Carbenes Add to Alkenes

In strong base, propene reacts with chloroform to yield 2,2-dichloro-1 -methylcyclopropane. 

the active reagent is believed to be singlet dichlorocarbene (CCI2). 

examine the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) of dichlorocarbene. Were the reaction a nucleophilic addition , how would you expect CCI2 to approach propene Were the reaction an electrophilic addition , how would you expect CCI2 to approach propene Which inteqDretation is more consistent with the geometry of the transition state  

HOMO and LUMO of dichlorocarbene characterize the molecule s electrophilic and nucleophilic behavior, respectively. 

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The reactivity of the singlet and triplet states should be different, as proposed by SkelL The triplet state is a diradical, and would be expected to exhibit a selectivity in its reactions toward olefins similar to that of other species having unpaired electrons. The singlet state, with its unfilled p-orbital, should be electrophilic and exhibit reactivity toward olefins similar to that of other electrophilic species. In general, carbenes add to alkenes to give cyclopropane derivatives. Dibromocarbene is typical in this respect. This reaction could involve the singlet dibromocarbene in a concerted addition, or the triplet state in a stepwise process ... 

Carbenes add to alkenes to form cyclopropanes. The mechanism of these insertion reactions depends on whether a singlet carbene (this contains a pair of reactive electrons in the same orbital) or a triplet carbene (this contains two unpaired and reactive electrons in different orbitals) adds to the double bond. 

Sometimes the barriers to triplet reactions are not effectively higher than those for singlets. In these cases, triplet carbene reactions can be observed. Like their singlet counterparts, triplet carbenes add to alkenes to give cyclopropanes (Fig. 10.45). 

Carbenes come in two flavors —spin-paired singlets and spin-unpaired triplets. Often the triplet is the more stable form, but the singlet state is usually more reactive. Both singlet and triplet carbenes add to alkenes to give cyclopropanes. In the concerted reaction of the singlets, any stereochemistry present in the alkene is preserved. In the nonconcerted additions of triplets, stereochemistry is lost. 

Decomposition of more complex diaziriries follows first order kinetics also. Chlorophenyl-carbene adds to cyclohexene to give a norcarane derivative. Substituent effects of m-Cl, m-NOa or m-Me groups, as well as solvent effects, are small. Chlorotrichloromethyldiazirine yields tetrachloroethylene chlorocyclooctyldiazirine also leads to an alkene 74CJC246). 

Carbenes and substituted carbenes add to double bonds to give cyclopropane derivatives ([1 -f 2]-cycloaddition). Many derivatives of carbene (e.g., PhCH, ROCH) ° and Me2C=C, and C(CN)2, have been added to double bonds, but the reaction is most often performed with CH2 itself, with halo and dihalocarbenes, " and with carbalkoxycarbenes (generated from diazoacetic esters). Alkylcarbenes (HCR) have been added to alkenes, but more often these rearrange to give alkenes (p. 252). The carbene can be generated in any of the ways normally used (p. 249). However, most reactions in which a cyclopropane is formed by treatment of an alkene with a carbene precursor do not actually involve free carbene... 

The mechanism of this nonspecific reaction must be different. In fact, a concerted reaction is impossible for triplet carbenes because of the spins of the electrons involved. After the carbene adds to the alkene in a radical reaction, the diradical (triplet) intermediate must wait until one of the spins inverts so that the second C-C bond can be formed with paired electrons. This intermediate also lives long enough for C-C bond rotation and loss of stereochemistry. 

IS the most popular, one-step method for m ag fluorinated cyclopropanes and cyclopropenes a-Fluorocarbenes are particularly well behaved, because they all have singlet ground states [/, 2] and therefore usually add stereospecifically to alkenes and do not insert into C-H bonds competitively with addition Moreover, quantitative competition studies of carbene additions to alkenes near room temperature show that a-fluorocarbenes are more selective than other a-halocarbenes, with difluorocarbene being the most selective electrophihc carbene known [3, 4] The relative selectivities, however, can be quite temperature dependent [5, d] The numerous preparations and cycloaddmons of fluorocarbenes have been reviewed thoroughly [7, 8 9,10 ... 

Metal-carbene complex B, also formed in Step [2], adds to alkene RCH=CH2 to form another metallocyclobutane, which decomposes to RCH=CHR, the second metathesis product, in Step [4]. Ru can bond to either the more or less substituted end of the alkene, but product is formed only when Ru bonds to the less substituted C, as shown. 

