Silylated cyclopropenes

Addition of the phenylthiolate ion to the silylated cyclopropene 9 resulted in overall reduction and the formation of the fra . -2-phenylsulfanyl-l-sulfonyl cyclopropane vide infra) in very high yield, apparently by initial addition followed by attack on silicon a similar reaction of the methoxide ion apparently occurs by initial attack on silicon, followed by addition to the derived sulfone. ... 

Addition of phenylthiolate to a silylated cyclopropene 15 led to an overall reduction, apparently by initial addition followed by further attack on silicon. ... 

Upon reaction with di- and trisilylated silyl anions, the tri-fert-butylcyclopropenylium ion 14 was also converted into the 3-silylated cyclopropene derivatives 16, which exhibited remarkable thermal stability owing to the large steric hindrance around the three-membered ring. The single bond between the cyclopropenyl carbon and the substituted silicon was shown by X-ray analysis to be elongated by 0.05... 

A platinum-catalysed rearrangement of silylated cyclopropenes to the corresponding allenes has been reported (Scheme 156). ... 

Calculations of the regio- and chemo-selectivities of C-H amination of silyl cyclopropenes by azodicarboxylates show that the initial Alder-ene reaction between the two reactants follows a stepwise mechanism and the subsequent allylic transposition proceeds via a concerted [1,3]-migration of hydrazodicarboxylate (Scheme 82). ... 

More recently, 3,3-dimethyl-1 -trimethylsilylcyclopropene was metallated using lithium diisopropylamide (LDA) in THF261. Methylation, silylation and thiomethylation of the lithiocyclopropene intermediate afforded the corresponding 2-substituted cyclopropenes (equation 191). 

Silylation of lithiated 2-methyl-l-methylenecyclopropane is reported to give selectively tra s-2-methyl-3-trimethylsilyl-1 -methylenecyclopropane361. However, when 2-trimethyl-silyl-l-methylenecyclopropane is again lithiated in situ, and subsequently reacted with Me3SiCl, silylation occurs mainly at the already silylated carbon, giving 2,2-bis-(trimethylsilyl)-l -methylenecyclopropane, along with minor amounts of the exo-silylated isomeric cyclopropene and syn and anti trisilyl derivatives (equation 296)368. 

Density functional theory computational studies have been used to determine die importance of secondary orbital interactions for the stability of transition-state structures for die 4 + 2-cycloaddition of furan with cyclopropene.175 Kinetic studies of die 2 + 4-cycloaddition of 2-cyclopropylidene acetates with furan and dimethylful-vene suggest a mechanism involving diradicals or zwitterions as intermediates.176 Cyclopropene, produced by die reaction of allyl chloride with sodium bis(bimediyl-silyl)amide, reacts with 1,3-diphenylisobenzofuran to produce both endo- and exo-Diels-Alder cycloadducts isolated for the first tune.177... 

In principle, the vinylcarbene-to-cyclopropene isomerization is reversible. While this has not been reported for 167 (where ring-opening could produce not only alkenyl(sulfonyl)carbene 166, but also the isomeric l-alkenyl(silyl)carbene), it was found that 1-trimethylsilylcyclopropenone acetal 168 (equation 49) by thermal ring-opening yields the (trimethylsilyl)vinylcarbene 169 besides traces of the isomeric vinylcarbene 170. Both carbenes are obviously nucleophilic since they are able to cyclopropanate the... 

In a similar manner, 3,3-disubstituted l,2-bis(trimethylsilyl)cyclopropenes rearrange to l,l-bis(trimethylsilyl)allenes, most likely by 1,2-silyl shift of primarily formed (1-silylvinyl)silylcarbenes. According to ab initio calculations, this reaction pathway is energetically more favorable than those including a 2,2-disilylcyclopropylidene or a 2,3-disilylpropylidene90b. 

Silyl-substituted cyclopropenes have been observed to rearrange photochemically giving high yields of allenes. A typical example is shown in equation 2 where the silylcyclo-propene 3 apparently ring-opened to the carbene 4 which, following 1,2-silyl migration, led to the allene 513. 

