Isomerization disilenes

Chart 3 provides a survey of the disilene isomerization processes known so far. 

Information on the E-Z isomerization of the disilenes sheds light on the nature of the chemical bonding of the silicon-silicon double bond. Kinetic parameters, which have been reported for cis-trans isomerization of five disilenes, are listed in Table IV. 

More recently, a new mode of cis-trans isomerization of a disilene has been suggested for the extremely hindered disilene 27. As will be detailed in Section VIII. B, 27 undergoes thermal dissociation into the corresponding silylenes. Monitoring the thermolysis of (Z)-27 at 50°C by H and 29Si NMR reveals a competitive formation of the isomerized ( >27 and benzosilacyclobutene 37, which is most likely formed by intramolecular insertion of silylene 36 into the C—H bond of the o-bis(trimethylsilyl)-methyl group (Scheme 3).22,59 This suggests the possible occurrence of cis-trans isomerization via a dissociation-association mechanism. 

Disilenes readily add halogens14,66 and active hydrogen compounds (HX), such as hydrogen halides,63,66 alcohols, and water,27 63 as well as hydride reagents, such as tin hydride and lithium aluminum hydride.66 These reactions are summarized in Scheme 9. The reaction of the stereo-isomeric disilene (E)-3 with hydrogen chloride and alcohols led to a mixture of E- and Z-isomers, but the reaction with chlorine gave only one of the two possible stereoisomers, thus indicating that the former two reactions proceed stepwise while the latter occurs without Si—Si rotation. 

However, at elevated temperatures, the disilene (/-Uu (Si)(k)SiSi( R)(Si/-Bu () 721 undergoes isomerization to give the 1,2-disilyl benzene derivative 730, which can be rationalized in terms of a C-H addition to the Si-Si double bond (Scheme 97). 

Before moving on to M=Si bonded compounds, mention should be made of the disilene complexes (Equation (6)), which are formed as (E)- and (Z)-isomers in 28% and 9% yields, respectively ( )-to-(Z)-isomerization follows first-order kinetics.173... 

Estimated from the experimental cis-trans isomerization barrier in a substituted disilene (84MI6). 

Thermal E-Z isomerization was also observed for kinetically stabilized disilenes 5-9 (equation l)19-22. The n bond strength was estimated to range from 24.7 to 30.6 kcalmol-1. These data are in good agreement with those of 1,2-dimethyl-1,2-diphenyldisilene and those predicted by ab initio calculations for H2Si=SiH2 (22-28 kcalmol-1)23,24. 

These E-Z isomerization studies of disilenes indicate that the n overlap between two 3p orbitals of silicon is sufficiently effective to retain the configuration around the double bond, although the 7r bonding of disilenes is significantly weaker than that of the C=C double bond. Therefore, it is expected that if appropriately substituted disilenes can be generated, regiochemistry as well as diastereochemistry of addition reactions to disilenes can be investigated even with transient reactive disilenes. 

Phenyltrimethyldisilene (15) and (E)- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (16) were also observed as transient absorption spectra by laser flash photolysis of the precursors in methylcyclohexanes28. The absorption band at 380 nm, assigned to the disilene 15, reached maximum intensity at ca 10 ns after the excitation and then started to decrease. The half-life assigned to 15 was 700 ns. The logarithm of the decay profile of the transient absorption at 380 nm versus time shows a very good linear relationship, indicating that the decay of the transient absorption fits first-order kinetics. This result shows that intramolecular isomerization or proton abstraction from the solvent is the origin for the decay of the disilene 15, which survives in solution only for several nanoseconds. 

Phenyltrimethyldisilene 15, produced by irradiation of the precursor 13 (X > 280 nm) in the presence of several alcohols, gives rise to the formation of 1 -alkoxy-2-hydrido-l,l,2-trimethyl-2-phenyldisilane (26) as the major product along with a small amount of the isomeric l-alkoxy-2-hydrido-l,2,2-trimethyl-l-phenyldisilane (27) (see Scheme 3). As shown in Table 3, very high regioselectivity was observed. This is the first example demonstrating a regioselective addition reaction to the unsymmetrically substituted disilenes. 

The high diastereoselectivity in the addition of i-PrOH, t-BuOH and EtOH (at low concentration) suggests that E Z photoisomerization of (E)- or (Z)-16 does not occur in solution at room temperature or that the trapping of (E)- or (Z)-16 by alcohols proceeds faster than the E Z isomerization. In addition, the results show that proton transfer in the intermediate adduct formed by the disilenes and alcohols occurs much faster than rotation around the Si—Si bond. However, in the reaction with ethanol, an appreciable amount of the anti addition product was formed. Thus, the diastereoselectivity remarkably depended on the concentration of ethanol. 

