Dmitry Bravo-Zhivotovskii

The Synthesis and Molecular Structure of the First Two-Coordinate, Dinuclear σ-Bonded Mercury(I) RHgHgR Compound.

A linear Si-Hg-Hg-Si arrangement and a Hg-Hg distance of 265.69 pm are exhibited by the first two-coordinate, dinuclear σ-bonded organomercury(I) compound 1. It was formed unexpectedly in the reaction of two equivalents of the silane (Me3 SiMe2 Si)3 SiH with tBu2 Hg. In contrast if the reagents are allowed to react in a 1:1 ratio the expected mercury(II) compound (Me3 SiMe2 Si)3 SiHgtBu is obtained.

A novel route to the trisilacyclobutane moiety. A possible silene-disilene reaction

Abstract The reaction of (Me 3 Si) 3 SiC(O)Ad with a two-fold excess of Et 3 GeLi in THF, gives a 1:2 mixture of: 1,1,2,2-tetra-(trimethylsilyl) -3-trimethylsilyl-3-triethylgermoxy, 4-(1-adamantyl)-trisilacyclobutane ( 4 ) and adamantoyladamantyl-carbinol ( 5 ). The crystal structure of 4 is described and discussed. It is suggested that the trisilacyclobutane skeleton is formed by a cycloaddition reaction between a silene and a disilene.

Synthesis and the electronic spectra of the first ß-ketoacylsilanes and their lithium enolates: new insights into hyperconjugation in acylsilanes and their enolates

Abstract The reaction of tetramethyl-1,3-cyclobutanedione (1) with R3SiLi (R = Me3Si or Et) and Et3GeLi results in the opening of the cyclobutanedione ring to give the corresponding s-ketoacylsilane lithium enolates 2a–2c, which after aqueous work-up gave the first known s-ketoacylsilanes 3a and 3b and s-ketoacylgermane 3c. The first X-ray structure of a s-ketoacylsilane, that of lithium enolate 2a, is reported and discussed. The UV-visible spectra of the lithium enolates 2 exhibits two new transitions: one absorption is “red” shifted and the other is “blue” shifted (each by about 40–50 nm) relative to the absorptions of the corresponding s-ketoacylsilanes. Ab initio molecular orbital calculations show that the “red-shifted” transitions result from the presence of a low-lying Rydberg-type antibonding OLi, orbital while the “blue-shifted” transition results from a weakening (due to Li+ complexation) of the destabilizing hyperconjugative interactions between the oxygen lone pair (no) and the σCSi orbital, which leads to a lowering of the energy of the filled (nOσCSi) orbital (relative to its energy in the acylsilanes), and thus to a higher ( n O −σ CSi )→π co ∗ excitation energy than in the corresponding acylsilanes.

Isolation of Silenolates (R3Si)2SiC(OLi)Ad with a Doubly Bonded Silicon Atom

Metal enolates are an important class of reactive intermediates widely employed in organic synthesis. In contrast, little is known about silenolates, the silicon analogues of enolates. Enolates exist in two tautomeric forms, the enol form and the keto form, and their reactions reflect the coexistence of these two forms. The dominant structure of alkali metal enolates is the enol form both in nonsolvating media and in various solvating media such as THF, N,N,N’,N’-tetramethylethylenediamine, and [18]crown-6. Silenolates also exist in two tautomeric forms: the keto form a (acyl silyl anion) and the enol form b (silene) [Eq. (1)], and they also show ambident reactivity. The first silenolate (solvated), recently isolated and characterized by X-ray crystallography, has the keto form a. An enol-form silenolate bwas not yet reported. Isolation of an enol-form silenolate is challenging, because it has a Si=C p bond which is thermodynamically and kinetically less stable than a C=C bond. In addition, enol-form silenolates can be regarded as functional silenes, which are reagents of growing importance in silicon chemistry. Here we report the synthesis, isolation, and X-ray molecular structure of the first enol-form silenolates (tBuMe2Si)2Si=C(OLi)Ad (1) and (tBu2MeSi)2Si=C(OLi)Ad (2). We show by DFT quantum-mechanical calculations that, in contrast to organic enolates, which exist predominantly in the enol form regardless of solvation, the structure of silenolates 1 and 2 is strongly dependent on the solvent. Silenolate 1 was synthesized by metal–halogen exchange between tBuMe2SiLi (in excess) and bromo acyl silane Br(tBuMe2Si)2SiC(O)Ad (3) in hexane at 78 8C. Upon warming to room temperature pale yellow crystals of silenolate 1 precipitated (10% yield). The major product is substitution product 4 [Eq. (2)].

