Disilenes with dienes

After review OW, the molecular structures in the solid state of 13 acyclic di-silenes, 13 endo- and exocyclic disilenes, 4 silicon-based dienes, 6 disilenes with radical, cation, and anion center, and one disilyne have been determined by X-ray analysis. Although the structural characteristics of a number of these new disilenes are summarized in a recent review by Weidenbruch,9 the structural parameters of all new disilenes reported after review OW are listed in Tables I-IV. 

Although disilenes do not undergo Diels-Alder reaction with 1,3-dienes,68 they react with heterodienes like benzil,67-89 acylimines,90 and 1,4-diazabutadienes91 to give [4 + 2] cycloadducts (Eqs. 26-28). 

While disilene 5 does not undergo Diels-Alder reactions with 1,3-dienes, the [4+2]-cycloaddition products are formed with heterodienes, e.g. 1,4-diazabutadienes [17] or a-ketoimines [19]. It can be deduced that the electron deficient properties of such dienes cause them to readily take part in hetero-Diels-Alder reactions, which have inverse electron demands. This is corroborated by theoretical calculations which predict an inverse electron demand of the Si-Si double bond it is strongly electron donating rather than electron accepting towards butadienes and other compounds [24,25]. 

This disilene undergoes Diels. Alder reaction with 1, 3 diene addition to acetylene and a single example of reaction with a carbonyl system. 

Thiadisilacyclopropanes result from the reaction of disilenes with sulphur and episul-phides. From the mechanistic standpoint, the reaction of RR,Si=SiRR/ is of great interest and the separation of the (E)- and (Z)-isomers (trans and cis) [R = 2,4,6-( -Pr)3C6H2, R = t-Bu] has led to the separate addition of sulphur to each. This occurs within a minute to give trans and ds-isomers, respectively, the latter with slightly different Si—S bond lengths60. Propylene sulphide reacted similarly, and the reaction of (i-Bu)2Si=Si(Bu-t)2 with thiophene leads to sulphur abstraction with the formation of the thiadisilacyclo-propane, with Si—S bonds of 217.1 pm, along with the l,2-disilacyclohexa-3,5-diene and 2,6-disilabicyclo[3.1.0]hex-3-ene (equation 29)61. 

Further cycloaddition reactions of silylenes generated by the photolysis of cyclotrisilanes have been published since Weidenbruch and coworkers summarized these reactions in an excellent review. Different siliranes were prepared by [2+1]-cycloaddition of di-t-butylsilylene to various alkenes and dienes (Scheme 6)46. Quite interesting results are obtained from the photolysis of hexa-i-butylcyclotrisilane in the presence of unsaturated five-membered ring compounds47 (Scheme 7). With cyclopentadiene and furane, [4 + 2]-cycloaddition of the photolytically generated disilene occurs only as a side reaction. Furthermore, [2 + 1]-cycloaddition of the intermediately formed silylene is highly favored and siliranes are primarily obtained. A totally different course is observed for the reaction in the presence of thiophene. The disilene abstracts the sulfur atom with the formation of the 1,2-disilathiirane as the major product with an extremely short Si—Si distance of 230.49 pm. 

Silylene and disilene adducts are obtained with dienes (equation 18). With thiophene, desulfurization takes place and the products are more complicated, as shown in equation 1949. 

In 1997, Weidenbruch et al. successfully synthesized the first silicon analog of butadiene, hexaaryltetrasila-1,3-diene 64.17 Treatment of disilene 2 with lithium in DME at rt followed by the addition of 0.5 equivalent of mesityl bromide at —30 °C gives 64 as reddish brown crystals [Eq. (29)]. In a proposed mechanism, a half of the lithiodisilene 68, which is formed by the reductive Si-Ar bond cleavage [Eq. (8)], is... 

The first structurally confirmed [2 + 4] adduct of a disilene and a 1,3-diene was compound 89, obtained from cyclopentadiene and 4174. The formation of the tricyclic compound 95 from furan and the cyclotrisilane 40 is probably initiated by a [2 + 4] cycloaddition of 41 to the five-membered ring to afford 94, which then undergoes a [2 + 1] addition at the newly formed double bond with the silylene 42 formed concomitantly in the photolysis of 40 (equation 16)74. 

The first and as yet only molecule with conjugated Si=Si double bonds is the tetrasilabuta-1,3-diene (tetrasila-1,3-diene) 139, which was prepared as follows. The disilene 13 is treated with excess lithium to give the putative disilenyllithium species 137. In the second step of the sequence mesityl bromide was added in the expectation that the bulk of the aryl group and the poor solubility of mesityllithium would favor halogenation over the competing transarylation. Indeed, the bromodisilene 138 does appear to be formed smoothly but, like 137, it has not yet been positively identified. Intermolecular cleavage of lithium bromide from the two intermediates then furnishes the tetrasilabuta-1,3-diene 139 in 60% yield (equation 32)130. 

