Dienes carbene insertion reactions

Somewhat similar considerations apply to the insertion of carbenes into a bonds, except that in this case the reaction can only involve four electrons, and there is no 6-electron alternative.739 We shall return to the carbene insertion reactions later when we discuss periselectivity why carbenes choose to react with a double bond by the nonlinear approach even with dienes, which would make a 6-electron linear approach analogous to the sulfur dioxide reaction 6.158 allowed. 

When 1,4-dienes are utilized, the activity of allylic C—H bonds to undergo carbene insertion reactions is further enhanced. For instance, a range of chiral Rh complexes is able to convert 1,4-cyclohexadiene into the corresponding C—H bond insertion product in a high yield and ee in the presence of methyl phenyldiazoacetate la (Scheme 1.6, eqn (1)). The synthetic utility of this methodology was further demonstrated by total synthesis of natural products (+)-indatraline (Scheme 1.6, eqn (2)) and (+)-cetiedil (Scheme 1.6, eqn (3)). ... 

The facile isomerization of simple alkylcarbenes by alkyl or hydride shifts is so rapid that these species cannot be conveniently studied even by laser-flash photolysis.128- 130 a-Fluori-nation of alkylcarbenes dramatically reduces the rate of rearrangements.131-134 e.g. fluoro(methyl)carbeneis sufficiently stabilized that it can be trapped by cyclopenta-1,3-diene.135 This type of insertion reaction is quite typical of a-fluorinated carbenes. oc-Fluorinated earbenes are less prone to CH insertion, generally adding stereoselectively to alkenes.136... 

Key to method (A) exchange reaction with tin heterocycle (B) hydride addition to diyne (C) oxidation of saturated ketone (D) bromination-dehydrobromination by pyrolysis (E) reaction of RLi or ArLi with exocyclic M-Cl of preformed diene (F) ring expansion reaction from cyclopentadiene derivative (G) LiAlHi reduction of exocyclic M-Cl (H) carbene insertion into five-membered cyclo-pentadiene derivative. Doering-Hoffman method (I) 1,6-cycloaddition of GeCU. 

Dienes. The reaction of Fischer carbene complexes with allenes gives 1,3-dienes into which the nonmetal moiety is incorporated. The transformation is a formal ene-type process. 1-Alkoxy-1,3-dienes arise when the carbene complexes and propargylsilanes are heated together. A 1,2-silicon shift accompanies the carbene insertion into the C-H bond. 

Intriguingly, while vinyldiazoacetates are utilized as carbene precursors in the allylic C—H bond insertion reaction catalyzed by Rh complexes, the combined C—H bond functionalization/Cope rearrangement occurs readily. This has emerged as a reliable methodology for the construction of 1,5-diene compounds bearing two vicinal stereogenic centers and will be discussed in detail in Section 1.1.2.6. 

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]. 

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. 

Another anomalous cycloaddition is the insertion of a carbene into an alkene. 6-Electron cheletropic reactions (p. 28) are straightforward allowed pericyclic reactions, which we can now classify with the drawings 3.47 for the suprafacial addition of sulfur dioxide to the diene 2.179 and its reverse. Similarly, we can draw 3.48 for the antarafacial addition of sulfur dioxide to the triene 2.180 and its reverse. The new feature here is that one of the orbitals is a lone pair, which is given the letter co to distinguish it from o- and n-bonds, with suprafacial and antarafacial defined by the drawings 3.45 and 3.46, which apply to all sp3 hybrids and p orbitals, filled or unfilled. 

An anomalous cycloaddition is the insertion of a carbene into an alkene. Some cheletropic reactions are straightforwardly allowed pericyclic reactions, which we can illustrate with the drawing 6.127 for the suprafacial addition of sulfur dioxide to a diene, and with the drawing 6.128 for the 8-electron antarafacial addition of sulfur dioxide to a triene. 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 (Section 4.6.2) page 149]. This is clearly a forbidden pericyclic reaction if it takes place in the sense 6.129. 

The reaction of alkyl-substituted tungsten-carbene complexes of the type (88b) have been reported by Macomber to react with alkynes to give dienes of the type (319). One mechanism that has been proposed to account for this product is a 3-hydride elimination from the metallacyclobutene intermediate (320) and subsequent reductive elimination in the metal hydride species (321). An additional example of this type of reaction has been reported by Rudler, also for an alkyl tungsten carbene complex. Chromium complexes have not been observed to give diene products of this type the reaction of the analogous chromium complex (88a) with diphenylacetylene gives a cyclobutenone as the only reported product (see Scheme 31). Acyclic products are observed for both tungsten and chromium complexes in their reactions with ynamines. These reactions produce amino-stablized carbene complexes that are the result of the formal insertion of the ynamine into the metal-carbene bond. ... 

Electrophilic addition reactions of carbenes to cyclopropane structures are rare and not synthetically important. Reaction of carbenes with bicyclo[2.1.0]pentane generally gave products arising from C —H insertion at one of the methylene groups of the four-membered ring. ° An exception was difiuorocarbene which gave 1,1-difluorohexa-l,5-diene (2) in very low yield by cleavage of two C —C bonds. - ... 

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. ... 

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