Dimethylvinylidene

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]. 

Experimental Procedure 3.1.2. Preparation of an Iron Vinylidene Complex by Electrophilic Abstraction Carbonyl(Ti -cyclopentadienyl)(dimethylvinylidene)-(triphenylphosphine)iron tetrafluoroborate... 

Methylation of Fe(C=CH)(dppeX C5H5) (3) affords an equimolecular mixture of the vinylidene (4) and dimethylvinylidene (5) complexes, together with spectroscopic amounts of the expected methylvinylidene derivative (6), implying that the ethynyl complex is more basic than the propynyl derivative (25) ... 

If methyllithium is used, only polymeric products result in the final step neutralization with MeOSOzF affords the dimethylvinylidene complex (Section III,A,2). 

Addition of dimethylvinylidene or a related carbenoid, generated from the reaction of propanone with a diazophosphonate and base, to alkylcyclopropenes leads largely to trienes, eg. (283) from 3,3-dimethylcyclopropene. Apparently the same products are obtained when the carbenoid is generated from 1,1 -dibromo-2-methylpropene and base, but it is not clear whether the reactions involve formation and rearrangement of a bicyclobutane or rather collapse of a polar intermediate such as (284)2U). 

The vast majority of work exploring the reactivity of ruthenium viny-lidene complexes has focused on the attack of alcohols at the electrophilic a carbon of monosubstituted vinylidenes, resulting in the formation of ruthenium alkoxycarbene complexes. Bruce and co-workers have determined, for example, that the phenylvinylidene complex 80 is slowly transformed in refluxing MeOH to the methoxycarbene complex 82 in good yield (73,83). The mechanism for this reaction must involve initial attack of the alcohol at the electrophilic Ca to form a transient vinyl intermediate 81 which is rapidly protonated at the nucleophilic Cp, generating the product carbene 82 [Eq. (79)]. In contrast to monosubstituted vinylidene complexes, disubstituted vinylidene complexes are generally unreactive to nucleophiles even the relatively small dimethylvinylidene complex 83 shows no reaction with MeOH after 70 hours at reflux [Eq. (80)]. 

Among the series of the parent systems 1-4, [5]radialene (3) is still unknown. The simplest derivative described so far is decamethyl[5]radialene (135) which has been obtained from l,l-dibromo-2-methyIpropene (22) by low temperature metalation with n-butyllithium followed by a metal exchange reaction with nickel or (better) copper salts and the thermal decomposition of the carbenoid thus formed (equation 10). The yield of 135 varies it is only 14% with CuBr SMe2, but it more than doubles (32%) when Cul PBu3 is employed . The formation of 135 is accompanied by di-, tri- and tetramer-ization of the dimethylvinylidene unit derived from 22 leading to tetramethylbutatriene and the respective permethylated [3]- and [4]radialenes. It is unlikely, though, that this... 

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]. 

Pentacarbonyl(dimethylvinylidene) chromium, generated in situ, also reacts with the butadiynyl complexes 34 at —60 °C by regiospecific cycloaddition of the bond of the... 

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 ... 

The parent [5]radialene is still unknown (see below, however note added in proof). As the simplest substituted derivative, decamethyl[5]radialene (84) has been obtained along with [4]radialene 71 by a formal cyclooligomerization of dimethylvinylidene units via copper [76] or nickel [10, 37] carbenoids (see Scheme 4.17, Section 4.2.2). It is unlikely, however, that other [5]radialenes will become accessible by the same method, because increasing steric demand of the substituents favors the formation of the [4]- and [3]radialene as well as the [3]cumulene structures (see Sections 4.2.1 and 4.2.2). 

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