Addition of carbenes

Attempts to synthesize water-soluble thioketals from 335 were not successful, but led to the isolation of the first fullerenecarboxylic acid derivative [382] 336 is prepared in 71% yield from 335 by treatment with trifluoroacetic acid at 110 °C. 

Carbene addition to C70 results in the formation of different monoadduct isomers [386]. Treatment with PhHgCCl2Br in reflmdng benzene produces a mixture of three isomers derived from addition to the C-l/C-2-bond, the C-5/C-6-bond and the C-7/C-8-bond in almost equimolar amoimts. Contrary to this result, addition of trichloroacetate yields almost exclusively the C-5/C-6-product [386]. 

Reactions of Cjq with metal carbene complexes also yield the [6,6] methano-fullerenes [392]. These adducts are probably not formed via a carbene addition, but via a formal [2-1-2] cycloaddition under formation of a metalla cyclobutane intermediate. The Fischer carbene complex [mefhyl(methoxymethylene)]pentacarbonyl chromium can be utilized to prepare l,2-mefhyl(methoxymethano)-fullerene in 20% yield [392]. A tungsten carbene complex was primarily used to initiate the formation of a polyacetylene polymer, but it was discovered that addition of to the complex-polymer-mixture improves the polymerization and dramatically increases the catalytic activity of the carbene complex [393]. can be integrated into the polymer via carbene addition. 

Polyisoprenes and polybutadienes can also be modified by reactions with carbenes. Dichlorocarbene adds to natural rubber dissolved in chloroform in a phase transfer reaction with aqueous NaOH [54]. A phase transfer reagent must be used with the aqueous NaOH. Solid sodium hydroxide can be used without a phase transfer reagent. There is no evidence of cis-trans isomerization and the distribution of the substituents is random [54]. 

Difiuorocarbene, generated under mild neutral conditions, adds to, A-cis- and 1,4-trans-polybutadienes to give materials containing cyclopropane groups [55]. The addition takes place randomly, to give atactic stereo sequence distributions [55]  

Fluorocarbene, formed from phenyl(fluoro,dichloromethyl)mercury by thermolysis in situ, also adds to 1,4 cis- and trans-polybutadienes. The carbene can add at various levels [57]. The addition 

Dichlorocarbene, generated in situ from an organomercury precursor, phenyl(bromo, dichloromethyl)mercuiy (Seyferth reagent), adds to polybutadiene in a similar maimer [56]. The reactions take place under homogeneous conditions. They can be carried out aal% solutions of the polymer in benzene, using 10-20% mol excess of the reagent. 

These are additions to double bonds, like the Prins reaction, and they can be carried out on natural and synthetic rubbers [59, 60]. They take place rapidly in the presence of acid catalysts. Aqueous formaldehyde [61], or paraform in CCLt [62], can be used. The catalysts are inorganic acids or anhydrous Lewis acids, like borrm trilluoride in acetic acid solution [63]  

Explain how a similar hydroboration reaction could be used to prepare (/ -2-butanol in good enantiomeric excess. 

A carbene is a reactive species having a carbon with only two bonds and an unshared pair of electrons. It is quite unstable and, like a carbocation, exists only as a transient intermediate in certain reactions. The simplest carbene, CH2, is called methylene. It is sp2 hybridized. The unshared pair of electrons occupies one of the. syr-hybridized AOs and the other two are used to form the bonds to the hydrogens. The remaining AO on the carbon is an unoccupied p orbital. 

Of course, because the electrophile and the nucleophile are the same atom, the addition must proceed in a syn manner. 

CHAPTER I I ADDITIONS TO CARBON-CARBON DOUBLE AND TRIPLE BONDS 

How are these reactive carbene species generated Two groups must be removed from a tetravalent carbon, leaving behind one pair of electrons. We will examine three of the more important methods that accomplish this. The first is the elimination of N2 from diazo compounds. The simplest diazo compound is diazomethane  

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By application of the Simmons-Smith reaction it is possible to synthesize a cyclopropane from an alkene by formal addition of carbene to the carbon-carbon double bond, without a free carbene being present in the reaction mixture the... 

The addition of carbene to a 3-(halopropyl)-5//-dibenz[7>./]azepine 5 (X = Cl, Br) in a Simmons—Smith reaction is more complex and results in a mixture of the tetracycle 6 (6%), its cyclopropano derivative 7 (48 %), the 5-allyl derivative 8 and the (cyclopropyl)methyl compound 9, the latter two products in a combined yield of 12%.31... 

