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Aryl 2-silyloxyprop-1-yl ethers underwent intramolecular C(sp2)–H/C(sp3)–H cross-dehydrogenative coupling in the presence of Ir/(S)-DTBM-SEGPHOS catalyst. The reaction proceeded enantioselectively to afford 3-methyl-3-silyloxy-2,3-dihydrobenzfurans with up to 99% ee.Aryl 2-silyloxyprop-1-yl ethers underwent intramolecular C(sp2)–H/C(sp3)–H cross-dehydrogenative coupling (CDC) in the presence of Ir/(S)-DTBM-SEGPHOS catalyst. The reaction took place enantioselectively to afford 2,3-dihydrobenzfurans bearing a stereogenic carbon center at the C3 position with up to 99% ee. This C(sp2)–H/C(sp3)–H CDC proceeded through a catalytic tandem process involving dehydrogenation of the alkyl group and enantioselective intramolecular hydroarylation.

Helically chiral poly(quinoxaline-2,3-diyl)s bearing 4-(dipropylamino)pyridin-3-yl pendants at the 5-position of the quinoxaline ring (PQXdpap) exhibited high catalytic activities and moderate to high selectivities (up to s= 87) in the acylative kinetic resolution of secondary alcohols. The solvent-dependent helical chirality switching of PQXdpapbetween pure toluene and a 1:1 mixture of toluene and 1,1,2-trichloroethane enabled the preparation of either compound of a pair of enantiomerically pure alcohols (>99% ee) from a single catalyst.

The mechanism of 4,4′-bipyridine-catalyzed diboration of pyrazines was studied by experimental observation of the intermediates and by theoretical calculations. Intermediary radical species were detected by ESR measurement of the reactions of 2,6-dichloro-4,4′-bipyridines with bis(pinacolato)diboron and identified by simulation. Based on these observations, radical processes involving 4,4′-bipyridine-stabilized boryl radicals were evaluated by DFT calculations combined with single-component artificial force induced reaction (SC-AFIR). The results of calculations indicate that a radical transfer process from 4,4′-bipyridine-stabilized boryl radical to pyrazine is a major pathway in the catalytic reaction. The origin of the high catalytic efficiency of 2,6-dichloro-4,4′-bipyridine is ascribed to the effect of the chlorine atom on the stability of the corresponding N,N′-diboryl-4,4′-bipyridinylidene.4,4′-Bipyridine-stabilized boryl radicals were observed by ESR study of the reactions of bis(pinacolato)diboron with 2,6-dichloro-4,4′-bipyridines. Based on these observations, the mechanism of 4,4′-bipyridine-catalyzed diboration of pyrazines was studied using a theoretical computational approach. The results revealed that a major path is radical transfer from 4,4′-bipyridine-stabilized boryl radical to pyrazine. The origin of the high catalytic efficiency of 2,6-dichloro-4,4′-bipyridine is found to destabilize the corresponding N,N′-diboryl-4,4′-bipyridinylidene.

Silylboronic esters bearing a N,N-dialkylaminomethyl group on the silicon atoms were synthesized and used in palladium-catalyzed reactions. Five silylboronic esters were synthesized through a reaction of the [(N,N-dialkylaminomethyl)diorganosilyl]lithiums with i-PrOB(pin). An addition reaction of the silylboronic ester to ethynylbenzene (silaboration) took place efficiently by a phosphine- and isocyanide-free palladium catalyst to give (E)-1-silyl-2-boryl-1-phenylethene in a regio- and stereoselective manner. On the other hand, the silylboronic esters underwent β-elimination reactions in the presence of a palladium catalyst and styrene to afford 1,3-disilacyclobutanes and aminoboranes in good yields. This may be due to the generation of silene intermediates during the reaction. It should be noted that use of styrene as a ligand was essential for the generation of the silene species.

Diphenylprolinol silyl ether is a widely used organocatalyst, and its immobilization on a solid support was investigated for the easy recycling and reuse of this catalyst. Because a silyl ether bond of the catalyst is weak, its alkyl ether was attached to polymers such as a polyquinoxaline-based polymer, a polystyrene polymer (PS) resin, and a polystyrene–poly(ethylene glycol) graft copolymer (PS-PEG) resin. Although a polymer-supported organocatalyst generally decreases its reactivity compared with the monomer catalyst, diphenylprolinol anthrylmethyl ether supported on PS-PEG was found to be a reactive organocatalyst; it catalyzed the Michael reaction of an aldehyde and a nitroalkene in water without an organic solvent being present, with excellent diastereo- and enantioselectivities.There is high demand for a highly reactive and reusable diphenylprolinol alkyl ether derivative. Diphenylprolinol anthrylmethyl ether supported on PS-PEG resin was found to be a polymer immobilized highly reactive organocatalyst in aqueous media; it catalyzed the Michael reaction of several aldehydes and nitroalkenes in water without an organic solvent being present, with excellent diastereo- and enantioselectivities.

