Cyclopropane, alkene-like reactions

The stabilization of chloromethoxycarbene (234) was intensively studied. It is formed from diazirine (233) in a first order reaction with fi/2 = 34h at 20 C. It reacts either as a nucleophile, adding to electron poor alkenes like acrylonitrile with cyclopropanation, or as an electrophile, giving diphenylcyclopropenone with the electron rich diphenylacetylene. In the absence of reaction partners (234) decomposes to carbon monoxide and methyl chloride (78TL1931, 1935). 

Vinylsilanes 132 have been reacted with bromomalononitrile to yield the intermediate 133, which was used for the synthesis of cyclopropane derivatives Addition reactions of dichloromalononitrile with substituted alkenes and alkadienes can also be used for the preparation of intermediates in carbo- and heterocyclic synthesis. 2-Arylmalononitriles 135 have been produced by coupling malononitriles with aryllead(IV) triacetates like 134163. 

We have met the breaking of C—C bonds by hydrogen already in Chapter 11, but the molecules considered there (cyclopropane and cyclobutane) had some degree of alkene-like character and reacted easily (especially the former). In this chapter we shall be involved with linear and branched alkanes having two, three or four carbon atoms. C—C bond fission is the principal process, but with the butanes skeletal isomerisation is also possible, and dehydrogenation sometimes happens at the same time. Reactions of acyclic and cyclic alkanes having five or more carbon atoms feature in the following chapter, where isomerisation and dehydrocyclisation are the important reactions. Some limited overlap between this chapters and the next is unavoidable. 

Two types of reactivity of bicyclobutanes with electron-deficient alkenes and alkynes have been known for many years ". These are an ene-like reaction and a cycloadditionlike reaction. The first is typified by the room-temperature addition of dicyanoacetylene (200) to 3-methylbicyclobutane-l-carbonitrile (224) to furnish 225 as the major product and the second by the reaction between 224 and acrylonitrile at elevated temperature to give the bicyclo[2.1.1]hexane 226 ". More recently a third mode of reaction has been describedfor which the prototype is addition of tricyclo[3.1.1.0 ]heptane ( Moore s hydrocarbon , 227) to tetracyanoethylene (228) to form cyclopropane 229 It is noteworthy that 227 is incapable of undergoing either the ene-type reaction, which would produce an olefin violating Bredt s Rule, or the... 

Because in metathesis reactions with most catalyst systems a selectivity of nearly 100% is found, a carbene mechanism seems less likely. Banks and Bailey ( ) reported the formation of small quantities of C3-C6-alkenes, cyclopropane, and methylcyclopropane when ethene was passed over Mo(CO)6-A1203, which suggests reactions involving carbene complexes. However, similar results have not been reported elsewhere most probably the products found by Banks and Bailey were formed by side reactions, typical for their particular catalyst system. 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

The reductive cyclization of readily available enol phosphates of 1,3-dicarbonyl compounds bearing pendant olefinic units has been explored [66,67]. The chemistry is exceptionally interesting, and provides a unique route to structures possessing a cyclopropyl unit which is suitable for structural elaboration. The reaction occurs in a manner wherein the phosphate-bearing carbon behaves like a carbene that adds to the pendant alkene to form a cyclopropane. While this provides a useful way of viewing the transformation, mechanistic studies indicate that a carbene is not an actual intermediate. Examples are portrayed in Table 11. 

Cyclopentenes behave differently and often act through radical mechanisms this can lead to photoreduction to cyclopentanes, or photoaddition of the kind exemplified by norborneneand propan-2-ol 12.57). The photoadduct in this process is linked through the carbon atom of the alcohol, and not the oxygen atom. A related addition to acetonitrile 12.58) takes place when norbornene is irradiated in the presence of a silver(i) compound. It is likely thal a metal complex of the alkene is the real irradiation substrate, and the same may be true for copper(i)-promoted additions of haloalkanes to electron-deficient alkenes (2.59). When dichloromelhane is used in such a reaction the product can be reduced electrochemically to a cyclopropane (2.60), which is of value because the related thermal addition of CH.I, to alkenes in the presence of copper does not succeed with electron-poor compounds. 

One of the earliest enantioselective carbon-carbon bond-forming processes catalyzed by chiral transition-metal complexes is asymmetric cyclopropanation discussed in Chapter 5, which can proceed via face-selective carbometallation of carbene-metal complexes. Some other more recently developed enantioselective carbon-carbon bond forming reactions, such as Pd-catalyzed enantioselective alkene-CO copolymerization (Chapter 7) and Pd-catalyzed enantioselective alkene cyclization (Chapter 8.7), are thought to involve face-selective carbometallation of acy 1-Pd and carbon-Pd bonds, respectively (Scheme 4.4). Similarly, the asymmetric Pauson-Khand reaction catalyzed by chiral Co complexes most likely involves face-selective cyclic carbometallation of chiral alkyne-Co complexes (Chapter 8,7). 

Dioximato-cobalt(II) catalysts are unusual in their ability to catalyze cyclopropanation reactions that occur with conjugated olefins (e.g., styrene, 1,3-butadiene, and 1-phenyl-1,3-butadiene) and, also, certain a, 3-unsaturated esters (e.g., methyl a-phenylacrylate, Eq. 5.13), but not with simple olefins and vinyl ethers. In this regard they do not behave like metal carbenes formed with Cu or Rh catalysts that are characteristically electrophilic in their reactions towards alkenes (vinyl ethers > dienes > simple olefins a,p-unsaturated esters) [7], and this divergence has not been adequately explained. However, despite their ability to attain high enantioselectivities in cyclopropanation reactions with ethyl diazoacetate and other diazo esters, no additional details concerning these Co(II) catalysts have been published since the initial reports by Nakamura and Otsuka. 

The three-component reaction of bis(phenylthio)-(trimethylsilyl)methyl lithium (243), phenyloxirane and a terminal alkene yields cyclopropanes 245142 (equation 82). It is assumed that a-elimination of LiSPh from the carbenoid-like species 243 generates phenylthio(trimethylsilyl)carbene (244) which is in equilibrium with 243 although this equilibrium is probably far on the side of the latter, trapping of the thiophenolate ion by the... 

We said that the formation of cyclopropanes by addition of substituted carbenes to alkenes was rare—in fact, alkyl-substituted carbenes undergo very few intermolecular reactions at all because they decompose very rapidly. When primary alkyl halides are treated with base, alkenes are formed by elimination. Having read Chapter 19, you should expect the mechanism of this elimination to be E2 and, if you started with a deuterated compound like this, the alkene product would be labelled with two deuterium atoms at its terminus. 

In practice, donor substituents make it possible actually to isolate a range of carbenes 4.105. With somewhat less stabilisation, the carbene 4.106, although it is only found as a reactive intermediate, is exceptionally easy to form. It is the key intermediate in all the metabolic steps catalysed by thiamine coenzymes, and its reactions are characterised by its nucleophilicity. Similarly, dimethoxycarbene 4.107 reacts as a nucleophile with electrophiles like dimethyl maleate to give the intermediate 4.108, and hence the cyclopropane 4.109, but it does not insert into unactivated alkenes. 

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