Polysilyl radicals: EPR study of the formation and decomposition of star polysilanes

Electron paramagnetic resonance (EPR) spectroscopy was fruitfully used for studying the formation and the reactions of the star polysilane radical (Me3SiMe2Si)3Si (1).1, which was successfully generated both thermally and photochemically from a variety of precursors, was found to be significantly more stable kinetically than the (Me3Si)3Si radical. Thus, (Me3SiMe2Si)3Si⋅ has a half-life time of ca. 6 min at 20°C, while (Me3Si)3Si⋅ can be observed only at −25°C. Density-functional quantum-mechanical calculations show that1 and (Me3Si)3Si⋅ have the same thermodynamic stability. The high kinetic stability of1 is attributed to its backfold “umbrella”-type conformation where the β-silyl groups point “inwards” towards the radical center. This conformation protects the radical center of1 from dimerization and other reactions. The EPR spectrum of1 and in particular the Si α-hyperfine coupling constant of 5.99 mT shows that1 is less pyramidal than (Me3Si)3Si⋅ but is more pyramidal than (i-Pr3Si)3Si⋅, with an estimated SiSiSi bond angle around the radical center of 118∘. Photolysis and thermolysis of [(Me3SiMe2Si)3Si]2 also involves the intermediacy of1. Photolysis of [(Me3SiMe2Si)3Si]2 leads to (Me3SiMe2Si)4Si, while thermolysis produced the less strained isomer of 1, (Me3SiMe2Si)3SiSi-Me2Si(Me3SiMe2Si)2SiMe3. In this study we provide the first direct evidence that silyl radicals are involved as intermediates in the reactions of silanes with di(tert-butyl)mercury.

The effective ‘size’ of the tris(trimethylsilyl)silyl group in several molecular environments

The effective size of the tris(trimethylsilyl)silyl group in several molecular environments has been estimated. 2,2-Dimesityl-1-tris(trimethylsilyl)silylethanol 1g has been prepared and its structure determined by X-ray crystallography. The Mes–CC torsional angles are 59.6 (φ2) and 63.3°(φ2) and the CC–Si bond angle α4 is 133.8°. The two-ring flip barrier for the correlated rotation of the two mesityl rings around the Mes–C bonds is ΔGc‡= 10.2 kcal mol–1. The structures of enols Mes2CC(OH)R, R = H, Me, Et, Pri, But(1a–1e), Me3Si (1f), (Me3Si)3Si (1g) and (Me3Si)3C (1h) and the two-ring flip barriers have been calculated by the MM2* force-field. The calculated and the experimental values are in good agreement, except for somewhat lower calculated α4 for 1b–1e and a shorter C–Si distance in 1g. From the linear correlations between the observed cos φ2 or ΔGc‡ values and Es values for the enols 1a–1e, and the values observed for 1g an average Es value of –1.46 has been calculated for (Me3Si)3Si. MM2* calculations gave an A value for (Me3Si)3Si of 4.89 kcal mol–1. These steric parameters resemble those for the But group (Es=–1.54; A= 4.9 kcal mol–1) indicating a similar effective size for the But and (Me3Si)3Si groups in these specific environments. (Me3Si)3C is significantly larger (A= 13.3 kcal mol–1; estimated Es=–3.7).

Synthesis, Isolation, and Characterization of 1,1‐DiGrignard and 1,1‐Dizincio Silanes

Geminal dimetallosilanes have already proved their great potential for the synthesis of novel silicon compounds. However, the variety and number of isolated geminal dimetallosilanes is very small. For example, all known geminal dimetallosilanes are alkali metal or mercury derivatives. Furthermore, only one example of a geminal dimetallosilane with two different metals, that is, a mercury bridged bis(silyllithium) species, was reported recently by our research group. This contrasts with the large variety of available geminal dimetallorganic reagents, which are very useful synthons in organic synthesis. In particular, diGrignard reagents and dizinc reagents are widely used in organic chemistry. In contrast, silicon analogues, (i.e. geminal diGrignard or dizincio silanes) are yet to be reported. The vast synthetic potential of such reagents remains to be explored. Herein we report the synthesis, isolation, and X-ray molecular structure of the first cyclic 1,1-dimagnesiosilane 1, the first diGrignard silane 1,1-di(chloromagnesio)silane 2, and the analogous 1,1-di(chlorozincio)silane 3. We also report the selective redox reactions of 1 with tBu2MeSiLi and with 1,1-dilithiosilane 4, thus leading to novel metallosilane species. We initially believed that geminal dimetallosilanes could be prepared by a transmetalation reaction of dilithiosilane derivatives with metal salts such as MgX2 or ZnX2, in analogy to the synthesis of metallosilanes. However, transmetalation reactions with strong electron donors often involve redox processes that lead to nonselective reactions. This is the case also with (tBuMe2Si)2SiLi2 (4). Thus, reaction of 4 with MX2 (MX2=MgCl2, MgBr2 ZnCl2, HgCl2, HgF2), leads to a complex mixture of products and a metallic M residue. In contrast, mixing 1,1-dilithiosilane 4with 1.5 equivalents of tBuMgCl·2MgCl2 (5) in THF [10] at 0 8C gave, after stirring for 30 minutes, cyclic 1,1-dimagnesiosilane 1 [Eq. (1); THF= tetrahydrofuran]. Tetrahydrofuran was

