The conversion of renewable biomass resources to synthetically valuable chemicals is highly
desirable, but remains a formidable challenge in regards to the substrate scope and reaction
conditions. Here we present the development of tris(pentafluorophenyl)borane–catalysed
conversion of furans via ring-opening and closing cascade processes to afford siliconfunctionalized
synthetic chemicals under transition metal-free conditions. The furan
ring-opening with hydrosilanes is highly efficient (TON up to 2,000) and atom-economical
without forming any byproduct to give rise to a-silyloxy-(Z)-alkenyl silanes. Additional
equivalents of silane smoothly induce a subsequent B(C6F5)3-catalysed cyclization of initially
formed olefinic silane compounds to produce anti-(2-alkyl)cyclopropyl silanes, another
versatile synthon being potentially applicable in the synthesis of natural products and
pharmacophores.
(c) The Author(s) 201611sciescopu

Chinmoy K. Hazra

Narasimhulu Gandhamsetty

Sehoon Park

Sukbok Chang

Synthesis

English

The conversion of renewable biomass resources to synthetically valuable chemicals is highly
desirable, but remains a formidable challenge in regards to the substrate scope and reaction
conditions. Here we present the development of tris(pentafluorophenyl)borane–catalysed
conversion of furans via ring-opening and closing cascade processes to afford siliconfunctionalized
synthetic chemicals under transition metal-free conditions. The furan
ring-opening with hydrosilanes is highly efficient (TON up to 2,000) and atom-economical
without forming any byproduct to give rise to a-silyloxy-(Z)-alkenyl silanes. Additional
equivalents of silane smoothly induce a subsequent B(C6F5)3-catalysed cyclization of initially
formed olefinic silane compounds to produce anti-(2-alkyl)cyclopropyl silanes, another
versatile synthon being potentially applicable in the synthesis of natural products and
pharmacophores.
(c) The Author(s) 201611Nsciescopu