Use of single-handed dynamic helical macromolecules as nucleophilic catalysts in asymmetric Steglich-type O-to-C aryloxycarbonyl rearrangement of 3-substituted indol-2-yl aryl carbonates is demonstrated. Among several single-handed poly(quinoxaline-2,3-diyl) copolymers (PQXap) bearing achiral 4-aminopyridin-3-yl groups at the 5-position of the quinoxaline rings, PQXmdppand PQXapy, containing N-methylpyrrolidine-fused pyridin-3-yl and 4-(1-azetidinyl)pyridin-3-yl groups, respectively, showed higher enantioselectivity and catalytic activity than PQXdmap, bearing 4-dimethylaminopyridine-3-yl groups. Substrates bearing p-(trifluoromethyl)phenyloxycarbonyl groups on both the nitrogen and oxygen atoms showed high reactivity, giving oxindoles with a quaternary stereogenic carbon center at their 3-positions in up to 97:3 enantiomeric ratio in THF. The macromolecular catalysts underwent inversion of their helix sense by solvent effect, allowing the same catalyst to give the opposite enantiomer in a mixture of methyl t-butyl ether and 1,1,2-trichloroethane (3:1). The macromolecular catalysts could be easily recovered by adding acetonitrile to the reaction mixture and were reused four times without reduction in enantioselectivity.Helically chiral poly(quinoxaline-2,3-diyl) derivatives bearing 4-aminopyridine pendants served as a highly enantioselective nucleophilic catalyst in Steglich-type rearrangement of 3-substituted indol-2-yl aryl carbonates, forming a quaternary stereogenic carbon center with up to 97:3 enantiomeric ratio. Solvent-dependent switching of helical chirality as well as solubility of the catalyst enabled divergent enantioselection and catalyst reuse.

Enantioconvergent intramolecular coupling of α-(2-bromobenzoylamino)benzylboronic esters was achieved using a copper catalyst having helically chiral macromolecular bipyridyl ligand, PQXbpy. Racemic α-(2-bromobenzoylamino)benzylboronic esters were converted into (R)-configured 3-arylisoindolinones with high enantiopurity using right-handed helical PQXbpy as a chiral ligand in a toluene/CHCl3mixed solvent. When enantiopure (R)- and (S)-configured boronates were separately reacted under the same reaction conditions, both afforded (R)-configured products through formal stereoinvertive and stereoretentive processes, respectively. From these results, a mechanism involving deracemization of organocopper intermediates in the presence of PQXbpy is assumed. PQXbpy switched its helical sense to left-handed when a toluene/1,1,2-trichloroethane mixed solvent was used, resulting in the formation of the corresponding (S)-products from the racemic starting material.

A rhodium-catalyzed [4 + 2 + 1] cycloaddition involving 1,3-diene, alkyne, and silylene to afford silicon-containing seven-membered rings was established. In the presence of a rhodium catalyst bearing bis(diphenylphosphino)methane (DPPM), nona-1,3-dien-8-yne derivatives reacted efficiently at 80–110 °C with boryl(isopropoxy)silane or boryl(diethyamino)silane, which reacts as the synthetic equivalent of silylene, to afford 1-silacyclohepta-2,5-dienes (2,5-dihydro-1H-silepines). Regiodivergent and chemo- and stereoselective functionalization of the seven-membered nonconjugated diene was achieved by hydroboration mediated by Cs2CO3or an iridium catalyst.

4,4′-Bipyridyl worked as an organocatalyst for the reduction of nitroarenes by bis(neopentylglycolato)diboron (B2nep2), followed by hydrolysis to give the corresponding anilines. This reduction proceeded under aerobic conditions without any prepurification of substrates and reagents. We found broad functional group tolerance and compatibility for O- and N-protecting groups under the reaction conditions. The key in this catalytic system was the addition of B2nep2to 4,4′-bipyridyl to form N,N′-bis[(neopentylglycolato)boryl]-4,4′-bipyridinylidene as a deoxygenating reagent of nitroarenes.

