David Scheschkewitz

A Tricyclic Aromatic Isomer of Hexasilabenzene

Aromatic Silicon Benzene has long intrigued chemists on account of the energy stabilization, termed aromaticity, which arises from π-electron delocalization around its ring framework. A persistent question has been how such stabilization would be impacted were the carbons to be replaced by heavier atoms such as silicon. Abersfelder et al. (p. 564) have prepared a benzene analog with Si atoms in place of all six-ring carbons, but a slightly altered bonding framework in which substituents outside the ring are no longer evenly distributed. Instead, the substituents pair up at two Si sites, leaving two other ring sites with no external appendages. The resulting compound no longer has a continuous network of π-electrons, but retains a degree of aromatic stabilization involving sigma and nonbonding electrons. A structural isomer of benzene in which carbon is replaced by silicon exhibits unexpected electronic stabilization. Benzene represents the showcase of Hückel aromaticity. The silicon analog, hexasilabenzene, has consequently been targeted for decades. We now report an intensely green isomer of Si6R6 (R being 2,4,6-triisopropylphenyl) with a tricyclic structure in the solid state featuring silicon atoms with two, one, and no substituents outside the ring framework. The highly dispersed 29Si nuclear magnetic resonance shifts in solution ranging from +125 to −90 parts per million indicate an inhomogeneous electron distribution due to the dismutation of formal oxidation numbers as compared with that of benzene. Theoretical analysis reveals nonetheless the cyclic delocalization of six mobile electrons of the π-, σ- and non-bonding type across the central four-membered ring. For this alternative form of aromaticity, in principle applicable to many Hückel aromatic species, we propose the term dismutational aromaticity.

Anionic Reagents with Silicon-Containing Double Bonds

E=Si transfer: Anionic compounds capable of transferring a silicon-containing double bond are reviewed (see figure), particularly reagents with Si=Si moieties (Tip=2,4,6-iPr(3)C(6)H(2), M=Li, Na, K) and their applications towards main-group and transition-metal electrophiles, as well as their reactivity towards organic compounds. A few recently reported derivatives with Si=C (Ad=1-adamantyl) and Si=P moieties are included for completeness.Anionic compounds capable of transferring a silicon double bond are summarized following an introduction to the differences between alkenes and their heavier homologues. The main focus is on reagents with Si=Si moieties and their applications towards main-group and transition-metal electrophiles, as well as their reactivity towards organic compounds, but a few recently reported derivatives with Si=C and Si=P bonds are also included.

A Stable Derivative of the Global Minimum on the Si6H6 Potential Energy Surface

The higharomatic stabilization that is conferred by the cyclic delocal-ization of six p electrons distinguishes benzene from itsapproximately 200 theoretically known isomers and accountsfor the ubiquitous occurrence of the benzene motif in manyareas of chemistry. Despite impressive progress regarding thesynthesis of stable compounds with Si Si p bonds,

Equilibrium between a cyclotrisilene and an isolable base adduct of a disilenyl silylene

In organic chemistry, compounds with adjacent alkene and carbene functionalities (vinyl carbenes) are studied widely as fleeting intermediates and in the coordination sphere of transition metals. Stable derivatives of vinyl carbenes remain elusive, including the corresponding heavier group 14 homologues. Here we report the isolation and full characterization of a base-stabilized silicon version of a vinyl carbene that features a silicon-silicon double bond as well as a silylene functionality, coordinated by an N-heterocyclic carbene (NHC). In solution, the intensely green disilenyl silylene adduct exists in equilibrium with the corresponding silicon analogue of a cyclopropene and free NHC, which was quantified by nuclear magnetic resonance spectroscopy and ultraviolet-visible spectroscopy. The reversibility of this process raises exciting possibilities for the preparation of extended conjugated π systems of silicon.