IBS Publications Repository

© Georg Thieme Verlag Stuttgart • New York – Synform 2017/03, A48–A52 • Published online: February 15, 2017 • DOI: 10.1055/s-0036-1590074Literature CoverageSynformA48A number of transition-metal complexes are known to effi-ciently catalyze hydrosilylation of unsaturated functionalities including C=O, C=N and C=C bonds, largely via inner- or outer-sphere pathways. Representatively, a series of platinum-based hydrosilylation catalysts (e.g. Karstedt’s catalyst) display powerful and selective catalytic performance, especially in hydrosilylation of alkenes, thus enabling large-scale syn - thes is of various alkyl silanes in industry. However, most of the presently available hydrosilylation processes rely on the use of expensive transition metals (Rh, Ir, Pt, or Pd). In this regard, certain Lewis acids such as B(C6F5)3 have drawn sig-nificant attention as catalysts due to their practical merits. In 1996, the Piers group first reported the B(C6F5)3-catalyzed hydrosilylation of aromatic aldehydes, ketones, and esters (for references see the original Nat. Commun. article). Since then, the B(C6F5)3 catalyst system has been shown to be effective not only for hydrosilylation of unsaturated functionalities but also for reductive sp3-C–X bond cleavage (X = O, S, or halides) using hydrosilanes. The Park and Chang group from the Institute for Basic Science and KAIST (Daejeon, South Korea) recently re-ported the B(C6F5)3-catalyzed dearomative silylative reduction of quinolines and pyridines leading to (partially) saturated azacyclic products having sp3-C–Si bonds beta to the nitrogen atom. Subsequently, they also showed that α,β-unsaturated nitriles and esters can undergo a selective silylative reduction.Continuing their efforts along these lines, Professor Chang and co-workers turned their attention to furans, one of the representative biomass-derived chemicals, mainly due to the fact that furans are predicted to undergo reductive cleav-age serving as various types of carbon sources. The Chang group envisioned that B(C6F5)3 would be capable of catalyz-ing a hydro silylative transformation of furans. Professor Chang said: “The unique reactivity of B(C6F5)3/hydrosilane toward the sp3-C–O and sp2-C=C bonds initially made us curious about which products could be generated from furans under the B(C6F5)3-mediated hydrosilylation conditions.” In a prelimina-ry reaction, 2-methylfuran (I) was subjected to the B(C6F5)3-catalytic conditions to reveal that I underwent ring-opening with PhMe2SiH, leading to the corresponding alkenyl silyl ether bear ing an sp3-C–Si bond alpha to the oxygen atom (II). Interestingly, the double bond in the product was deter mined to be exclus ively Z. “Such an unprecedented ring-opening pro-duct with excellent chemo-, regio-, and stereoselectivities under mild metal-free conditions is considered to be excep-tional, and it also caught our attention with regard to the me-chanistic path way,” remarked Professor Chang. Through a set of optimization studies, the authors found that as little as 2.0 mol% of B(C6F5)3 with 2.05 equivalents of PhMe2SiH allowed for quantitative silylative ring opening of I at room tempera-ture within ten minutes (Scheme 1).“More interestingly, when one more equivalent of  PhMe2SiH was added into the reaction mixture, we observed an exothermic reaction with a new product formation,” said Professor Chang. He continued: “The structure of this new compound was identified to be a silylated cyclopropane (III) with exclusive anti-diastereoselectivity with the formation of a stoichiometric amount of disiloxane by-product.”To gain mechanistic insights, the Chang group conduct-ed an NMR study in a reaction of 2-methylfuran (I) with  PhMe2SiH (4.0 equiv, Scheme 2). “Low-temperature NMR monitoring was a useful analytical technique especially for a rapid cascade transformation as in this case,” remarked Pro-fessor Chang. He continued: “The reaction was observed to proceed smoothly at –70 °C leading to (Z)-α-silyloxy alkenyl Borane-Catalyzed Ring-Opening and Ring-Closing Cascades of  Furans Leading to Silicon-Functionalized Synthetic IntermediatesNat. Commun. 2016, 7, 13431Scheme 1 B(C6F5)3-catalyzed silylative ring-opening and ring-closing cascade of 2-methylfuran (I)Downloaded by: Korea Advanced Institute of Science and Technology. Copyrighted material.© Georg Thieme Verlag Stuttgart • New York – Synform 2017/03, A48–A52 • Published online: February 15, 2017 • DOI: 10.1055/s-0036-1590074Literature CoverageSynformsilane (II) quantitatively over 3.5 hours. Upon further warming to room temperature, the in situ generated intermediate II was converted into the corresponding silylated cyclopropane (III). These results clearly indicated that the ring-opening and ring-closing cascade of 2-methylfuran (I) proceeded under perfect kinetic differentiation.”With this mechanistic depiction, Chang and co-workers explored the substrate scope (Scheme 3). Professor Chang said: “We were pleased to see that a variety of 2-substituted furans were transformed into a single product of α-silyloxy-(Z)-homoallylsilanes in high yields under standard conditions with excellent stereoselectivity (Z/E > 99:1, Scheme 3; Condi-tions A).” Professor Chang also said: “In agreement with the kinetic behavior observed in the low-temperature NMR study, a range of 2-substituted furans were smoothly converted into anti-2-alkylcyclopropyl silanes at room temperature in good to high yields irrespective of their electronic and steric varia-tions when PhMe2SiH (4.0 equiv) was used in the presence of B(C6F5)3 (5.0 mol%) catalyst (>99% anti-selectivity, Scheme 3; Conditions B).”Subsequently, Professor Chang and co-workers found that the present B(C6F5)3 catalysis was applicable for the silyla tive ring opening of additional furan derivatives, providing the corresponding silylated products in good yields (Scheme 4). “It is notable that the chemoselectivity was altered depend-ing on the position of substituents on the furan substrates, thus delivering a range of various ring-opening products,”  remarked Professor Chang.In addition, Professor Chang and co-workers demonstrat-ed the synthetic utility of two types of products obtained through the present B(C6F5)3-catalyzed hydrosilylation cas-cade of furans (Scheme 5). Professor Chang explained: “The A49Scheme 2 Borane-catalyzed ring-opening and ring-closing cascade of furans giving rise to synthetically valuable silicon compounds with 1H NMR monitoring of this process (Si = SiMe2Ph)Downloaded by: Korea Advanced Institute of Science and Technology. Copyrighted material.