Pyrazolylaniline serves as a temporary directing group attached to the boron atom of alkylboronic acids in Ir-catalyzed C(sp3)–H borylation. The reaction takes place at α-, β-, and γ-C–H bonds, giving polyborylated products including di-, tri-, tetra-, and even pentaborylalkanes. α-C–H borylation was generally found to be the preferred reaction of primary alkylboronic acid derivatives, whereas β- or γ-borylation also occurred if β- or γ-C–H bonds were located on the methyl group.

Use of chiral solvents in asymmetric synthesis as a sole source of enantioselection remains largely unexplored in organic synthesis. We found that the use of a helical macromolecular catalyst of which helical chirality is dynamically formed in chiral solvents allowed several mechanistically different reactions to proceed with high enantioselectivity. In this system, the chirality of the solvent, such as limonene, induces a configurational imbalance to the helical macromolecular scaffold of the catalyst, and in turn to the reaction products through palladium-catalyzed asymmetric reactions including Suzuki-Miyaura cross-coupling (up to 98% ee), styrene hydrosilylation (up to 95% ee), and silaboration (up to 89% ee). Not only enantiomerically pure limonene but also limonene with low enantiomeric excesses induce single-handed helical structures with majority-rule-based amplification of homochirality. The helical conformation of the macromolecular catalyst was retained even in the absence of limonene in the solid state, enabling asymmetric cross-coupling in achiral solvent with high enantioselectivity.

An organocatalytic silaboration of terminal alkynes and allenes was established using pyridine‐based catalysts. In the presence of 4‐cyanopyridine (1–2 mol%), alkyl propiolates underwent regio‐ and stereoselective addition of silylboronic esters in toluene at 135 °C to afford (Z)‐3‐boryl‐2‐silylacrylates in good yields. 2,6‐Dichloro‐4,4′‐bipyridine and 4‐(3,5‐dichlorophenyl)pyridine also exhibited high catalyst efficiency for the 1,2‐silaboration of ethyl propiolate, whereas 1,1‐silaboration was induced by P(n‐Bu)3, t‐BuOK, and ICy to afford ethyl 3‐boryl‐3‐silylacrylates as Z/Emixtures. The silaboration of ethynylbenzenes and terminal allenes was also catalyzed by 4‐cyanopyridine to afford (Z)‐β‐boryl‐α‐silylstyrenes and β‐borylallylsilanes in a regioselective manner. Free from TM: Silaboration of terminal alkynes and allenes takes place under organocatalytic conditions. Pyridine‐based catalysts including 4‐cyanopyridine promote the silaboration efficiently in regio‐ and stereoselective manners. This method is useful as a cost‐effective preparation method of (Z)‐1‐boryl‐2‐silylalkenes and β‐borylallylsilanes.

Lyotropic liquid crystallinity was investigated for a concentrated solution of linear and 3-arm star poly(quinoxaline-2,3-diyl), of which the main chain has a rigid helical nature in tetrahydrofuran. Four samples for both linear and star chains were prepared with the weight-average molar mass Mwranging from 50 to 250 kg mol–1. Two phase boundary concentrations, cIbetween the isotropic and biphasic regions and cAbetween the biphasic and anisotropic regions, were determined as a function of Mw. The resultant cIand cAfor the linear chain increased gradually with lowering Mw. This behavior was successfully explained by the latest theory for lyotropic liquid crystallinity based on the scaled particle theory (SPT) for the wormlike spherocylinder. The phase diagram for star chains, on the other hand, has similar cIand cAat the high Mwregion but increased abruptly with decreasing Mw. It can be explained by a modified SPT theory in which we assumed that two or three arm chains align parallel in the nematic phase. A small-angle X-ray scattering pattern for the concentrated solutions indicated that the smectic superstructure was contained at least a little in the liquid crystalline phase and the diffraction peak of the linear chains corresponded to the total chain length, whereas the d-spacing for the star chain is almost the same as that for the arm length, supporting the abovementioned assumption for the modified SPT.

An alkene–alkyne–silylene [2 + 2 + 1] cycloaddition takes place in the rhodium-catalyzed reaction of 1,6-enynes with borylsilanes bearing an alkoxy group on the silicon atoms, which react as synthetic equivalents of silylene. The reaction proceeds efficiently in 1,2-dichloroethane at 80–110 °C in the presence of a rhodium catalyst bearing bis(diphenylphosphino)methane (DPPM) as a ligand to afford 1-silacyclopent-2-enes in good to high yields.

