In recent decades, it has generally been recognized that carbenes play an important role as transient intermediates. As a result of a number of stable carbenes having been isolated and investigated in detail, it is not an exaggeration to say that the chemistry of carbenes has been thoroughly investigated and is now well-understood.1 In addition, much attention has also been paid to the heavier analogues of carbenes, i.e., silylenes (R2Si:), germylenes (R2Ge:), stannylenes (R2Sn:), and plumbylenes (R2Pb:). These so-called metallylenes are monomeric species of the polymetallanes. This is especially true of the silylenes, which are believed to be monomers of polysilane. The metallylenes could be expected to be of great importance in fundamental and applied chemistry as a result of their many differences and similarities to carbenes. The valency of the central atom of the heavier carbene analogues (R2M:, M ) Si, Ge, Sn, Pb) is two. That is, its oxidation state is MII and its stability increases as the principal quantum number (n) increases. In fact, dichloroplumbylene and dichlorostannylene, PbCl2 and SnCl2, respectively, are very stable ionic compounds. However, these dihalides exist as polymers or ion pairs both in solution and in the solid state. The dichlorogermylene complex GeCl2 · (dioxane)3 is also known to be stable and isolable, whereas the dihalosilylenes are barely isolable compounds.2 The early silylene research was concerned largely with comparing the chemistry of the dihalosilylenes with that of carbenes. Hence, the chemistry of the metallylenes has been considered mainly from the molecular chemistry point of view.4 In contrast to the carbon atom, the heavier group 14 atoms have a low ability to form hybrid orbitals. They therefore prefer the (ns)2(np)2 valence electron configurations in their divalent species.5 Since two electrons remain as a singlet pair in the ns orbital, the ground state of H2M: (M ) Si, Ge, Sn, Pb) is a singlet, unlike the case of H2C:, where the ground state is a triplet (Figure 1).1a On the basis of theoretical calculations, the singlet-triplet energy differences ∆EST for H2M, [∆EST ) E(triplet) E(singlet)], are found to be 16.7 (M ) Si), 21.8 (M ) Ge), 24.8 (M ) Sn), and 34.8 (M ) Pb) kcal/mol, respectively. That of H2C: is estimated as -14.0 kcal/mol.6 Furthermore, the relative stabilities of the singlet species of R2M: (M ) C, Si, Ge, Sn, Pb; R ) alkyl or aryl) compared to the corresponding dimer, R2MdMR2, are estimated to increase as the element row descends, C < Si < Ge < Sn < Pb. It follows, therefore, that one can expect that a divalent organolead compound such as plumbylene should be isolable as a stable compound. However, some plumbylenes, without any electronic or steric stabilization effects, are known to be thermally unstable and undergo facile disproportionation reactions, giving rise to elemental lead and the corresponding tetravalent organolead compounds.7 On this basis, it could be concluded that it might be difficult to isolate metallylenes as stable compounds under ambient conditions, since they generally exhibit extremely high reactivity toward other molecules as well as themselves. Metallylenes have a singlet ground state with a vacant p-orbital and a lone pair of valence orbitals. This extremely high reactivity must be due to their vacant p-orbitals, since 6 valence electrons is less than the 8 electrons of the “octet rule”. Their lone pair is expected to be inert due to its high s-character. In order to stabilize metallylenes enough to be isolated, either some thermodynamic and/or kinetic stabilization of the reactive vacant p-orbital is required (Figure 2). A range of “isolable” metallylenes have been synthesized through the thermodynamic stabilization of coordinating Cp* ligands, the inclusion of heteroatoms such as N, O, and P, * To whom correspondence should be addressed. Phone: +81-774-38-3200. Fax: +81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp. Chem. Rev. 2009, 109, 3479–3511 3479