The mechanism of this type of reaction depends on whether the carbene is a singlet or a triplet, and the outcome of the reaction can provide our first chemical test of the conclusions we came to in the previous section. Singlet carbenes, like this one here (remember that electron-rich substituents stabilize the singlet spin state), can add to alkenes in an entirely concerted manner the curly arrows for the process can be written to show this. 

A carbene, RjC , is a neutral molecule containing a divalent carbon with only six valence electrons. Carbenes are highly reactive toward alkenes, adding to give cyclopropanes, Dichlorocarbene adds to alkenes to give 1,1- dichlorocyclopropanes, Nonhalogenated cyclopropanes are best prepared by treatment of the alkene with CH l2 and zinc-copper alloy—the Simmons-Smith reaction. 

The spin state of the reacting acylcarbene has a decisive influence on the reactivity of this short-lived species (Houben-Weyl, Vol.4/5b, pp 1158-1257 Vol. E19b, pp 1052 and 1231 refs 67,100-103). As far as cyclopropanation is concerned, carbene addition to a (Z)- or ( )-alkene can occur with retention or with loss of the stereochemical relationship present in the alkene (stereospecific and nonstereospecific cyclopropanation, respectively). Singlet carbenes add to C-C double bonds in a concerted manner, and therefore, stereospecifically. In contrast, a triplet carbene undergoes stepwise addition via an intermediate triplet 1,3-diradieal. Sinee spin-in-version must occur before ring closure of the latter species, the extent to which stereospecificity... 

The thermal decomposition of (l,l-difluoroalkyl)trifluorosilanes generates alkylfluoro-carbene which similarly adds to alkenes to give l-alkyl-l-fluorocyclopropanes (see Houben-Weyl, Vol. E19b, pp 1038-1039, 1466-1467). 

Historically one of the first asymmetric methods to be explored, cyclopropanation came of age32 with box and salen ligands on Cu(I). Diazo compounds, particularly diazoesters 138, react with Cu(I) to give carbene complexes 140 that add to alkenes, particularly electron-rich alkenes to give cyclopropanes 141. The reaction is stereospecific with respect to the alkene -1runs alkenes giving trans cyclopropanes - and reasonably stereoselective as far as the third centre is concerned. Any enantioselectivity comes from the chiral ligand L. You have already seen the Ru carbene complexes are intermediates in olefin metathesis (chapter 15). 

In addition reactions, where more than one alkene moiety is present in a molecule, there is the potential for formation of isomers via addition to one or both of the double bonds. In general, carbenes add to the more... 

The rhodium chemistry just presented actually involves C=Rh species rather than a free carbene. In this section, we will look at a general class of compounds called carbenoids. A carbenoid is a reactive intermediate that reacts similarly to a carbene but does not actually involve formation of a carbene. The most commonly used carbenoid is generated by reaction of diiodomethane and a Zn/Cu couple this reaction, when it adds to alkenes, is called the Simmons-Smith reaction.316 A simple example is the conversion of cyclohexene to bicyclo[4.1.0]heptane (norcarane, 336). Initial reaction of diiodomethane with zinc gave an iodozinc compound (399-see sec. 9.8.B for a brief discussion of organozinc compounds), which added to the alkene to give 400. Loss of zinc iodide (Znl2) gave the cyclopropane derivative (343 in this case).3ll A one-step... 

The reactivity of carbene-metal complexes, amongst others the reactivity with respect to alkenes and alkynes, has been reviewed by Dotz Just like free carbenes the coordinated carbenes add to triple bonds to give cyclopropene derivatives. Other reaction products, however, are also possible. For instance, the carbene ligand of chromium complex 23 reacts with diphenylacetylene to a mixture of products, including naphthalene derivative 24 and furan derivative 25 (equation 18). A carbonyl ligand has participated. Molecular orbital calculations by Hofmann and Hammerle " on this system reveal that the reaction would pass through an y-vinylcarbene type of complex (26) instead of through a planar chromacyclobutene 27. The subsequent steps to yield either phenol or furan could involve vinylketene 28, but this still is a matter of debate. Similar, but more selective, furan syntheses have been observed for carbene complexes based on iron and cobalt. ... 

Cyclopropylchlorocarbene (8) should have 0-73 as calculated from Eq. 4. However, as determined experimentally from the measured relative reactivities, w xy 0.41. [34] Close analysis, including ab initio calculations, reveal that in order to add to alkenes, steric demands require the cyclopropyl unit of carbene 8 to twist away from the bisected conformation shown in 8 (where stabilizing electronic interactions of cyclopropyl a bonds and the vacant... 

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