymmetric Synthesis of Isomerically Pure Allenyl Boranes from Alkynyl Boranes through a 1,2‐Insertion–1,3‐Borotropic Rearrangement

2006

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Very recently, we described a variety of new 10-trimethylsilyl-9-borabicyclo[3.3.2]decane (10-TMS-9-BBD) reagents for the asymmetric allyl-, crotyl-, allenyl-, and propargylboration of aldehydes.[1] Prepared through adaptations of known organoborane transformations, these rigid and robust trialkyl borane systems are exceptionally stable and selective. Moreover, the related 10-Ph-9-BBD reagents, which are quite effective for the related addition reactions to ketones and ketimines, can also be prepared readily. [2] Simple Grignard procedures can be used to prepare many of these reagents, including the B-alkynyl 10-TMS-9-BBDs (2), the asymmetric Michael addition of which to N-acyl aldimines provides nonracemic N-propargyl amides.[3] On the basis of modeling studies with 2, we envisaged the highly stereoselective insertion of TMSCHN 2 into the alkynyl BÀC bond of 2 to give 4 via 3 (Scheme 1). An antiperiplanar 1,2-alkynyl migration (Matteson-type homologation) [4a] followed by a sterically driven suprafacial 1,3-borotropic rearrangement were expected to proceed with a minimum of TMS-TMS repulsions and ultimately convert the propargyl borane 4 into the novel chiral allenyl borane 1. [1c, 2a, 4b] The thermally stable, optically pure alkynyl boranes 2 (a: R = Me; b: R = (CH 2 ) 4 Cl; c: R = c-Pr; c-Pr = cyclopropyl) are readily prepared in either enantiomeric form through the reaction of the corresponding alkynyl Grignard reagents with the pseudoephedrine complexes 6 (96-99 %). At room temperature, TMSCHN 2 reacts cleanly with 2 with the evolution of nitrogen to form 1 ( 11 B NMR: d = 78 ppm) in 3 h. We view this process as occurring through the reversible addition of TMSCHN 2 to the least hindered side of 2, followed by the insertion of CHTMS into 2 with inversion. The chiral a-borylallenes 1 are then produced in enantiomerically and diastereomerically pure form in essentially quantitative yield by a suprafacial 1,3-borotropic rearrangement (Scheme 1).Asymmetric allenylboration [1b, 2c, 5] is a highly useful organoborane transformation. Compound 1 was tested as an asymmetric allenylboration reagent with representative aldehydes (Scheme 2). The addition was rapid at À78 8C (3 h), and the intermediate borinic esters 5 produced were either oxidized or converted into the pseudoephedrine complexes 6 for recycling back to 2 by the Grignard methodology. The syn b-TMS homopropargylic alcohols 7 were isolated in 80-96 % yield with d.r. 99:1 and 94-99 % ee (Table 1). This process is more enantioselective than allenylboration with the parent B-allenyl system (93-95 % ee).[1b] The enhanced enantioselectivity can be attributed to the additional a substituent Scheme 1. The preparation of 1 from 2 through insertion-borotropic rearrangement. TMS = trimethylsilyl.Scheme 2. Asymmetric allenylboration with 1.

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Asymmetric Synthesis of Isomerically Pure Allenyl Boranes from Alkynyl Boranes through a 1,2‐Insertion–1,3‐Borotropic Rearrangement

2006

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“…[8,9] We converted 1-Cl to the methoxy derivative 1,2-Fc(BMe(OMe))-(SnMe 2 Cl) (1-OMe) and treated the latter with 0.5 equivalents of (1R,2R)-(À)-N-methylpseudoephedrine ((À)-MPE) or (1S,2S)-(+)-N-methylpseudoephedrine ((+)-MPE, Scheme 1). [10,11] The crude product was extracted with hexanes, and the chelate complexes (R p )-2-(À)-MPE and (S p )-2-(+)-MPE were isolated from the corresponding solid fractions; the characteristics of the (S p )-2-(+)-MPE enantiomer are described below, and similar considerations apply to the R p isomer.…”

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“…[9,14] The boron-allylated compounds (R p )-3 and (S p )-3 are obtained selectively from (R p )-1-Cl and (S p )-1-Cl, respectively, by metathesis with Me 3 SnAll (Scheme 2, All = allyl). Alternatively, the bis-allyl compounds (R p )-4 and (S p )-4 can be generated by reaction of the methoxy derivatives (R p )-1-OMe/(S p )-1-OMe or the ephedrine complexes (R p )-2-(À)-MPE/(S p )-2-(+)-MPE with 1.95 equivalents AllMgBr, as illustrated in Scheme 2 for the S p enantiomer.…”

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Allylation of Ketones with a Ferrocene‐Based Planar Chiral Lewis Acid

2008

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Flach wie eine Hand: Die Enantiomere der zweizähnigen Lewis‐Säure 1,2‐Fc(BMeCl)(SnMe2Cl) können durch stereoselektive Komplexierung mit N‐Methylpseudoephedrin leicht getrennt werden. Die resultierenden planar‐chiralen Lewis‐Säuren, deren optische Reinheiten 97 % übersteigen, können nach Umwandlung in die abgebildeten Allylboranderivate schnell und mit Enantioselektivitäten bis 80 % an Ketone addieren.

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(+)-9-(1S,2S-Pseudoephedrinyl)-(10R)-(trimethylsilyl)-9-borabicyclo[3.3.2]decane

Soderquist

2009

Encyclopedia of Reagents for Organic Synthesis

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Asymmetric Allyl- and Crotylboration with the Robust, Versatile, and Recyclable 10-TMS-9-borabicyclo[3.3.2]decanes

Cited by 151 publications

References 62 publications

Asymmetric Synthesis of Isomerically Pure Allenyl Boranes from Alkynyl Boranes through a 1,2‐Insertion–1,3‐Borotropic Rearrangement

Asymmetric Synthesis of Isomerically Pure Allenyl Boranes from Alkynyl Boranes through a 1,2‐Insertion–1,3‐Borotropic Rearrangement

Allylation of Ketones with a Ferrocene‐Based Planar Chiral Lewis Acid

(+)-9-(1S,2S-Pseudoephedrinyl)-(10R)-(trimethylsilyl)-9-borabicyclo[3.3.2]decane

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