Cyclopropanes electrophilic addition

Calculations [28] on the formation of cyclopropanes from electrophilic Fischer-type carbene complexes and alkenes suggest that this reaction does not generally proceed via metallacyclobutane intermediates. The least-energy pathway for this process starts with electrophilic addition of the carbene carbon atom to the alkene (Figure 1.9). Ring closure occurs by electrophilic attack of the second carbon atom... 

In qualitative terms, the rearrangement reaction is considerably more efficient for the oxime acetate 107b than for the oxime ether 107a. As a result, the photochemical reactivity of the oxime acetates 109 and 110 was probed. Irradiation of 109 for 3 hr, under the same conditions used for 107, affords the cyclopropane 111 (25%) as a 1 2 mixture of Z.E isomers. Likewise, DCA-sensitized irradiation of 110 for 1 hr yields the cyclopropane derivative 112 (16%) and the dihydroisoxazole 113 (18%). It is unclear at this point how 113 arises in the SET-sensitized reaction of 110. However, this cyclization process is similar to that observed in our studies of the DCA-sensitized reaction of the 7,8-unsaturated oximes 114, which affords the 5,6-dihydro-4//-l,2-oxazines 115 [68]. A possible mechanism to justify the formation of 113 could involve intramolecular electrophilic addition to the alkene unit in 116 of the oxygen from the oxime localized radical-cation, followed by transfer of an acyl cation to any of the radical-anions present in the reaction medium. 

In marked contrast to those of cyclopropanes, the addition reactions of cyclobutanes usually require drastic conditions and activating substituents. Notwithstanding these shortcomings, the cleavage of cyclobutanes by addition reactions has been widely employed in the synthesis of organic molecules of moderate complexity. In this section, electrophilic addition, as well as nucleophilic addition, of cyclobutanc derivatives are reviewed with the aim of demonstrating their synthetic profile. 

Diazocarbonyl compounds, especially diazo ketones and diazo esters [19], are the most suitable substrates for metal carbene transformations catalyzed by Cu or Rh compounds. Diazoalkanes are less useful owing to more pronounced carbene dimer formation that competes with, for example, cyclopropanation [7]. This competing reaction occurs by electrophilic addition of the metal-stabilized carbocation to the diazo compound followed by dinitrogen loss and formation of the alkene product that occurs with regeneration of the catalytically active metal complex (Eq. 5.5) [201. 

Among the transition-metal catalysts that have been used, only those of Pd(II) are productive with diazomethane, which may be the result in cyclopropanation reactions [7,9,21] of a mechanism whereby the Pd-coordinated alkene undergoes electrophilic addition to diazomethane rather than by a metal carbene transformation in any case, asymmetric induction does not occur by using Pd(II) complexes of chiral bis-oxazolines [22],... 

Fluorinated carbocations play an important role as intermediates in electrophilic reactions of fluoroolefins and other unsaturated compounds. For example, F-allyl cation 1 was proposed as a reactive intermediate in reactions of HFP with fluoroolefins catalyzed by Lewis acids [7]. The difference in stability of the corresponding allylic cations was suggested as the explanation for regio-specific electrophilic conjugated addition to CF2=CC1CF=CF2 [11]. Allylic polyfluorinated carbocations were proposed as intermediates in the reactions of terminal allenes with HF [53] and BF3 [54], ring-opening reactions of cyclopropanes [55], Carbocations are also an important part of the classic mechanism of electrophilic addition to olefins (see Eq. 2). This section deals with the questions of existence and stability of poly- and perfluorinated carbocations. 

The Simmons-Smith reaction is an efficient and powerful method for synthesizing cyclopropanes from alkenes [43]. Allylic alcohols are reactive and widely used as substrates, whereas a,j8-unsaturated carbonyl compounds are unreactive. In 1988, Ambler and Davies [44] reported the electrophilic addition of methylene to a,/3-unsaturated acyl ligands attached to the chiral-at-metal iron complex. The reaction of the racemic iron complex 60 with diethylzinc and diiodomethane in the presence of ZnCl2 afforded the c/s-cyclopropane derivatives 61a and 61b in 93 % yield in 24 1 ratio (Sch. 24). 

Ionic additions which transform cyclopropenes into cyclopropanes are well known. The electrophilic addition of sulphenyl halides proceeds by a two-step mechanism to give trans-disubstituted products but with little regioselectivity. Cyclic sulphonium ion intermediates are probably involved as illustrated for 1-methylcyclopropene. Electrophilic... 

A-Acyliminium ions are suitable partners for electrophilic addition reactions of allylstannanes. Although the corresponding silanes have been more widely studied in these cases/ allylic stannanes are fully competent reaction participants, as illustrated by formation of the l-azabicyclo[3.1.0]pentane 159 via intra-molecular cyclization of the fi-allylic stannane 157 (Scheme 5.2.33). The reaction produces the vinyl cyclopropane under protic acid conditions with complete stereocontrol. s... 

Homoallylstannanes are also potential starting materials for cyclopropanes. Selective addition of electrophiles to but-3-enyltin compounds leads to a y-methyl ion which then undergoes destannylation to give cyclopropylmethyl derivatives 11 in moderate to high yield. This reaction is mechanistically similar to the methyl ion induced 1,3-eliminations of silicon compounds. 

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. - ... 

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