Acetylenedicarboxylates undergo trans-addition of tetraalkoxydiboron in THF at 60 °C in the presence of 4,4′-bipyridine (5 mol %) as a catalyst to give 2,3-diborylfumarates in good yields with high stereoselectivity.Acetylenedicarboxylates undergo trans-addition of tetraalkoxydiboron in THF at 60 °C in the presence of 4,4′-bipyridine (5 mol %) as a catalyst to give 2,3-diborylfumarates in good yields with high stereoselectivity.

Temporary modification of a boronyl group with pyrazorylaniline allowed insertion of arylpropiolates and diphenylacetylenes into the o-C–H bond of arylboronic acids in the presence of rhodium catalysts, giving 3,3-diarylacrylates and triarylethene containing aryl groups bearing an o-boryl group stereoselectively. The boronyl group in the 3,3-diarylacrylates was converted into various functional groups, including chlorine, hydrogen, hydroxy, and aromatic groups.Using pyrazorylaniline as an ortho-directing group on boron, 3,3-diarylacrylates and triarylethenes containing an o-borylated aryl group were synthesized via rhodium-catalyzed C(sp2)–H addition of arylboronic acids to alkynes followed by replacement of the directing group with pinacol. A versatile conversion of the boronyl group afforded 3,3-diarylacrylates bearing an o-functionalized benzene ring containing hydrogen, phenyl, chloro, and hydroxy groups.

Oligo(naphthalene-2,3-diyl)s were synthesized by iterative cross-coupling of 1,8-diaminonaphthalene-modified 3-iodo-2-naphthylboronic acids prepared from naphthalenes via Ir-catalyzed C–H borylation, Ru-catalyzed ortho-C–H silylation directed by an anthranilamide-modified boronyl group, and subsequent iododesilylation. The introduction of a chiral diol to the retained terminal boronyl group allowed selective induction of P- or M-helical conformations.1,8-Diaminonaphthalene (DAN)-masked 3-iodo-2-naphthylboronic acids were prepared from their corresponding naphthalenes via Ir-catalyzed nondirected C–H borylation, Ru-catalyzed ortho-C–H silylation directed by an anthranilamide (AAM)-modified boronyl group, and iododesilylation followed by replacement of the AAM group with DAN. Iterative Suzuki–Miyaura cross-coupling of the DAN-masked 3-iodo-2-naphthylboronic acids afforded helical oligo(naphthalene-2,3-diyl)s bearing a convertible terminal boronyl group.

Double take: β‐Borylallylsilanes have been synthesized by the regioselective silaboration of terminal alkynes followed by palladium‐catalyzed double‐bond migration of the resulting β‐borylalkenylsilanes (see scheme). The stereoselectivity of the double‐bond migration can be controlled by additives, thus leading to the stereocomplementary synthesis of (E)‐ and (Z)‐β‐borylallylsilanes.

WahlweiseEoderZ: Die regioselektive Silaborierung endständiger Alkine zu β‐Borylalkenylsilanen mit anschließender palladiumkatalysierter Doppelbindungsverschiebung ergibt β‐Borylallylsilane (siehe Schema). Die Stereoselektivität der Doppelbindungsverschiebung lässt sich über das Additiv steuern, sodass ein stereokomplementärer Weg zu E‐ und Z‐β‐Borylallylsilanen offensteht.

Katalytische Spirale: In Gegenwart helical‐chiraler Polychinoxalin‐basierter Phosphane (PQXphos) mit hohem Molekulargewicht gelingen hoch enantioselektive Suzuki‐Miyaura‐Kupplungen von 1‐Brom‐2‐naphthalinphosphonsäureestern mit o‐Methylphenylboronsäuren.

Taking a new turn: An asymmetric Suzuki–Miyaura coupling of 1‐bromo‐2‐naphthalenephosphonic esters with o‐methyl‐substituted phenylboronic acids proceeds with high enantioselectivity in the presence of high‐molecular‐weight helically chiral polyquinoxaline‐based phosphines (PQXphos) bearing pendant diarylphosphino groups.

