Reversible Intramolecular Cycloaddition of Phosphaalkene to an Arene Ring

Arene extrusion as an approach to reductive elimination at boron: implication of carbene-ligated haloborylene as a transient reactive intermediate

Herein, we report boron-centered arene extrusion reactions to afford putative cyclic(alkyl)(amino) carbene (CAAC)-ligated chloroborylene and bromoborylene intermediates. The borylene precursors, chloro-boranorbornadiene (ClB(C6Me6), 2Cl) and bromo-boranorbornadiene (BrB(C6Me6), 2Br) were synthesized through the reaction of the corresponding 1-halo-2,3,4,5-tetramethylborole dimer (XBC4Me4)2 (X = Cl, 1Cl; X = Br, 1Br) with 2-butyne. Treatment of 2Cl with CAACs resulted in the release of di-coordinate chloro-borylene (CAAC)BCl from hexamethylbenzene (C6Me6) at room temperature. In contrast, the reaction of 2Br with CAAC led to the formation of a boronium species [(CAAC)BC6Me6]+Br- (7) at room temperature. Heating 7 in toluene promoted the release of di-coordinate bromo-borylene (CAAC)BBr as a transient species. Surprisingly, heating 7 in dichloromethane resulted in the C-H activation of hexamethylbenzene. The conversion of a CAAC-stabilized bromo-borepin to a borylene, a boron-centered retro Büchner reaction, was also investigated.

Enantiopure P-Chiral Secondary Phosphines (P*HRR') from the Catalytic Asymmetric Hydrogenation of P═C Bonds

We report the first bottleable enantiopure P-chiral secondary phosphines from the rhodium-catalyzed asymmetric hydrogenation of phosphaalkenes. Catalytic asymmetric hydrogenation, a reaction of broad academic and industrial importance for C═C, N═C, and O═C bonds, has not previously been reported for the P═C bond. The hydrogenation of ArP═CR2 (Ar = Mes, m-Xyl and TMOP; R = Ph, 4-C6H4F) affords four unprecedented P-stereogenic secondary phosphines in 76%-90% isolated yields with 91%-97% enantiomeric excess (ee). These isolable P-chiral secondary phosphines are configurationally stable indefinitely in the solid state and show only modest loss in ee when kept in solution for over a month at room temperature.

Reversible [4 + 1] Cycloaddition of Arenes by a "Naked" Acyclic Aluminyl Compound

The large steric profile of the N-heterocyclic boryloxy ligand, -OB(NDippCH)2, and its ability to stabilize the metal-centered HOMO, are exploited in the synthesis of the first example of a "naked" acyclic aluminyl complex, [K(2.2.2-crypt)][Al{OB(NDippCH)2}2]. This system, which is formed by substitution at AlI (rather than reduction of AlIII), represents the first O-ligated aluminyl compound and is shown to be capable of hitherto unprecedented reversible single-site [4 + 1] cycloaddition of benzene. This chemistry and the unusual regioselectivity of the related cycloaddition of anthracene are shown to be highly dependent on the availability (or otherwise) of the K+ countercation.

Dearomatization of C6 Aromatic Hydrocarbons by Main Group Complexes

The dearomatization reaction is a powerful method for transformation of simple aromatic compounds to unique chemical architectures rapidly in synthetic chemistry. Over the past decades, the chemistry in this field has evolved significantly and various important organic compounds such as crucial bioactive molecules have been synthesized through dearomatization. In general, photochemical conditions or assistance by transition metals are required for dearomatization of rigid arenes. Recently, main-group elements, especially naturally abundant elements in the Earth's crust, have attracted attention as they have low toxicity and are cost-effective compared to the late transition metals. In recent decades, a variety of low-valent main-group molecules, which enable the activation of stable aromatic compounds under mild conditions, have been developed. This minireview highlights the developments in the chemistry of dearomatization of C6 aromatic hydrocarbons by main-group compounds leading to the formation of seven-membered EC6 (E=main-group elements) ring or cycloaddition products.

