1.3- Dienes reaction with dihalocarbenes

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]. 

Conjugated dienes yield mono-adducts with dihalocarbenes at the more electron-rich C=C bond further reaction at the less reactive bond may also occur [e.g. 4,8,19, 23, 31, 37, 49, 62, 69, 94]. Cycloheptatriene yields the syn- and nnti-1,2 5,6-bis-adducts (14.5 and 22.9%) and the syn-1,2 5,6-ann -3,4-tris-adduct with dichloro-carbene [62], The facile reaction of cyclopropylethenes with dihalocarbenes produces dicyclopropyl compounds [53, 117]. Isoprene reacts with chloro(phenylthio)carbene across the more reactive 1,2-bond (51%) [146]. 

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. 

The addition of dihalocarbenes to double bonds occurs stereospecifically to give cyclopropane derivatives [68J. The reaction appears to have electrophilic character even though the electronic state of the carbene is uncertain (cf. p. 341). Again the direction of attack on the double bond is subject to steric control. The diene (2) for example, reacts with di-bromocarbene to give first the 2ct,3[Pg.294]

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