The addition of alkoxycarbonylcarbene derived by catalysed decomposition of methyl diazoacetate to several simple, and in particular terminal, alkynes leads to low yields S7), but the reaction with 1 -trimethylsilylalkynes proceeds reasonably efficiently subsequent removal of the silyl-group either by base or fluoride ion provides a route to l-alkyl-3-cyclopropenecarboxylic acids. In the same way 1,2-bis-trimethylsilyl-ethyne can be converted to cyclopropene-3-carboxylic acid itself58 . The use of rhodium carboxylates instead of copper catalysts also generally leads to reasonable yields of cyclopropenes, even from terminal alkynes 59). 

Although a l-(alkylthio)cyclopropene could not be isolated in the above reactions, the corresponding silylated derivative (162) does ring open either on heating or on photolysis, leading to an allene the reaction may involve a 1,2-silyl-shift in an intermediate carbene (163), though in this case the latter could not be trapped by added alkene 82). 

In the latter case this has been shown to involve an intramolecular silyl-shift. In the case of 3-azido-cyclopropenes, a similar migration apparently occurs through ionic intermediates 278). 

Cyclopropenes undergo an oxidative cleavage to yidd substituted enones, as shown in equation (27). The reaction is believed to proceed through the unstable qwxide. The regioselectivity is genoaOy low if R 9 R, but if one of the substituents is trimethylsilyl a hi y selective conversion to the a-silyl enone takes place. There is one report of a similar oxidative cleavage that takes place with thallium(ni). ... 

In contrast to the carbene and carbenoid chemistry of simple diazoacetic esters, that of a-silyl-a-diazoacetic esters has not yet been developed systematically [1]. Irradiation of ethyl diazo(trimethylsilyl)acetate in an alcohol affords products derived from 0-H insertion of the carbene intermediate, Wolff rearrangement, and carbene- silene rearrangement [2]. In contrast, photolysis of ethyl diazo(pentamethyldisilanyl)acetate in an inert solvent yields exclusively a ketene derived from a carbene->silene->ketene rearrangement [3], Photochemically generated ethoxycarbonyltrimethyl-silylcarbene cyclopropanates alkenes and undergoes insertion into aliphatic C-H bonds [4]. Copper-catalyzed and photochemically induced cyclopropenation of an alkyne with methyl diazo(trimethylsilyl)acetate has also been reported [5]. 

Cyclopropen 2,3-Dimethyl-l-(dimethyl-phenyl-silyl)- E17d, 2717 (— Br2 - en)... 

The reaction of a 1,1-dibromocyclopropane with an alkyllithium generally leads to 1,1-elimination producing a cyclopropylidene, which normally rearranges to an allene, but may be trapped by other carbenic reactions. When there is a silyl substituent on C2, the carbene is apparently trapped by a 1,2-silicon shift giving a silyl-substituted cyclopropene. ... 

Chloro-3.3-dimelhyl-l-trimethyl-silyl- 27S9 C H.sCISiZn Cyclopropene... 

It is possible for organosilyl compounds with a leaving group at the fi position to undergo E2-type reactions induced by cesium fluoride. In deprotonation-ehmina-tion, a strong base such as amide or alkoxide is required the conjugated acid produced by deprotonation (e.g. amine or alcohol) may react with the elimination product. In the case of cesium fluoride-induced desilylation-elimination, the formed conjugated acid is silyl fluoride which is unreactive. The production of unstable products such as cyclopropene (Scheme 2.13), cyclohexene-3-yne (Scheme 2.14), or orthoquinodimethane has been reported [24—29]. 

Silyl and geimyl functionalised vinyl cyclopropenes ring-open thermally to give allene and alkyne derivatives with the former dominating, a-trimethylsilyl a-allenyl ketones couple to give enediynes related to the neocarzinostatin chromophore and also result fnnn Y-(t-BuMe2Si)allenylborane and acetylenic aldehydes. Allenylsilanes couple with AdQ and... 

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