Isolable Photoreactive Polysilyl Radicals

Reaction of silyl substituted dichlorosilanes with lithiosilanes in hexane leads exclusively to the corresponding stable silyl radicals. Two radicals, the new (t-Bu(2)MeSi)(2)HSi(t-Bu(2)MeSi)(2)Si* (1) and the previously isolated (t-Bu(2)MeSi)(3)Si* (2), were isolated and fully characterized including by X-ray crystallography. This one-step method is general and was applied for the synthesis of other silyl radicals. Upon irradiation radical 1 (yellow solution in hexane) decays to yield the corresponding disproportionation products, silane and disilene (blue colored). In contrast, radical 2 is photostable in the absence of additives, but it abstracts hydrogen from triethylsilane and 2-propanol upon irradiation. DFT calculations and irradiation experiments with lambda > 400 nm suggest that SOMO-1 --> SOMO excitation, which provides better electron accepting properties to the radical, is responsible for the photoreactivity of 1 and 2.

Radical Activation of SiH Bonds by Organozinc and Silylzinc Reagents: Synthesis of Geminal Dizinciosilanes and Zinciolithiosilanes**

Organozinc compounds are very useful reagents in synthesis, and in analogy to organomagnesium and organolithium compounds are used mainly as nucleophiles. Some reports are also available for silylzinc compounds. At the same time, as Zn and Hg are both Group 12 elements, organozinc compounds are expected also to exhibit some of the characteristic reactions of organomercury compounds. For example, organomercury and silylmercury compounds undergo radical reactions both thermally and photochemically and activate Si H bonds, leading to silylmercury compounds. Such compounds are also excellent precursors for the preparation of silyl anions by transmetalation. The possibility to replace in these reactions the highly toxic organomercury compounds by the less toxic organozinc compounds is attractive. The radical chemistry of organozinc compounds, initiated by dioxygen, is a vigorously developing field of organic chemistry with many applications in synthesis. In contrast, to the best of our knowledge, there are no reports on radical reactions of silylzinc compounds. Activation of Si H bonds is a very useful reaction synthetically and therefore it is intensively studied, both in academia and in industry. There are two common strategies for Si H bond activation: by radical initiators [7] and by transition-metal catalysts. Silylzinc reagents became recently popular synthons in organic chemistry owing to their selective and mild nucleophilic nature. Therefore, a single-pot reaction leading to zinciosilanes by activation of Si H bonds has a great synthetic potential. Furthermore, in analogy to the synthesis of bismercuriosilanes, activation of dihydridosilanes by diorganozinc or disilylzinc compounds can lead, in a single-step reaction, to new geminal dizinciosilanes. These interesting dimetallic reagents are potentially very attractive reagents, in a similar fashion to geminal dizinciomethanes, their organic analogues. Herein, we present the first examples of radical activation of a Si H bond in R3SiH (R=Me3Si, Me3SiMe2Si) and Me3SiMe2SiH by R’2Zn (R’= tBu, Et) and by (tBuMe2Si)2Zn (1) (for experimental details see Supporting Information). Using this reaction we synthesized the silylenoid-type compound [Cl(tBuMe2Si)2Si]2Zn (2), which upon lithiation yields the zinc-bridged bis(silyllithium) [(thf)2Li(tBuMe2Si)2Si]2Zn (3), the first known compound with lithium and zinc bonded to the same Group 14 atom. We have also synthesized and fully characterized the novel tetrametallic bis(zinciosilyllithium)silane [(thf)3Li(tBuMe2Si)2SiZn]2Si(SiMe2tBu)2 (4). The reactions of Et2Zn, which is commercially available, and of tBu2Zn with tBuMe2SiH (5), Me3SiMe2SiH (6), and (Me3Si)3SiH (7) were first studied. The reaction of Et2Zn or of tBu2Zn with 5 was disappointing; 5 remained unchanged after 3 h at 90 8C, even in the presence of azobisisobutyronitrile (AIBN) or tBu2Hg as radical initiators. However, this situation changes upon silyl substitution of the silane. Thus, reaction of Me3SiMe2SiH (6) with tBu2Zn for 3 h at 90 8C yielded (Me3SiMe2Si)2Zn (8) in 30% yield. When a minute amount of tBu2Hg, a radical initiator, was added the yield of 8 increased to 95%. In contrast, 6 was recovered unreacted after reaction with neat Et2Zn and yielded only 10% of 8 in the presence of tBu2Hg at 90 8C for 3 h (Scheme 1).

Synthesis of the first long-lived bis-silene

Abstract The sila-olefination reaction has been used to synthesize the first long-lived bis-silene t -BuMe 2 Si(Me 3 Si)Si(2,6-Ad)Si(SiMe 3 )SiMe 2 Bu- t ( 1 ). In this paper, we report the synthesis, spectroscopic data and some reactions of 1 . Our experiments show that the elimination step of sila-olefination reaction is strongly accelerated in diethyl ether (and other ethers) relative to toluene and by increasing the steric bulk around the silicon center undergoing elimination.