Dimethylvinylidene carbene

Generation and reaction of dimethylvinylidene carbene and related compounds... 

As noted with the reactions between terpenes and dihalocarbenes, mono-insertion adducts at the more electron-rich sites can be isolated from the reaction of non-conju-gated acyclic and cyclic dienes although, depending on the reaction conditions, the bis-adducts may also be formed. Norbomadiene produces both 1,2-endo and 1,2-exo mono-insertion adducts with dichlorocarbene, as well as a 1,4-addition product (Scheme 7.4) [67]. The mono adduct produced from the reaction with dimethylvinylidene carbene rearranges thermally to yield the ring-expanded product (Scheme 7.4) [157] a similar ring-expanded product is produced with cyclo-hexylidene carbene [149]. 

Cycloocta-2,5-diene yields the mono adduct with dimethylvinylidene carbene ( 19%) [156]. With dichloro- and dibromocarbene, the syn- and cmh-bis-adducts are obtained in a ratio which favours the syn-isomer [55, 104] whereas, with bromochlorocarbene, the mono-adduct is reported to be the major product (55%) with only 9% of the bis-adduct [142]. In contrast, cyclo-octatetraene is converted into the mono-, syn-1,2 5,6-bis-, tris-, and tetra-adduct with dichlorocarbene depending on the reaction conditions [4, 17, 25, 55] and the 1,2 5,6-bis-adduct with... 

Alkynes tend to be much less reactive than alkenes. For example, 1,2-diphenylethyne produces only 23% of the dichlorocyclopropene from its reaction with dichlorocarbene, compared with 96% of the dichlorocyclopropane obtained from rrans-stilbene under analogous conditions [4]. Conjugated eneynes react preferentially at the C=C bond with dihalocarbenes [18-20, 22, 38] and with dimethylvinylidene carbene [158],... 

Both acridone and dibenzo[6,/]azepine produce unexpected products (Scheme 7.39) when reacted with dimethylvinylidene carbene (7.1.18.A). Acridone reacts initially at the nitrogen atom to produce the 10-(3,3-dimethylallenyl) derivative (13%) and a pyrroloacridone (10%) which, if the structure is correct, could be derived from the allene by sigmatropic shifts [16]. The dibenzoazepine reacts as expected to produce a cyclopropyl derivative but, under the reaction conditions, the adduct rearranges spontaneously to yield a 1,6-methanodibenzo[b,/]cyclo-prop [J]azepine, the structure of which was confirmed by X-ray crystallography [17]. 

Except for 3-bromoindole, it is necessary to proceed via the indoline, because treatment of indole with 1,1-dimethyIpropargyl chloride (31) in the presence of base leads to product mixtures resulting from reaction of in situ formed dimethyl-vinylidene carbene. Sheu and co-workers isolated the three products 32, 33 and 34 on reaction of indole-3-carbaldehyde (30) (Scheme 7) [53]. In the main reaction, indole C3 attacks at dimethylvinylidene carbene forming a vinyl anion, which is protonated to give the 3-allenylindolenine. Hydroxide then attacks the carbal-dehyde moiety with loss of formic acid and formation of a 3-allenylindole. Addition of a second equivalent of indole-3-carbaldehyde (30) affords 32 and 33. 

After treatment of Boc-protected tryptophan methyl ester (20) with 1,1-dimethylpropargyl chloride (31) / NaH in DMF, we isolated the five products 35, 36, 37, 38 and 39 (Scheme 8) [54]. Both the indole nitrogen and C3 had reacted as nucleophiles towards Cl and C3 of the intermediate dimethylvinylidene carbene, followed by cychsation to the pyrrolo[2,3-b]indoles. Earlier, Hino and co-workers had obtained a similar result when starting from achiral A b tncthoxycarbonyl-tryptamin [55]. With 3-unsubstituted indoles, Wenkert and co-workers observed formation of quinolines as side products, presumably via cyclopropanation of the indole 2,3-double bond by dimethylvinylidenecarbene [56]. 

Dimethylvinylidene carbene generated from 3-chloro-3-methylbut-l-yne using aqueous KOH and a phase-transfer catalyst, has been added to cis,cis-cyclo-octa-l,5-diene to give the mono-adduct. ... 

A number of unsaturated carbenes have been generated under phase transfer conditions. The most extensive work has been reported for dime thy Ivinylidene carbene. Dimethylvinylidene carbene has been generated by base catalyzed 7-elimination of HCl from 3-chloro-3-methyl-l-butyne and by a-elimination of HBr from l-bromo-3-methyl-1,2-butadiene (see Eq. 4.13). Both quaternary alkylammonium salts and various crown ethers have proved to be effective catalysts for this reaction. Yields of dimethylvinylidenecyclopropane products obtained under phase transfer catalytic... 

QH NaOH or crown Table 4.5. Addition of dimethylvinylidene carbene to olefins ... 

The addition of dimethylvinylidene carbene to three types of non-olefinic substrates has also been examined. Dimethylvinylidene carbene reacts with azobenzene to yield l-phenyl-2-isobutenylbenzimidazole (10%) (Eq. 4.14). This product may arise from a rearrangement of the initial carbene adduct of the azo linkage [29]. 

Reaction of dimethylvinylidene carbene with 3-methyl-2-butenyl-methylsulfide leads to 4-methylthio-3,3,6-trimethylhept-5-ene-l-yne (36%). The reaction sequence, illustrated in equation 4.15, has been proposed to account for the product obtained. Apparently, the unsaturated carbene favors reaction with sulfur rather than the carbon-carbon double bond [28]. 

An attempt to insert dimethylvinylidene carbene into the tertiary benzylic C—H bond of isopropylbenzene was unsuccessful [28]. 

The reaction of tetramethylallene with isopropylidene carbene affords the [2+1] cycloadduct in 12% yield. Likewise, reaction with dimethylvinylidene carbene 89 affords the [2+1] cycloadduct 90 in 35 % yield ... 

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