https://pubs.acs.org/doi/pdf/10.1021/cr900294f

Stable Heavier Carbene Analogues

The chemistry of silicon is mainly based on silicon(IV), whereas that of silicon(II) is still in its infancy. Silylene (R2Si:) is a molecule with a divalent neutral silicon atom having a lone pair of electrons. Silylenes are key intermediates in various photochemical, thermal, and metal reduction reactions of organosilicon compounds. Therefore, silylenes play a vital role in the field of silicon chemistry. Until 1994, silylenes were generally considered to be very reactive unstable species that decompose or polymerize readily at temperatures above 77 K. This situation changed when West et al. reported the first N-heterocyclic silylene that is stable at room temperature. Since then, a fair number of cyclic silylenes and a dialkyl silylene have been reported. Only one example of a monochlorosilylene that is stable at room temperature has been published, and it was characterized by X-ray diffraction; no dihalosilylenes that are stable at room temperature have been reported to date. Gaseous dichlorosilylene has been known for many years; at room temperature, it condenses to polymeric (SiCl2)n. Its properties were already studied by Schmeisser and Schenk in 1964. Some reactions of condensed SiCl2 with acetylene and with benzene were carried out by Timms in 1968, and resulted in brown polymeric products of unknown composition. A literature survey reveals that there have been few reports on the synthesis and properties of cyclic and linear polydichlorosilanes. A polymeric perchloropolysilane (SiCl2)n was reported by West et al. in 1998 and was studied by single crystal Xray diffraction. However, access to dichlorosilylenes that are stable at room temperature and with electronic structure and properties are quite similar or close to those of unstable examples, such as SiCl2, is still a challenge. Of course, efforts of making a stable version of a very reactive species may cause a change of its behavior. How an unstable species can be stabilized is a matter of keen interest for the scientific and industrial community. Addition reactions of HSiCl3 with different organic compounds in the presence of various tertiary amine bases were reviewed by Benkeser, and the existence of the trichlorosilyl anion as an intermediate was postulated. Moreover, Jung et al. reported the trapping of dichlorosilylene, which was generated by the reaction of HSiCl3 in the presence of phosphonium chloride in a stainless steel cylinder at 150 or 180 8C, by conjugated dienes and by an alkyne. In their proposed mechanism, species of dichlorosilylene or the trichlorosilyl anion as intermediates were discussed but not isolated. In 1995, Kuhn et al. prepared N-heterocyclic carbene (NHC) adducts of SiCl4. We have used N-heterocyclic carbenes as dehydrochlorinating agents for the preparation of germylene and metal hydroxides. Very recently, Robinson and co-workers reported a Lewis base stabilized low-valent silicon compound in which the silicon atom is present in the formal oxidation states + 1 or zero, but no dichlorosilylene was isolated. Almost all silylenes or low-valent silicon compounds reported to date were prepared by reductions of their parent compounds using strong reducing agents, such as potassium metal or KC8. Herein, we report the synthesis of the first basestabilized dichlorosilylene that is stable at room temperature, LSiCl2 (1; L 1 = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). This compound is formed under mild reaction conditions by reductive elimination of HCl from trichlorosilane in the presence of the NHC (L; Scheme 1), and was

Lewis Base Stabilized Dichlorosilylene

Isolating stable compounds with low-valent main group elements have long been an attractive research topic, because several of these compounds can mimic transition metals in activating small molecules. In addition, compounds with heavier low-valent main group elements have fundamentally different electronic properties when compared with their lighter congeners. Among group 14 elements, the heavier analogues of carbenes (R2C:) such as silylenes (R2Si:), germylenes (R2Ge:), stannylenes (R2Sn:), and plumbylenes (R2Pb:) are the most studied species with low-valent elements. The first stable carbene and silylene species were isolated as N-heterocycles. Among the dichlorides of group 14 elements, CCl2 and SiCl2 are highly reactive intermediates and play an important role in many chemical transformations. GeCl2 can be stabilized as a dioxane adduct, whereas SnCl2 and PbCl2 are available as stable compounds.In the Siemens process, which produces electronic grade silicon by thermal decomposition of HSiCl3 at 1150 ...

Dichlorosilylene: a high temperature transient species to an indispensable building block.