Optically pure, helical poly(quinoxaline-2,3-diyl)s (degree of polymerization, n=10–100), prepared by asymmetric living polymerization of 1,2-diisocyano-3,6-dimethyl-4,5-di(propoxymethyl)benzene with an optically pure organopalladium initiator, formed stable monolayers on a water surface. As the polymerization degree increased, more closely packed monolayers were formed, as indicated by surface pressure-area (p-A) isotherms. Up to 54 monolayers were deposited onto the solid support using the Langmuir–Blodgett (LB) technique. Polarized ultraviolet spectra of LB films showed that the helical axis was orientated parallel to the dipping direction. This optical property indicates that polymer molecules are organized anisotropically in LB films.

Changing places: Intramolecular BpinH exchange took place in the presence of a platinum–phosphane catalyst, giving synthetically useful cisβmethylsubstituted alkenylboronates stereoselectively see scheme; Bpin=4,4,5,5tetramethyl1,3,2dioxaborolan2yl.

Die Plätze getauscht: In Gegenwart eines PlatinPhosphanKatalysators findet ein intramolekularer BpinHAustausch statt siehe Schema; Bpin=4,4,5,5Tetramethyl1,3,2dioxaborolan2yl. Dabei bilden sich stereoselektiv cisβMethylsubstituierte Alkenylboronate, die nützliche Synthesebausteine sind.

Mannichtype reactions of aldehydes with secondary amines and a ketene silyl acetal were promoted by trimethoxyborane in DMSO to afford the corresponding βamino esters in good yields. BOMe3also promoted Ugitype reactions ofaldehydes with secondary amines and isocyanides in 1,2dichloroethane, which leads to the formation of αamino amides. In these reactions, trimethoxyborane serves as an inexpensive, commercially available, and virtually nonacidic iminium ion generator.© WileyVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009

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New reactivities of silylboranes are described with their applications to organic synthesis. The silylboranes add to unsaturated organic compounds such as alkynes, alkenes, 1,3-dienes, and allenes in the presence of nickel, palladium, and platinum catalysts. Reactions with isocyanide proceed in the absence of catalysts, giving 1,1-addition products. The silaborations proceed in highly regio- and stereoselective manners, leading to the formation of organic compounds bearing silyl and boryl groups. Silaborations accompanied by regioselective C–C bond cleavage in three-membered rings take place in the reactions of methylenecyclopropane and vinylcyclopropane derivatives. Unsaturated organic molecules, e.g., 1,3-diene and aldehyde, undergo silaborative C–C coupling reactions in the presence of transition-metal catalysts. Enantioselective silaboration of terminal allenes and silaborative C–C cleavage of meso-methylenecyclopropanes have been achieved. Silaborations in which the stereochemical course, i.e., cis and trans stereoselectivities, is governed by the ligand have been established in platinum-catalyzed intramolecular silaborations of alkenes. Synthetic applications of the silaboration products through Suzuki–Miyaura coupling, one-carbon homologation, oxidation, allylation, and Rh-catalyzed conjugate addition are also described. Synthesis of silylboranes, including those having heteroatom functional groups at the silicon atoms, is summarized.

A highly enantioface-selective silaboration of allenes having stereogenic centers at the -positions of the double bonds has been achieved using a combination of a chiral silylborane (−)-2and a chiral Pd/(R)-3catalyst. The chiral reagent system efficiently controlled the stereochemistry of the new stereogenic centers even in the reactions of mismatched combinations.

The utilization of aminoborane derivatives in amination reactions such as Strecker-type aminative cyanation, Mannich-type reaction, and reductive amination is described. Bis(dialkylamino)cyanoboranes and bis(dialkylamino)boron enolates underwent the concurrent transfer of the amino group and either the cyano or the enoxy group from the boron to carbonyl carbon atom in their reaction with aldehydes, leading to the formation of α-amino nitriles and β-amino ketones, respectively. Bis(dialkylamino)borane derivatives that lack the additional nucleophilic groups on the boron atoms were found to serve as effective generators of iminium ions from aldehydes. In the presence of these aminoboranes, reactions of aldehydes with ketene silyl acetals or silyl enol ethers take place, giving Mannich-type products, i.e., β-amino esters and ketones, selectively. Using diisopropylamino-substituted boron compounds, which are designed as "universal" iminium ion generators, reactions of free sec-amines, aldehydes, and ketene silyl acetals proceed efficiently, giving β-amino esters in which the amino groups are derived exclusively from the free amines. The use of the universal iminium ion generator is also effective for reductive amination of aldehydes, in which NaBH4is used as a hydride donor.