Lithio Siloles: Facile Synthesis and Applications

Selective Si-C(sp3) bond cleavage of a silyl-bridged amido alkyl ligand in an yttrium complex

Compared with Si-C(sp² and sp) bonds bearing neighboring π-bond hyperconjugative interactions, the activation of robust Si-C(sp³) bonds has proved to be a challenge. Herein, two distinct Si-C(sp³) bond cleavages have been realized by rare-earth-mediated and nucleophilic addition of unsaturated substrates. The reactions of Tp^Me2Y[κ²-(C,N)-CH(SiH₂Ph)SiMe₂NSiMe₃](THF) (1) with CO or CS₂ gave two endocyclic Si-C bond cleavage products, Tp^Me2Y[κ²-(O,N)-OCCH(SiH₂Ph)SiMe₂NSiMe₃](THF) (2) and Tp^Me2Y[κ²-(S,N)-SSiMe₂NSiMe₃](THF) (3), respectively. However, 1 reacted with nitriles such as PhCN and p-R'C₆H₄CH₂CN in a 1 : 1 molar ratio to yield the exocyclic Si-C bond products Tp^Me2Y[κ²-(N,N)-N(SiH₂Ph)C(R)CHSiMe₂NSiMe₃](THF) (R = Ph (4); R = C₆H₅CH₂ (6H); R = p-F-C₆H₄CH₂ (6F); and R = p-MeO-C₆H₄CH₂ (6MeO)), respectively. Moreover, complex 4 can continuously react with an excess of PhCN to form a Tp^Me2-supported yttrium complex with a novel pendant silylamido-substituted β-diketiminato ligand, Tp^Me2Y[κ³-(N,N,N)-N(SiH₂Ph)C(Ph)CHC(Ph)N-SiMe₂NSiMe₃](PhCN) (5).

(3 + 2)-Annulation of 1,3-N,Si-tetraorganosilane reagents TsHNCH2SiBnR1R2 with arynes for efficient synthesis of 3-silaindolines

1,3-N,Si-Tetraorganosilane reagents TsHNCH2SiBnR1R2 were developed as robust synthons to prepare 3-silaindolines via a Cs2CO3-promoted (3 + 2)-annulation reaction with arynes.

Nucleophilic Activation of Hydrosilanes via a Strain-Imposing Strategy Leading to Functional Sila-aromatics

Carefully designed cyclic hydrosilanes enable trans-selective hydrosilylation of unactivated alkynes without transition metal catalysts via silicate formation. Employment of sterically demanding bidentate ligands of silicon increases steric congestion upon silicate formation, and this strain-imposing strategy facilitates hydride transfer. This hydrosilylation provides efficient access to diverse benzosiloles, silaphenalenes, and related silacycles.

Aromatic Metamorphosis of Thiophenes by Means of Desulfurative Dilithiation

A new mode of aromatic metamorphosis has been developed, which allows thiophenes and their benzo-fused derivatives to be converted to a variety of exotic heteroles. This transformation involves 1) the efficient generation of key 1,4-dianions by means of desulfurative dilithiation with lithium powder and 2) the subsequent trapping of the dianions with heteroatom electrophiles in a one-pot manner. Via the desulfurative dilithiation, the sulfur atoms of thiophenes are replaced also with a carbon-carbon double bond or a 1,2-phenylene for the construction of benzene rings.

2,5-Bis-trimethylsilyl substituted boroles

This manuscript includes a comprehensive study of the synthesis and spectroscopic features of 2,5-disilyl boroles.

Preparation of Thioanisole Biscarbanion and C-H Lithiation/Annulation Reactions for the Access of Five-Membered Heterocycles

The synthesis, isolation, and X-ray structure of a thioanisole-based trilithium complex are reported. On the basis of the double-lithiation strategy, two novel synthetic methodologies have been developed under mild reaction conditions (room temperature): (1) reactions of lithiated thioanisoles with nitriles give benzoisothiazoles via a [3 + 2]-type of approach with two new bond formations and (2) formation of benzothiophenes from thioanisoles and amides through a [4 + 1] pattern forming 4 new chemical bonds.

Conversion of zirconacyclopentadienes into metalloles: Fagan-Nugent reaction and beyond

Metalloles are derivatives of cyclopentadiene in which the methylene unit is replaced by a heteroatom, such as S, Se, Te, N, P, As, Sb, Bi, Si, Ge, Sn, B, Al, Ga, and so on. Many metallole derivatives have been widely used as photovoltaic cells, organic light emitting diodes (OLEDs), chemical sensors, electrochromic devices, microelectronic actuators, and organic field effect transistors (OFETs). In the meantime, many of them showed promising biological actives. Due to the similarity to cyclopentadiene, the anionic forms of metalloles were also widely explored in coordination chemistry. As a result, development of a general method for the formation of metalloles from available starting materials is highly desired. In this Account, we outline formation of various p-block element metalloles from zirconacyclopentadienes. The zirconacyclopentadienes can be easily prepared from two molecules of alkynes and a low-valent zirconocene species "Cp2Zr(II)" (Cp = cyclopentadienyl). Fagan and Nugent first reported the formation of main group metalloles from zirconacyclopentadiene, which provided a versatile approach for the construction of metalloles, especially for the formation of metalloles in heavier p-block elements. To further expand the substrate scope, a number of stepwise conversions were developed, which involve 1,4-dimetallo- or dihalo-1,3-butadiene as intermediates from zirconacyclopentadienes. Here, four processes are classified based on direct and indirect conversion of zirconacyclopentadienes into metalloles. Direct reaction of zirconacyclopentadienes with element halides afforded heterocycles of main group elements, which provided a versatile method for the synthesis of metalloles. Nonetheless, the reaction scope was restricted to heavier p-block elements such as S, Se, P, As, Sb, Bi, Ge, Sn, Ga, and In. And these reactions usually suffered low yields and long reaction time. Transmetalation of zirconacyclopentadiene with copper chloride greatly enriched the zirconacyclopentadiene chemistry. The synthesis of stannoles and pyrroles from zirconacyclopentadienes has been developed in the presence of CuCl. The direct reaction of the zirconacyclopentadienes with SiCl4 or R2SiCl2 does not give the desired silacyclopendadiene derivatives, even in the presence of CuCl. It can be circumvented by using dilithiated dienes from diiododienes, which are easily prepared by the iodination of zirconacyclopentadienes using CuCl as an additive. Finally, an umpolung strategy, reaction of electrophilic 1,4-diiodo-1,3-butadiene with nucleophilic amine or sulfide reagents, was successfully used in the formation of pyrroles and thiophenes.