In view of the preference of the tetrasilabuta-1,3-diene 139 for the s-cis form, it seemed worthwhile to examine its behavior in [4 + 2] cycloadditions of the Diels-Alder type. Since 139, like many disilenes, should behave as an electron-rich diene, we attempted to react it with various electron-poor and also with some electron-rich olefins. No reaction was detected in any case. Only in the presence of water did 139 react with quinones to furnish the unsymmetrically substituted disilenes 36 and 37 (see Section III.A). The effective shielding of the double bonds by the bulky aryl groups and, above all, the 1, 4-separation of the terminal silicon atoms of about 5.40 A appear to be responsible for these failures. Thus, it was surprising that treatment of 139 with the heavier chalcogens afforded five-membered ring compounds in a formal [4 + 1] cycloaddition (see below). 

Absolute kinetic data have been reported for four of the characteristic bimolecular reactions of disilenes 1,2-addition of alcohols and phenols (equation 72), [2 + 2]-cycloaddition of ketones (equation 73), [2 +4]-cycloaddition of aliphatic dienes (equation 74) and oxidation with molecular oxygen (equation 75). As with silenes, the addition of alcohols has been studied in greatest detail. 

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. 

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. 

Tetrazines with electron-withdrawing substituents in the 3 and 6 positions constitute extremely electron-poor dienes [19]. In order to test their reactivity towards the disilene 3 we have selected the compound 18 because its CF3 groups should not be so susceptible to the otherwise readily occurring halogen abstraction by 1 and its photolysis products. However, irrarliation of 18 in the presence of an excess of 1 did not lead to the expected product 17 instead the compound 19 was isolated and its structure confirmed by X-ray crystallography. The structure of 19 clearly reveals that not only has one... 

Differing disilene reaction products were obtained from the reactions with o-methylstyrene and allylbenzene The reaction of o-methylstyrene gave rise to the [2+2] cycloaddition product [3], On the other hand, the reaction of allylbenzene furnished the ene reaction product [8], Further information on the cycloaddition behavior of the disilene 3 has been obtained from photolysis of 1 in the presence of the carbocyclic dienes 18-20. It was initially assumed that the fixation of the syn-geometry of the conjugated dienes would favor the Diels-Alder reaction However, cyclohexadiene 19 and norbornadiene 20 reacted to form the [2+2]-adducts 29 and 30 and no indications for the formation of Diels-Alder adducts were seen. 

Bis(diisopropylamino)silylene has been generated by photolysis of the precursor (76), and trapped chemically by reaction with triethylvinylsilane and 2,3-dimethylbuta-l,3-diene.Contrary to theoretical prediction, its dimer seems to have a disilene structure and not a bridged structure. 

Silanones, the silicon analogs of ketones, are produced via rDA reactions. One attempt to prepare a suitable DA precursor for retrograde decomposition to a silanone met with unexpected results. The desired DA reaction between 2,2-dimethyl-l-oxa-2-silacyclohexa-3,S-diene (251) and perfluoro-2-but-yne was complete in one day at room temperature. The observed product was o-bis(trifluoromethyl)ben-zene, as the initial adduct apparently underwent retrodiene decomposition to yield the intermediate dimethylsilanone (252) (equation 111). The occurrence of this retro ene process at room temperature was not consistent with the analogous extrusions of silenes and disilenes that require elevated temperatures. However, the reaction sequence was substantiated by comparison with its carbon analog in which tetramethylpyran and dimethyl acetylenedicarboxylate react at room temperature to afford only acetone and the corresponding phthalate. Stable adducts that extrude silanones are also known. Reactions of 2-sili yrans and nudeic anhydride provide stable adducts, such as (253), that decompose upon thermolysis... 

A new and potentially valuable photochemical route to tetra-methyldisilene (175) has been reported and involves irradiation of 7,7,8,8-tetramethyl-7,8-disilabicyclo[2.2.0]octa-2,5-diene(176)in an argon matrix at 10 the disilene readily undergoes [ 4 + 2] cycloaddition to benzene to regenerate the precursor. The silane-selenones (177), reactive intermediates with a silicon-selenium double bond, can be photochemically generated and trapped with hexamethylcyclotrisiloxane as shown in Scheme 9. Irradiation of hexamesitylcyclotrisilane (178) in the presence of azobenzene... 

Silylene and Disilene Reactions with some 1,3-Dienes and Diynes... 

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