Reversible Base Coordination to a Disilene

During the last few decades, alkene and alkyne analogues of the heavier Group 14 elements have attracted considerable interest. Their isolation as stable derivatives has become possible by the use of carefully designed bulky substituents that provide kinetic (and to some extent thermodynamic) stabilization. The considerable differences in structure, bonding, and reactivity of such compounds in comparison to the carbon-based species have prompted various experimental and theoretical studies. By and large, the lower electronegativity of heavier elements and the increasing spatial extension of their valence electron shells are responsible for many of these differences. One of several rationalizations for structure and reactivity of such doubly and triply bonded species is based on zwitterionic (Ib and IIb in Scheme 1) and

Syntheses of Trisila Analogues of Allyl Chlorides and Their Transformations to Chlorocyclotrisilanes, Cyclotrisilanides, and a Trisilaindane

The rearrangements of (chlorosilyl)disilenes R2(Cl)Si-(Tip)Si=SiTip2 (5a,b: Tip = 2,4,6-iPr3C6H2, a: R = Me, b: R = Ph) quantitatively yield the isomeric chlorocyclotrisilanes (6a,b). The disilene precursors 5a,b are, in turn, accessible from the reactions of the disilenide Tip2Si=Si(Tip)Li (1), that is, a disila analogue to vinyl anions, with dichlorosilanes R2SiCl2. This novel approach to cyclotrisilanes potentially allows for the facile variation of the substitution pattern and grants access to the first anionic derivatives; while the rearrangement of 5a,b to 6a,b is slow at room temperature and additionally requires the presence of THF or other n-donors, reduction of 5b with lithium instantly yields the corresponding cyclotrisilanide (7b) without detection of any open-chained isomer. Heating of a neat sample of 5b to 150 degrees C provides a completely characterized 1,2,3-trisilaindane derivative (13), strongly supporting the intermediacy of a disilanyl silylene species that inserts into an ortho-CH bond of the phenyl substituents. The X-ray diffraction studies on single crystals of 6a,b and 7b reveal that the Si-Si bond distance in cyclotrisilanes depends significantly on the electronegativity of the opposing silicon atoms substituents, which is rationalized by density functional theory (DFT) calculations on model systems.

Phosphide Delivery to a Cyclotrisilene

The reactivity of the 2-phosphaethynolate anion (PCO−) towards a cyclic trisilene (cSi3(Tip)4) is reported. The result is the net activation of the P=C and Si=Si multiple bonds of the precursors affording a heteroatomic bicyclo[1.1.1]pentan-2-one analogue ([P(CO)Si3(Tip)4]−; 1). This reaction can be interpreted as the formal addition of a phosphide and a carbonyl across the Si=Si double bond. Photolytic decarbonylation of 1 results in the incorporation of the phosphide vertex into the cyclotrisilene scaffold, yielding a congener of the cyclobutene anion with considerable allylic character.

Transfer of a disilenyl moiety to aromatic substrates and lateral functional group transformation in aryl disilenes.

The reaction of 1 equiv of the disilenide Tip2Si═Si(Tip)Li (5; Tip = 2,4,6-(i)Pr3C6H2) with para-substituted phenyl iodides, 4-X-PhI, transfers the Tip2Si═Si(Tip) moiety with elimination of lithium iodide to yield the laterally functionalized disilenes Tip2Si═Si(Tip)(4-X-Ph) [X = H (6a), F (6b), Cl (6c), Br (6d), I (6e)]. The UV-vis absorptions of 6a-d suggest a linear correlation with electronic Hammett parameters. In addition, X-ray structural analyses of 6a-d verified the theoretically predicted linear dependence of the Si═Si bond length and trans-bent angles. The p-bromophenyl-substituted disilene 6d undergoes a metal-halogen exchange reaction to give 6f (X = Li), which was trapped with Me3SiCl to afford 6g (X = SiMe3). In the case of simple phenyl halides PhX without additional functionality, the reaction with 5 proceeded smoothly for X = Br, but phenyl chlorides and fluorides did not react at room temperature even after one week, hinting at an S(N)2-type aromatic substitution mechanism. Reactions of p- and m-diiodobenzene with 5 afford the corresponding phenylene-bridged tetrasiladienes p-7 and m-7. While red p-7 (λ(max) = 508 nm) exhibits efficient conjugation of the two Si═Si bonds with the phenylene linker, the conjugation in yellow m-7 (λ(max) = 449 nm) is much less effective. Electrochemical studies of m-7 and p-7 as well as density functional theory calculations and electron paramagnetic resonance studies of their respective radical anions provided further support for the notion of conjugation.