An ab initio study on the stability of isolated phosphaalkene synthons

Using ab initio methods and flexible basis sets, we examined the electronic, geometric, and thermodynamic stabilities of selected phosphaalkene synthons matching the (PCR2)- formula (R = H, CH3, C6H5, C6F5, and Mes). All isolated synthons considered were found to be electronically stable and susceptible to neither fragmentation nor isomerization processes. The structures corresponding to the most stable isomers of the studied phosphaalkene synthons contain a PC double-bond (whose presence was confirmed by natural bond orbital occupancies of σ(P-C) and π(P-C) approaching 2 electrons) and two R substituents connected to the carbon atom in either (PCR2)- (for R = H, CH3, C6H5, and Mes) or (PCF-R-R)- (for R = C6F5) manner. Vertical electron detachment energies (spanning the 0.924-3.118 eV range) characterizing the phosphaalkene synthons were predicted and discussed.

2H-Phosphindole-Enabled Dearomatization and [4+2] Cycloaddition of (Hetero)Arenes

The heavier main group multiple bonds offer an effective tool for small molecule activation. Transient 2H-phosphinidole working as a reactive phosphadiene system undergoes phospha-Diels-Alder reaction with a wide range of non-activated aromatic carbocycles and heterocycles, including naphthalene, anthracene, phenanthrene, furan, thiophene, pyrrole, pyridine, and benzo-fused heterocycles, affording concise access to a range of polycyclic fused rings feature with phosphorus at the bridgehead. These results demonstrate that non-activated (hetero)arenes are capable of acting as 2π systems in [4+2] cycloaddition with highly reactive 2H-phosphindole complex.

An Air-Stable "Masked" Bis(imino)carbene: A Carbon-Based Dual Ambiphile

Carbenes, once considered laboratory curiosities, now serve as powerful tools in the chemical and material sciences. To date, all stable singlet carbenes are single-site ambiphiles. Here we describe the synthesis of a carbene which is a carbon-based dual ambiphile (both single-site and dual-site). The key is to employ imino substituents derived from a cyclic (alkyl)(amino)carbene (CAAC), which imparts a 1,3-dipolar character to the carbene. Its dual ambiphilic nature is consistent with the ability to activate simple organic molecules in both 1,1- and 1,3-fashion. Furthermore, its 1,3-ambiphilicity facilitates an unprecedented reversible intramolecular dearomative [3 + 2] cycloaddition with a proximal arene substituent, giving the carbene the ability to "mask" itself as an air-stable cycloadduct. We perceive that the concept of dual ambiphilicity opens a new dimension for future carbene chemistry, expanding the repertoire of applications beyond that known for classical single-site ambiphilic carbenes.

Phosphorus-Involved Wagner-Meerwein Rearrangement of Phosphiranes: An Entry to Four-Membered Phosphacycles

Phosphenium ions [R₂P]⁺ are important and highly reactive dicoordinate phosphorus species. Herein, we report a rearrangement of the carbocation into the phosphenium cation driven by ring strain. This phosphorus-involved Wagner-Meerwein rearrangement pathway converted the 1-acylphosphirane complex into phosphetane and 1,2-dihydrophosphete derivatives depending on the reaction temperature. The generation of the intermediate phosphenium cation was identified by the intramolecular reaction with ether, which also disclosed its strong Lewis acidity. This work expands the boundary of the phosphorus-carbon analogy.

Activation of the C[double bond, length as m-dash]P bond in phosphanylphosphaalkenes (C[double bond, length as m-dash]P-P bond system) in the reaction with nucleophilic reagents: MeLi, nBuLi and tBuLi

Three reactions of phosphanylphosphaalkene (1) with nucleophiles were performed to activate the diphosphorus monomer. We observed similar results in the reactions with MeLi and nBuLi, in which the P-P bond is cleavaged and triphosphorus systems [P(Me)₂-CH(biph)-CH(biph)-P-(PtBu₂)]^- (1a'') and [P(nBu)₂-CH(biph)-CH(biph)-P-(PtBu₂)]^- (1b''), respectively, are formed depending on the nucleophilic reagent (biph = biphenyl). In the case of tBuLi, the P-P bond remains intact; on the phosphorus atom, only one -tBu group is substituted, and as a result, [(biph)(H)C-P(tBu)-PtBu₂]^- (1c) is generated as a stable carbanion. We additionally investigated the effect of substitution in the phenyl ring in the course of these reactions by involving two other phosphanylphosphaalkenes (3 and 4). All initial reactions were conducted in tetrahydrofuran (THF) solution at ambient temperature.

