Dienes dichlorocarbene addition

Although the gem-dihalocyclopropanes are fairly stable compounds, they can participate — as has been shown in the above sections — in quite a number of chemical transformations. Several reactions between dihalocarbenes and alkenes have been described in which no dihalocyclopropane formation could be observed that these intermediates might have been produced was only inferred from the type of products finally isolated. A typical process of this type is the e/ufo-addition of dihalocarbenes to norbomene and norbomadiene as discussed above. Comparable rearrangements have been observed, when dichlorocarbene additions either lead to aromatic products or when they cycloadd to rather inert aromatic systems. In the latter case a ring-enlargement takes place. A reaction related to the concerted opening of two cyclopropane rings in a bicyclopropyl system as discussed above takes place when dichlorocarbene is added to spiro[2.4]hepta-4,6-diene [227]. 

The regioselectivity of dichlorocarbene addition to 1-alkyl-1,3-dienes is sometimes only slightly marked. Furthermore, the addition of dichlorocarbene to the double bond more distant from the aryl group in 1-aryl-1,3-dienes and particularly 1-arylpolyenes, assures energetically suitable Tiaryr vinyi Conjugation in the products. The examples of 3 and 4," 5 and 6, and 8 support these conclusions. 

Alkoxy-1,3-dienes and 2-alkoxy-l,3-dienes undergo addition of dichlorocarbene to the more remote double bond or to the oxygen-substituted double bond, respectively. For example, 1 -alkoxy-1,3-diene 5 gives cyclopropane 6, and 2-alkoxy-l, 3-diene 7 gives cyclopropane 8. ... 

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... 

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). 

Dramatically differing effects of phase-transfer catalysts on the cyclopropanation of cw,trans,trans-cyclododecatriene (61) and a series of dienes have been reported. Addition of dichlorocarbene to (61) results in tris-cyclopropanation when cetyltri-methylammonium bromide (i) is employed, whereas with benzyl-P-hydroxyethyl-dimethylammonium ion (ii) as catalyst only monocyclopropanation (of the more strained bond) is observed (Scheme 7). From the extensive study it may be concluded that, for dichlorocarbene addition, the P-hydroxyethyl catalyst restricts potential polycyclopropanation to monocyclopropanation at the most highly substituted (or strained) double bond. With dibromocarbene a different situation results. Catalyst (i) does not effect the addition of dibromocarbene to styrene, cyclohexene, or allyl bromide while catalyst (ii), with the P-hydroxyethyl function, effects dibromocyclo-propanation, in yields of up to 80 %. 

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 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 mechanism of the cycloaddition of singlet dibromocarbene to formaldehyde was studied using DFT at the B3LYP/6-31G level of theory 47 The energy barrier is estimated as 13.7 kJ mol-1. Reaction paths for the addition of dichlorocarbene to 1,2-disubstituted cyclopropenes were studied using the same level of theory as above.48 The addition gives 1,3-dienes or bicyclobutanes and was predicted to be concerted following an asymmetric approach. 

The addition of dichlorocarbene to (+)-trans-isolimonene (mentha-2,8-diene) gives predominantly the diastereoisomers from reaction at the least substituted... 

Dehydrogenatioa, Vogel ei al. present procedures for the synthesis of tricyclo-[4.4.1.0 ]undeca-3,8-diene (C) by reduction of naphthalene to isotetralin (A), addition of dichlorocarbene (B), and dechlorination (C). Dehydrogenation with DDQ in... 

In the case of the dicyclopropanation of dienes with relatively congested double bonds, steric effects must be considered the second molecule of dichlorocarbene undergoes addition from the less hindered face to give the less strained product, e.g. formation of 15 and 16. ... 

Bicyclic dienes of general structure 22, usually undergo addition of dichlorocarbene to the tetrasubstituted double bond to give the product 23. 

Dichlorocarbene, as a rule, undergoes 1,2-addition to the great majority of conjugated di- and polyenes. Investigation of the competitive cycloaddition of dichlorocarbene to alkenes showed that conjugated dienes are more reactive than alkenes due to stabilization of the transition state by the adjoining double bond. The same effect created by a suitable substituent at C2 in a 1,3-diene, directs the addition of dichlorocarbene to the 1,2-double bond, e.g. formation of 1 - and 2. ... 

The monocyclopropanation of conjugated dienes is relatively easily realized, in comparison to unconjugated dienes (Section 1.2.1.4.2.1.3.), since the remaining double bond is deactivated after the addition of the first molecule of dichlorocarbene. It is also possible to prepare diadducts by using excess of carbene source to diene and by the correct selection of method for the generation of the carbene. The chloroform/base/phase-transfer catalyst method is suitable for the synthesis of either mono- or diadducts, while the chloroform/potassium tert-butoxide method allows the monoadducts to be obtained, often in high yields. The examples of the reactions of 16,and 1849,84,85 illustrate the problems. 

Small- and medium-ring conjugated dienes undergo smooth addition of dichlorocarbene to give the diadducts, often in high yield, particularly if the chloroform/base/phase-transfer catalyst method is applied (for the reaction of cyclopentadiene and fulvenes, see Houben-Weyl, Vol. El9b, p 1545 for the first example of the isolation of the stable monoadduct of dichlorocarbene to cyclopentadiene derivative, see ref 4). The reaction conditions, the type of catalyst and the solvent, as well as their relative amounts, must be considered if the monoadducts are the synthetic target, e.g. formation of monoadducts 24 and 26 and diadducts 25 and 27. ... 

Apart from 1,2-addition, 1,4-addition of dichlorocarbene was observed in the case of dienes with fixed cw-l,3-diene structure only (see Houben-Weyl, Vol.E19b, p 1546), e.g. formation of 29. ... 

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... 

One of the earliest useful applications of eliminations in the cyclopropane series is the two-step synthesis of l-alkenyl-2-alkylidenecyclopropanes from simple alkenes. The first step consists of addition of dichlorocarbene to the alkene to give a 1,1-dichlorocyclopropane. This is then followed by double dehydrochlorination with potassium /ert-butoxide in dimethyl sulfoxide at room temperature (Table 6). The product diene contains one double bond exo to the cyclopropane ring and the other in the vinyl position, presumably through the intermediacy of an undetected, highly strained methylenecyclopropene. The method is illustrated by the synthesis of l-methylene-2-vinylcyclopropane (3) starting from pent-2-ene (1). ... 

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. ... 

More generally, cycloproparenes are available via dihalocarbene addition to cyclohexa-1,4-diene or synthetic equivalents, followed by base-induced twofold dehydrochlorination and aromatization. When the dichlorocarbene adduct of cyclohexa-1,4-diene, 7,7-dichlorobicyclo-[4.1.0]hept-.3-ene (3), was reacted with potassium /ert-butoxide in dimethyl sulfoxide a twofold elimination of hydrogen chloride occurred which was accompanied by migration of the resulting double bonds into the cyclohexene ring to give benzocyclopropene. This sequence, sometimes named Billups synthesis, provides the most convenient access to benzocyclopropene. ... 

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. 

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