Tetrakis disilenes

The first synthesis of a disilene by photolysis of the corresponding cyclic trisilane was reported by Masamune et al. in 1982.14 This method has been adapted for the synthesis of a variety of stable and marginally stable disilenes (Eq. 6).15-20 More recently, Kira et al. synthesized the first example of a stable tetrakis(trialkylsilyl)disilene 22 by this method.21... 

Each of the three tetrakis(trialkylsilyl)disilenes 22-24 has an absorption band near 420 nm but compound 24, which bears the largest substituents, also exhibits a rather strong absorption at exceptionally low energy, max 480 nm.21 The likely explanation of this anomaly is that a strongly twisted conformation of 24 may exist in solution. Among the tetraalkyldisilenes, the unusually long wavelength absorption at 433 nm for 21 may also result from a twisted conformation.4... 

The unusually low-field 29Si NMR resonances (S 142-155) observed for tetrakis(trialkylsilyl)disilenes 22-24 are interesting,21 but whether these effects result from the trialkylsilyl substituents is open to question in view of their similarity to the chemical shifts between diarylbis(trimethylsilyl)-disilene 9 and dialkyldiaryldisilene 10 ((Z)-9 (8 97.68) vs. (Z)-10 (8 96.93)). 

Two A2Si = SiB2-type tetrakis(triakylsilyl)disilenes 36 and 37 were synthesized using Method D [Eq. (4)].14 In the synthetic route, the preparation of hydridosilyl-lithiums 80 by the silametallation of the corresponding dihydridosilanes 79 is involved.15... 

The four-membered cyclic disilene with mixed group-14 elements 56 is synthesized in 95% yield by the reaction of the dipotassium salt of tetrakis(di-tert-butyl-methylsilyl)-l,2-disila-3,4-digermacyclobutadiene dianion 74 with Me2S04 the two Me groups in 56 are bound selectively to Ge atoms 28... 

Structural parameters of tetrakis(trialkylsilyl)disilenes depend significantly on the steric bulkiness of the silyl substituents. Disilenes 23 and 24 are slightly trans-bent without twisting, while disilenes 22, 35, and 37 are twisted with x of 8.9, 10.4 (avg.), and 28.0°, respectively no bending was found around the unsaturated silicon atoms... 

As a typical unimolecular reaction of disilenes, the /i,Z-isomcrization is discussed first. In contrast to the isomerization of an alkene that occurs via the rotation around the C = C double bond with an activation energy of ca. 60kcalmol-1 the E,Z-isomerization of disilenes is known to occur more easily. As shown in review OW, the E,Z-isomerization in aryl-substituted disilenes 3,4,20,26, and 27 proceeds under mild conditions to allow the kinetic studies at 40-80 °C by NMR spectroscopy. Recently, the T,Z-isomerization between tetrakis(trialkylsilyl)disilenes ( )- and (Z)-33 was found to occur more rapidly with the rates of the NMR time scale at 30 °C 63... 

Tetrakis(trialkylsilyl)disilenes 22 and 23 react with excess lithium in THF giving the corresponding 2,3-dilithiotetrasilanes 161 and 162 as thermally stable off-white powders [Eq. (72)].115 Interestingly, under the same conditions, the reduction of tetrakis(triisopropylsilyl)disilene 24 provides quantitatively bis(triisopropylsilyl) dilithiosilane 78 [Eq. (73)] 115 dilithiosilane 78 is known to be obtained also by the lithium reduction of 3,3-bis(triisopropylsilyl)-l,2-bis(trimethyislyl)silacyclo-propene.12... 