BNN-1,3-dipoles: isolation and intramolecular cycloaddition with unactivated arenes

The mono-base-stabilized 1,2-diboranylidenehydrazine derivatives featuring a 1,3-dipolar BNN skeleton are obtained by dehydrobromination of [ArB(Br)NH]₂ (Ar = 2,6-diphenylphenyl (Dpp), Ar = 2,6-bis(2,4,6-trimethylphenyl)phenyl (Dmp) or Ar = 2,4,6-tri-tert-butylphenyl (Mes*)) with N-heterocyclic carbenes (NHCs). Depending on the Ar substituents, such species can be isolated as a crystalline solid (Ar = Mes*) or generated as reactive intermediates undergoing spontaneous intramolecular aminoboration of the proximal arene rings via [3 + 2] cycloaddition (Ar = Dpp or Dmp). The latter reactions showcase the 1,3-dipolar reactivity toward unactivated arenes at ambient temperature. In addition, double cycloaddition of the isolable BNN species with two CO₂ molecules affords a bicyclic species consisting of two fused five-membered BN₂CO rings. The electronic structures of these BNN species and the mechanisms of these cascade reactions are interrogated through density functional theory (DFT) calculations.

Ge=B π-Bonding: Synthesis and Reversible [2+2] Cycloaddition of Germaborenes

Phosphine-stabilized germaborenes featuring an unprecedented Ge=B double bond with short B⋅⋅⋅Ge contacts of 1.886(2) (4) and 1.895(3) Å (5) were synthesized starting from an intramolecular germylene-phosphine Lewis pair (1). After oxidative addition of boron trihalides BX3 (X=Cl, Br), the addition products were reduced with magnesium and catalytic amounts of anthracene to give the borylene derivatives in yields of 78 % (4) and 57 % (5). These halide-substituted germaborenes were characterized by single-crystal structure analysis, and the electronic structures were studied by quantum-chemical calculations. According to an NBO NRT analysis, the dominating Lewis structure contains a Ge=B double bond. The germaborenes undergo a reversible, photochemically initiated [2+2] cycloaddition with the phenyl moiety of a terphenyl substituent at room temperature, forming a complex heterocyclic structure with GeIV in a strongly distorted coordination environment.

Synthesis of an Isolable Bis(silylene)-Stabilized Silylone and Its Reactivity Toward Small Gaseous Molecules

The first bis(N-heterocyclic silylene)-stabilized zero-valent silicon compound, [Si^II(Xant)Si^II]Si⁰ (4, Xant = 9,9-dimethyl-xanthene-4,5-diyl), has been synthesized via the reduction of the corresponding chlorosilyliumylidene chloride precursor {[Si^II(Xant)Si^II]SiCl}⁺Cl^- (2). The electronic structure of silylone 4, whose molecular structure is confirmed spectroscopically and crystallographically, is investigated by DFT calculations and Natural Bond Orbital analysis, showing two perpendicular lone-pairs of electrons on the central Si⁰ atom, i.e., an sp^0.41-type lone-pair and a delocalized p lone-pair. With the electron-rich and oxophilic Si⁰ center, silylone 4 exhibits a striking reactivity toward small gaseous molecules. Remarkably, the oxidation of silylone 4 by N₂O can be controlled to generate distinct products by regulating the amount of added N₂O. Exposing 4 to an excess or two molar equivalents of N₂O yields the unexpected oxidation product 5, bearing a central six-membered Si₄O₂ ring. When 4 is mixed with one molar equivalent of N₂O, the unique compound 6 is obtained, resulting from a rare 1,4-addition of two central silicon atoms to a phenyl ring of an amidinate ligand coordinated to the Si^II atom. In addition, cleavage of the strong N-H bond in ammonia is also readily accomplished by silylone 4, representing the first example of NH₃ activation in silylone chemistry. In the presence of the Lewis acid BPh₃, silylone 4 achieves heterolytic dihydrogen cleavage and ethylene addition to form the corresponding hydridosilyliumylidene hydroborate salt 8 and the zwitterionic compound 9, respectively, which represent a new type of frustrated Lewis pair based on an electron-rich Si⁰ donor and a borane acceptor.

Oligomerization of phosphaalkynes mediated by bulky N-heterocyclic carbenes: avenues to novel phosphorus frameworks

Reactions of RCP (R = tBu or Ad (admantyl)) with NHCs (SIMes, 1a; IMes, 1b and IDipp, 1d), leading to 1,2,3-triphosphetenes 2 and 3, a triphosphole 4, and a di-1,2-dihydro-1,2-diphosphete-substituted diphosphene 5, are reported.