© Georg Thieme Verlag Stuttgart • New York – Synform 2017/03, A48–A52 • Published online: February 15, 2017 • DOI: 10.1055/s-0036-1590074Literature CoverageSynformA50Scheme 3 B(C6F5)3-catalyzed cascade silylative transformation of furans (Si = SiMe2Ph)Scheme 4 B(C6F5)3-catalyzed silylative ring opening of alkyl furans and benzofurans (Si = SiMe2Ph, R3 = 4-TIPSO-C6H4, TIPS = triiso-propylsilyl)Downloaded by: Korea Advanced Institute of Science and Technology. Copyrighted material.© Georg Thieme Verlag Stuttgart • New York – Synform 2017/03, A48–A52 • Published online: February 15, 2017 • DOI: 10.1055/s-0036-1590074Literature CoverageSynformA51obtained products of α-silyloxy homoallylsilanes and anti-2-alkylcyclopropyl silanes possess synthetic building units which are readily transformed into other synthetically valu-able functional groups. Therefore, the synthetic utility of the present method could be potentially broad in synthetic and medicinal chemistry.”“In conclusion, chemodivergent catalytic transformations of furans have been developed to furnish synthetically valu-able silicon-functionalized products, α-silyloxy-(Z)-alkenyl silanes and anti-cyclopropyl silanes with excellent diastereo-selectivity,” said Professor Chang. He also noted: “The mechan-istic pathway of this cascade reaction was well elucidated by a series of mechanistic experiments.” Finally, he commented: “The present procedure showcases an example of biomass conversion to provide synthetically valuable chemicals under extremely mild and convenient conditions without requiring transition-metal species.”Scheme 5 Enrichment and elaboration of products (Si = SiMe2Ph, Si = SiPh2H/SiMe2Ph)Downloaded by: Korea Advanced Institute of Science and Technology. Copyrighted material.© Georg Thieme Verlag Stuttgart • New York – Synform 2017/03, A48–A52 • Published online: February 15, 2017 • DOI: 10.1055/s-0036-1590074Literature CoverageSynformA52Chinmoy Kumar Hazra is currently working as a postdoctoral scientist under Professor Sukbok Chang at the Institute for Basic Science (IBS) (South Korea). He obtained his Ph.D. from the Westfälische Wilhelms-Uni-versität Münster (Germany) in 2013 (Professor Martin Oestreich) and his M.S. degree in chemistry from the In-dian Institute of Technology Bombay (India) in 2010. He also stayed at the University of Strasbourg (France) for a postdoctoral experience with Professor Françoise Colobert. His research interests include the development of metal-free catalysis with mechanistic understanding and synthetic appli-cations.Narasimhulu Gandhamsetty was born in 1980 and raised in Sathupalli (Kadapa, India). He obtained his Ph.D. at the Indian Institute of Chemical Technology (India) under Professor Jhillu S. Yadav in 2014. Currently, he is working as a postdoctoral scientist under Professor Sukbok Chang at the Institute for Basic Science (IBS) (South Korea). He received an ‘outstanding research award’ from IBS in 2016. Hisresearch interests are the develop-ment of new synthetic methods, silylative reductions, and orga-nocatalytic methodologies for general applications in synthetic organic chemistry.Sehoon Park was born in Seoul  (South Korea) in 1977 and received his Ph.D. (2008) in chemistry from the Tokyo Institute of Technology (Japan) under Professor Kohtaro  Osakada, where he received a Monbukagakusho schol-arship (2004–2008). He then joined the University of North Carolina at Chapel Hill (USA) as a postdoctoral fellow (Professor Maurice Brookhart, 2009–2012). In 2013, he joined Pro-fessor Chang’s group at the Institute for Basic Science (IBS) (South Korea), where he is a senior re-search fellow. He was a recipient of the outstanding research award in 2014. Currently, he is also an Adjunct Professor at the Korea University of Science and Technology. His research inter-ests are synthetic and mechanistic organometallic chemistry as well as synthetic methodology in catalysis.Sukbok Chang is Director at the Cen-ter for Catalytic Hydrocarbon Func-tionalizations in a program of the In-stitute for Basic Science (IBS)  (South Korea) and also Professor at the Korea Advanced Institute of  Science & Tech-nology (KAIST). In 1996, he earned his Ph.D. at Harvard University (USA) un-der Professor Eric N.  Jacobsen. After postdoctoral work at Caltech (USA) with Professor Robert H. Grubbs,he joined Ewha Womans University in Seoul (South Korea) as an Assistant Professor in 1998, and moved to KAIST in 2002. His research interests in clude the de-velopment and mechanistic understanding of metal-catalyzed organic transformations.About the authorsDr. C. K. Hazra Dr. S. ParkProf. S. ChangDr. N. GandhamsettyDownloaded by: Korea Advanced Institute of Science and Technology. Copyrighted material.

Borane-Catalyzed Ring-Opening and Ring-Closing Cascades of Furans Leading to Silicon-Functionalized Synthetic Intermediates

https://pr.ibs.re.kr/bitstream/8788114/4813/1/c_Synthesis-0036-1590074%20%ed%8e%98%ec%9d%b4%ec%a7%80.pdf

Borane-Catalyzed Ring-Opening and Ring-Closing Cascades of Furans Leading to Silicon-Functionalized Synthetic Intermediates

Abstract

Similar works

Full text

Available Versions

IBS Publications Repository