Dichlorocarbene reaction with dienes

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]. 

The M kosza reagent also is superior for reaction of dichlorocarbene with dienes bis-adducts can be obtained in reasonable yicld. ... 

Difiuorocarbene underwent addition to quadricyclane to give enr/u-6-(2,2-difluorovinyl)bi-cyclo[3.1.0]hex-2-ene together with three other products in 3% yield.The proposed formation of an insertion product into one of the cyclopropane C — C bonds was not verified in this reaction, but was observed in the reaction with other carbenes such as dichlorocarbene and bis(methoxycarbonyl)carbene. The major product with these carbenes possessed a bi-cyclo[3.2.1]octa-2,6-diene structure as the result of the insertion of the carbene into the cyclopropane bond next to the methylene bridge. ... 

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... 

Vinylbuta-1,3-diene produces the 1,2- and 3,4-mono-insertion adducts with dichlorocarbene in a 4 1 ratio [37]. A similar preference in reactivity is observed with 3-methylene-cyclohexenes [90]. 1,2-Dienes react with two equivalents of dichlorocarbene to form spiropentanes [21, 90] (Scheme 7.6). Spiropentanes (50-95%) are also obtained from methylenecyclopropanes [36,106] and by the reaction of electron-deficient alkenes with an excess of chloroform [31] (see Scheme 7.12)... 

A number of cheletropic reactions also appear to be anomalous, including the best known of all cheletropic reactions, the stereospecific insertion of a carbene into a double bond, as in the reaction of dichlorocarbene 2.173 with alkenes. Here we have a reaction involving only four electrons, which is known to be suprafacial on the alkene, preserving the geometry of the substituents in the starting alkenes in the cyclopropanes 2.174 and 2.175. Furthermore, the [2+2] reaction takes place even with a diene, which could. undergo an allowed [4+2] reaction, but chooses not to. 

The production of buta-1,3-dienes (37) by reaction of 1,2-diarylcyclopropenes with dihalocarbenes is thought to involve electrophilic attack of the carbene to give a dipolar intermediate (38).51 The addition of carbene to CO and H2C=0 has been studied by MNDO calculations.52 Photolysis of diethyl diazomalonate in a CO matrix at low temperature gave rise to ketenes by immediate trapping of the postulated carbene (39).53 The major products of reaction between dichlorocarbene and cyclone were CO and the gem-dichloro species (40).54 The predominance of this pathway over formation of the dichlorooxirane or the cyclopropane is attributed to the aromatic nature of the carbonyl ylide and its twist geometry. 

Steric eflfects must be considered if the reaction of dichlorocarbene with bicyclic conjugated dienes is to be used on preparative scale. Thus, the less sterically congested dienes may, primarily, form diadducts in the meso form, while the more hindered dienes, form mainly the monoadduct, e.g. the formation of 21 and 22, 23. ... 

Thus, investigation of the reaction of dichlorocarbene with 2-phenylsulfonyl-2-aza-bicyclo[3.2.1]octa-3,6-diene showed that this carbene primarily undergoes addition to the en-amide double bond, and that the stereochemistry of the product, e.g. 7-11, depends on the substituent at... 

The double elimination of 1,1-dichlorocyclopropanes with potassium tert-butoxide to give cycloproparenes is an important variant of the reaction discussed in Section 5.2.2.1.2.3. When a six-membered ring fused to the cyclopropane contains a double bond, the elimination is accompanied by isomerization of the new double bonds to give an aromatic ring (Table 7). The reaction is illustrated by the synthesis of bicyclo[4.1.0]hepta-l,3,5-triene (l//-cyclo-propabenzene, 3) in two steps from cyclohexa-1,4-diene (1), the first step being dichlorocarbene addition to the diene to give 7,7-dichlorobicyclo[4.1.0]hept-3-ene... 

Another anomalous cycloaddition is the cheletropic insertion of a carbene into an alkene. Six-electron cheletropic reactions are straightforwardly allowed pericyclic reactions, which we can now classify with the drawing 6.158 for the suprafacial addition of sulfur dioxide to a diene.733 Similarly, we can draw 6.159 for the 8-electron antarafacial addition of sulfur dioxide to a triene.734 The problem comes with the insertion of a carbene into a double bond, which is well known to be stereospecifically suprafacial on the alkene with singlet electrophilic carbenes (see p. 201) like dichlorocarbene.735 This is clearly a forbidden pericyclic reaction if it takes place in the sense 6.160. 

Although coordination of CCl2 with a dioxolane function was suggested as responsible for the selective cyclopropanation of a diene (64), negative evidence (65) has been accumulated to refute the hard-soft O C interaction during the reaction. Definitive influence by a dithioacetal group on dichlorocarbene addition to the n bond is indicated (66). 

The Reaction of Dichlorocarbene With Olefins Table 2.5. Addition of dichlorocarbene to 1,3-dienes... 

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