Whereas the analogous carbenes easily isomerize wherever possible to compounds containing doubly bonded carbon atoms even under the conditions of matrix isolation, silylenes are almost as stable as the corresponding substances with doubly bonded silicon atoms. For example, methyl- and silylsilylene lie just 4 and 8 kcalmol-1 above silaethene and disilene, whereas the difference between ethene and methylcarbene is as high as 70 kcalmol-1 149-151 As a consequence, silylenes are often key intermediates on the way to other highly reactive silicon compounds discussed above. 

Subsequent studies by Sakurai and coworkers of the photolysis of disilyl-bridged aromatic compound 78, indicated that two interesting (not necessarily independent) processes were occurring, namely elimination of the disilyl group as a disilene, 79, another reactive intermediate of silicon chemistry, and secondly, Si-C homolysis to a diradical 80, which overall underwent a 1,2-silyl shift yielding the isomeric disilyl-bridged compound 8141-42 (Scheme 11). 

The trans isomers of two disilenes were observed to undergo photoisomerization to mixtures enriched in the cis isomers when irradiated at either 254 or 350 nm (equation 28), and the kinetics of the thermal cis-trans isomerization have been determined164. 

Studies of the kinetics of cis-trans interconversion in disilene 33 indicate that it takes place by dissociation to the silylene and recombination, rather than by Si=Si bond rotation as is the case for other disilenes187. This isomerization occurs even at 50 °C. 

The first cyclic disilene, hexakisUerf-butyldimethylsilyljcyclotetrasilene (51), is synthesized in 13.6% yield by the reduction of a 1 2 mixture of tetrabromodisilane 86 and dibromosilane 87 with lithium naphthalenide in THF [Eq. (12)].23 Interestingly, the initial product of the reductive condensation is not cyclotetrasilene 51 but bicyclo[1.1.0]tetrasilane 88. Thus, reduction of a mixture of 86 and 87 with lithium naphthalenide at -78 °C followed by hydrolysis at 0 °C affords mainly a hydrolysis product of bicyclotetrasilane 88, 89 [Eq. (13)]. As discussed in Section IV.A.4 in more detail, compound 51 should be formed by the thermal isomerization of 88 during workup at higher temperatures. 

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... 

Migration of a substituent on a disilene giving the corresponding silylene [pathway (3)] should be considered as a pathway for the /AZ-isomerization but occurs usually with much higher activation energies than pathways (1) and/or (2). The pathway (3) and related dyotropic rearrangement are discussed in detail in Section IV.A.3. 

As discussed in the previous section, thermal dissociation of disilenes into the corresponding silylenes may occur if the BDE of the disilenes is small. As shown in review OW, a facile thermal dissociation of disilene 27 into silylene 127 occurs at 50 °C [Eq. (49)],61,91 The formation of silylene 127 is evidenced by its trapping by methanol, triethylsilane, and 2,3-dimethyl-1,3-butadiene. The activation enthalpy and entropy for the dissociation of (Z)-27 to 127 are 25.5kcalmol-1 and 7.8 cal mol-1 K-1 respectively.91 The activation free energy for the dissociation at 323 K (22.9 kcal mol-1) is much smaller than that for the Z-to-E isomerization of 26 (27.8 kcal mol-1), indicating that the E,Z-isomerization of 27 should occur via the pathway (2) in Eq. (47) rather than pathway (1) in Eq. (48). 

The ene reaction of cyclic disilene 50 with enolizable ketones such as biacetyl and acetophenone gives initially a.v-adduct 199, which gradually isomerizes to the corresponding traKi-isomer 200 104,133... 

Cyclotrisilene 49 reacts with 2 mol of phenylacetylene to give unique bicyclic product 203 in good yield [Eq. (96)].138 On the basis of the results of deuterium labeling experiments, compound 203 is proposed to form via two consecutive [2 + 2] cycloadditions of 49 to phenylacetylene, i.e., the initial [2 + 2] cycloaddition giving 204 followed by the isomerization to disilene 206 via 205, and then the second [2 + 2] cycloaddition of 206 [Eq. (97)]. The reaction of 1-disilagermirene 50 with phenylacetylene proceeds in a similar way.139... 

For the Z-to-E isomerization, a mechanism via silylsilylene complex 238 [Eq. (112)] has been proposed. Mechanisms including the following processes are excluded on the basis of the theoretical calculations (1) dissociation-association equilibrium between the corresponding disilene and Fe(CO)4, (2) removal of one CO from 236Z and then Si-Si bond cleavage forming the corresponding bis-silylene... 

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