A Ni/Cu-catalyzed silylation of unactivated C–O electrophiles derived from phenols or benzyl alcohols is described. This transformation is characterized by its wide scope and mild conditions, providing a direct access to synthetically versatile silylated compounds. The protocol allows for the coupling of C(sp2)−O and even C(sp3)–O bonds with similar efficiency.

A mild Ni/Cu-catalyzed silylation via C-O cleavage.

Interstellar molecules: guides for new chemistry

Phosphonium Chloride Induced Dichlorosilylene Transfer from Trichlorosilane

Novel phosphonium chloride-catalyzed dehydrohalogenative Si-C coupling reaction of alkyl halides with trichlorosilane.

Phosphonium Chloride-Catalyzed Dehydrochlorinative Coupling Reactions of Alkyl Halides with Hydridochlorosilanes

The present invention relates to a process for preparing organochlorosilanes and more particularly, to the process for preparing organochlorosilanes of formula I by a dehydrohalogenative coupling of hydrochlorosilanes of formula II with organic halides of formula III in the presence of quaternary phosphonium salt as a catalyst to provide better economical matter and yield compared with conventional methods, because only catalytic amount of phosphonium chloride is required and the catalyst can be separated from the reaction mixture and recycled easily, wherein R1 represents hydrogen, chloro, or methyl; X represents chloro or bromo; R2 is selected from the group consisting of C1-17 alkyl, C1-10 fluorinated alkyl with partial or full fluorination, C2-5 alkenyl, silyl containing alkyl group represented by (CH2)nSiMe3-mClm wherein n is an integer of 0 to 2 and m is an integer of 0 to 3, aromatic group represented by Ar(R′)q wherein Ar is C6-14 aromatic hydrocarbon, R′ is C1-4 alkyl, halogen, alkoxy, or vinyl, and q is an integer of 0 to 5, haloalkyl group represented by (CH2)pX wherein p is an integer of 1 to 9 and X is chloro or bromo, and aromatic hydrocarbon represented by ArCH2X wherein Ar is C6-14 aromatic hydrocarbons and X is a chloro or bromo; R3 is hydrogen, C1-6 alkyl, aromatic group represented by Ar(R′)q wherein Ar is C6-14 aromatic hydrocarbon, R′ is C1-4 alkyl, halogen, alkoxy, or vinyl, and q is an integer of 0 to 5; and R4 in formula I is the same as R2 in formula III and further, R4 can also be (CH2)pSiR1Cl2 or ArCH2SiR1Cl2, when R2 in formula III is (CH2)pX or ArCH2X, which is formed from the coupling reaction of X—(CH2)p+1—X or XCH2ArCH2X with the compounds of formula II; or when R2 and R3 are covalently bonded to each other to form a cyclic compounds of cyclopentyl or cyclohexyl group, R3 and R4 are also covalently bonded to each other in the same fashion.

Process for preparing organochlorosilanes by dehydrohalogenative coupling reaction of alkyl halides with chlorosilanes

Herein, we report the development of an iron-catalyzed olefin oxyamidation by utilizing tethered dioxazolones as the nitrenoid precursor to produce valuable β-lactam scaffolds. Mechanistic studies revealed that a relatively strong π-accepting ability of the phthalocyanine ligand is critical in generating the key triplet iron-imidyl radical intermediate to enable the 4-exo-trig-lactamization with the incorporation of oxygen nucleophiles in high diastereoselectivity. This cyclization approach was readily extended to the highly efficient γ-lactam synthesis (TON > 300).

Seung-Hyun Kang

Papers

Phosphonium Chloride Induced Dichlorosilylene Transfer from Trichlorosilane

Novel phosphonium chloride-catalyzed dehydrohalogenative Si-C coupling reaction of alkyl halides with trichlorosilane.

Phosphonium Chloride-Catalyzed Dehydrochlorinative Coupling Reactions of Alkyl Halides with Hydridochlorosilanes

Process for preparing organochlorosilanes by dehydrohalogenative coupling reaction of alkyl halides with chlorosilanes

Access to β-Lactams via Iron-Catalyzed Olefin Oxyamidation Enabled by the π-Accepting Phthalocyanine Ligand.