Styrene and substituted styrenes react with tetramesityldisilene 1, tetra-tert-butyl-disilene 21, and tetrakis(tert-butyldimethylsilyl)disilene 22 to afford the corresponding disilacyclobutane derivatives.127,134 Similarly, [2 + 2] additions occur between the disilenes with a C = C double bond in an aromatic ring135 and acrylonitrile.136 Bains et al. have found that the reaction of disilene 1 with trans-styrene- provides a 7 3 diastereomeric mixture of [2 + 2] adducts, 201 and 202 [Eq. (95)] the ratio is changed, when czs-styrene-Ji is used.137 The formation of the two diastereomeric cyclic adducts is taken as the evidence for a stepwise mechanism via a diradical or dipolar intermediate for the addition, similar to the [2 + 2] cycloaddition of phenylacetylene to disilene ( )-3, which gives a 1 1 mixture of stereoiso-meric products.116,137... 

The NMR resonances for the central 29Si nuclei of tetrakis(trialkylsilyl)disilene complexes in solution vary from —160 ppm for 241 to 127 ppm for 240, depending on the bonding characteristics of the disilene ligands to the metals for reference, the 29Si NMR resonances for hexakis(trialkylsilyl)cyclotrisilanes and free disilene 22 are... 

Only four transient disilenes have been studied to date by fast time-resolved spectroscopic techniques l,l,2-trimethyl-2-phenyldisilene (103), ( )- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (104) and tetrakis(trimethylsilyl)disilene (35). The first three compounds were generated by photolysis of the 7,8-disilabicyclo[2.2.2]octa-2,5-diene derivatives 101 and 102 (equation 76)148 while 35 was generated, together with 106, by photolysis of the 1,2-disilacyclobutane derivative 33 (equation 77)68. 

FIGURE 15. Arrhenius plots for the decay of tetrakis(trimethylsilyl)disilene (35) in 85% MeOH/THF ( ) and 85% MeOD/THF (o)... 

Stable disilenes generally do not undergo Diels-Alder cycloaddition with conjugated dienes16, but the reaction is well known in the cases of more reactive disilene derivatives8. The stereochemistry of the reaction has not been widely studied, but isolated examples such as those shown in equations 86-88 show that the reaction proceeds with retention of the original stereochemistry of both the diene (equation 88)161 and the disilene (equations 86 and 87)162. Reaction of tetrakis(trimethylsilyl)disilene (35) with 1,3-buta-diene in solution yields the expected Diels-Alder adduct 116 (equation 89)68. 

The data of Table 19 indicate there to be very little difference between the absolute rate constants for trapping of 103, ( )-104 and (Z)-104 by 2,3-dimethyl-l,3-butadiene158. As with alcohol and 02-additions, the tetrakis(trialkylsilyl)disilene 35 exhibits substantially lower reactivity than 103 and 10468. None of the products of these reactions has been isolated68,158. 

Reduction of tetrakis(trialkylsilyl) disilene with Li in THF or K in DME gives the corresponding 1,2-dianions, together with the anion radicals [Eq. (31)1 (72). 

Tetrakis[bis(trimethylsilyl)methyl]disilene undergoes dissociation... 

Tetrakis(trialkylsilyl)disilenes formally made by the dimerization of the corresponding... 

It was assumed that an a-fluorosilyl potassium species was formed initially, and that this subsequently underwent a self-condensation reaction. The eventual product (1) displayed both nucleophilic as well as electrophilic character, which was demonstrated in various derivatization reactions. Although it contains fluorine and potassium atoms in close proximity, the compound displayed a remarkable thermal stability. Even at 80 °C, potassium fluoride elimination occurred only sluggishly. Attempted transmetalation reactions with various metal halides, though, caused an immediate elimination of metal fluoride and the formation of tetrakis(trimethylsilyl)disilene. The latter can be trapped in cycloaddition reactions [5] or, in the absence of trapping reagents, it dimerizes to a cyclotetrasilane (Scheme 2) [6]. 

A liquid-phase pyrolysis of neat methoxytris(trimethylsilyl)silane, a bis-(trimethylsilyl)silylene precursor, produces octakis(trimethylsilyl)cyclotetrasilane62. Almost inevitably, tetrakis(trimethylsilyl)disilene is involved in the mechanism of this transformation and is possibly formed by dimerization of the silylene. However, the mechanistic details of this transformation remain speculative. 

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