An experimental charge density study of two isomers of hexasilabenzene.

The similarities and differences between carbon and its heavier congener silicon generate challenging synthetic targets, especially when multiply bonded systems are concerned. The first stable compound with a Si=Si bond goes back to West et al. in 1981, and conjugated systems with Si= Si double bonds were pioneered in 1997 by Weidenbruch et al. Whether silicon analogues of benzene can show aromatic character is still a point of constant debate. The aromatic nature of silabenzenes has been predicted theoretically, but the synthesis of a stable silabenzene was not accomplished until Tokitoh et al. reported the sterically encumbered 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl-substituted monosila derivative. At the same time, Ando et al. independently reported the synthesis of 1,4-disila Dewar benzene. Only two years later, Sekiguchi et al. accomplished the synthesis of 1,2-disilabenzene by reacting RSi SiR (R = Si(CH(SiMe3)2iPr) with PhC CH in a formal [2+2+2] cycloaddition reaction. Recently, in a cooperative effort with the Roesky group, we reported the synthesis of a 1,4-disilabenzene by reacting [{PhC(NtBu)2}Si]2 with diphenyl alkyne. [8] With regards to homonuclear systems, the Scheschkewitz group recently made groundbreaking progress with the isolation of ring and cage isomers of hexasilabenzene (Scheme 1), which prompted the present experimental charge-density study. The dark-green-colored six-membered ring system 1 rearranges upon heating or UV irradiation to the red silicon cage compound 2 with a bridged propellane structure. An analogous transformation for fully saturated silicon compounds under irradiative conditions has been described by Kira and co-workers. We propose a transition of 1 to 2 by the reaction pathway in Scheme 1 (bottom). The transformation proceeds by the breaking of the Si1 Si3 and Si2 Si4 bonds in 1, followed by a twist of the four-membered silicon ring and the formation of the new Si1 Si2 and Si3 Si4 bonds (see Scheme 1 and video in the Supporting Information). In the following, we analyze the bonding situation in the ring (1) and cage (2) isomer of hexasilabenzene (TipSi)6 on the basis of experimental charge-density investigations (Figure 1). High-resolution X-ray data with (sinq/l)max =

From Disilene (SiSi) to Phosphasilene (SiP) and Phosphacumulene (PCN)

The generation of heavier double-bond systems without by- or side-product formation is of considerable importance for their application in synthesis. Peripheral functional groups in such alkene homologues are promising in this regard owing to their inherent mobility. Depending on the steric demand of the N-alkyl substituent R, the reaction of disilenide Ar2Si=Si(Ar)Li (Ar = 2,4,6-iPr3C6H2) with ClP(NR2)2 either affords the phosphinodisilene Ar2 Si=Si(Ar)P(NR2)2 (for R = iPr) or P-amino functionalized phosphasilenes Ar2(R2N)Si=Si(Ar)=P(NR2) (for R = Et, Me) by 1,3-migration of one of the amino groups. In case of R = Me, upon addition of one equivalent of tert-butylisonitrile a second amino group shift occurs to yield the 1-aza-3-phosphaallene Ar2(R2N)Si=Si(NR2)(Ar)-P=C=NtBu with pronounced ylidic character. All new compounds were fully characterized by multinuclear NMR spectroscopy as well as single-crystal X-ray diffraction and DFT calculations in selected cases.