Synthesis of silafluorenes and silaindenes via silyl radicals from arylhydrosilanes: intramolecular cyclization and intermolecular annulation with alkynes

Effective synthetic methods, involving silyl radical intermediates and furnishing silafluorenes and silaindenes via direct Si–H and C–H cleavage in one step, have been developed.

Organometallic intermediate-based organic synthesis: organo-di-lithio reagents and beyond

Metal-mediated organic reactions have become one of the great frontiers of organic synthesis.

1,4-Dilithio-1,3-dienes: reaction and synthetic applications

The development of organometallic reagents remains one of the most important frontiers in synthetic chemistry. Commonly used organometallic reagents (such as RLi and RMgBr) are typically monometallic compounds, although they aggregate in many cases. When two carbon-metal bonds are in the same molecule in close proximity, however, these two carbon-metal moieties may exhibit novel reactivity. In this Account, we outline our work on new reactions and synthetic applications of the organo-dilithio reagents 1,4-dilithio-1,3-butadienes. The 1,4-dilithio-1,3-butadienes can be accessed readily in high efficiency with a wide variety of substitution patterns on the butadienyl skeleton. The configuration has been predicted and demonstrated to favor a double dilithium bridging structure in both solution and solid states. The two Li atoms are bridged by a butadiene moiety and are in close proximity. By taking advantage of this unique configuration, we have developed useful and interesting synthetic methodologies. Three types of reactions of 1,4-dilithio-1,3-butadienes, termed dilithio reagents here, have been developed and are discussed. An intramolecular reaction is introduced in the first section. The reaction is a result of the intracooperative effect among the two C-Li moieties, the butadienyl bridge, and the substituents. A useful transformation from silylated 1,4-dilithio-1,3-butadienes to α-lithio siloles is described. Second, we discuss an intermolecular reaction that results from the intercooperative effect of the two C-Li moieties toward substrates. As an example of the formation of functionalized cyclic dianions from the linear dianions of the dilithio reagents and organic substrates, we describe the isolation and structural characterization of a novel type of cyclic dianion; that is, fully substituted oxy-cyclopentadienyl dilithium formed via the reaction of dilithio reagents with CO. We also describe diverse reactions of dilithio reagents with nitriles to form substituted pyridines, tricyclic 1-bipyrrolines, and siloles, demonstrating the remarkable effect of substituents on the butadienyl skeleton. Third, we discuss transmetalation of dilithio reagents to generate other organo-dimetallic compounds. This section focuses on organo-dicopper compounds and their reactivity toward the synthesis of strained ring systems, such as semibullvalenes and twisted four-membered rings, with the metal-mediated C-C bond-forming approach. In addition to these three representative reactions, other useful applications are also briefly introduced. The dimetallic 1,4-dilithio-1,3-butadienes and their transmetalated derivatives provide unique synthetic organometallic reagents that are very different from monometallic reagents, both in terms of reactivity and synthetic application. These organo-dimetallic reagents provide access to interesting and useful compounds that are not available by other means. Moreover, given the possibilities afforded, the study of organo-dimetallic and organo-polymetallic compounds should yield further synthetic applications in the near future.

Synthesis of 1,4-dichloro-1,3-butadienes by rhodium complex catalyzed reaction of terminal alkynes with trichloroacetyl chloride

Chlorinative dimerization of terminal alkynes with trichloroacetyl chloride as chlorine donor proceeds in the presence of rhodium catalysts to give (Z,Z)-1,4-dichloro-1,3-butadienes stereoselectively. Ligand screening has revealed that reactions using sterically bulky and electron-donating ligands like trimesitylphosphine are high yielding. The reaction is compatible with a range of functional groups to give the title compounds nearly quantitatively in most cases. A mechanistic possibility involving coupling of beta-chloroalken-1-yl intermediate has been discussed.

1-Lithio-1,3-dienes as useful building blocks in organic synthesis

Both intermolecular reactivities toward C-O and C-N unsaturated substrates and intramolecular reactions of various 1-lithio-1,3-dienes are briefly summarized. Results reveal that intramolecular synergy between the alkenyllithium moiety and the butadienyl skeleton make 1-lithio-1,3-diene derivatives useful and unique as building blocks in organic synthesis. It is demonstrated that substitution patterns and the nature of the substituents on the butadienyl skeleton have remarkable effects on reaction pathways.