Those reactions that have found general use for the preparation of aziridines can be grouped into two broad classes addition and cyclization processes, and each of these categories can be further divided. Addition processes can be classified as being C2+N1 reactions (addition of nitrenes, or nitrene equivalents [ nitrenoids ], to alkenes Scheme 4.1) or (J N1+C1 reactions (addition of carbenes or carbenoids to imines Scheme 4.2). 

Woodworth, based on the common reaction of addition of carbenes to double bonds to form cyclopropane derivatives (15-50). If the singlet species adds to cis-2-butene, the resulting cyclopropane should be the cis isomer since the movements of the two pairs of... 

The Addition of Carbenes and Carbenoids to Double and Tripie Bonds... 

Reactions involving free carbenes are very exothermic since two new theoretical treatment of the addition of singlet methylene to ethylene suggests that there is no activation barrier.168 The addition of carbenes to alkenes is an important method for synthesis of many types of cyclopropanes and several of the methods for carbene generation listed in Scheme 10.8 have been adapted for use in synthesis. Scheme 10.9, at the end of this section, gives a number of specific examples. 

The addition of carbenes is also a stereospecific reaction, each geometrical isomer forms the cis addition product. Thus carbene generated from the photolysis of diazomethane adds in a cis manner to both cis and trans butene-2. 

This property is relatively rare in the very large number of reactions for which substituent effects were evaluated quantitatively106. It seems to be common, however, for all dediazoniations of arenediazonium ions and of related compounds, e.g. of substituted phenyl azides forming nitrenes, as well as for additions of carbenes to alkenes. 

Addition of carbenes to Jt-electron excessive aromatic compounds, or those which possess a high degree of bond fixation, is well established. Dihalocarbenes react with naphthalenes with ring expansion to produce benztropylium systems (Scheme 7.8). Loss of hydrogen halide from the initially formed product leads to an alkene which reacts with a second equivalent of the carbene to yield the spirocyclopropyl derivatives in high yield (>95%) [14, 50]. Insertion into the alkyl side chain (see Section 7.2) also occurs, but to a lesser extent [14]. Not unexpectedly, dichlorocarbene adds to phenanthrenes across the 9,10-bond [9, 10, 14], but it is remarkable that the three possible isomeric spiro compounds could be isolated (in an overall yield of 0.05% ) from the corresponding reaction with toluene [14]. 

Cycloadditions are one class of reactions that provide a wide variety of differently functionalized nanotubes (Fig. 3.8). As forhalogenations, cycloadditions are generally carried out directly on the nonoxidized pristine CNTs, which requires the use of highly reactive species or/and harsh conditions. The most common cycloaddition reactions include the addition of carbenes, nitrenes, 1,3-dipolar cycloadditions and Diels Alder reactions. 

Photolysis of diazoaJkanes in liquid phase yields carbenes in a vibrationally relaxed state, since deactivation in solution immediately removes all excess vibrational energy. The addition of carbenes to the olefins, which results in nonstereospecific formation of cyclopropanes, must therefore result from the different multiplicity of carbenes — singlet or triplet. Since most of these multiplicity... 

Photolysis of 3-diazoindole 59a in cycloalkenes, via the addition of carbene 65 to the cycloolefine, gave rise to cyclopropane adduct 66, which can rearrange to indolenines 68 or 69 and then to indoles 70 (66LA17). 

In this method two, or occasionally more, open-chain compounds are merged into a ring. Examples are the common syntheses of cyclopropanes by the addition of carbene (CHj) or substituted carbenes to alkenes (Section 6.4). 

The proposed catalytic cycle starts from the nucleophilic addition of carbene 17 to aldehyde 18 (Scheme 14.4) to afford acyl azolium intermediate 19, which takes part in an acyl transfer event with co-catalyst HOAt to deliver the activated car-... 

The two nonbonded electrons of a carbene can be either paired or unpaired. If they are paired, the species is spectrally a singlet, while, as we have seen (p. 193), two unpaired electrons appear as a triplet. An ingenious method of distinguishing between the two possibilities was developed by Skell,202 based on the common reaction of addition of carbenes to double bonds to form cyclopropane derivatives (5-50), If the singlet species adds to cis-2-butene, the resulting cyclopropane should be the cis isomer since the movements of... 

Additions to carbon-carbon double bonds have already been mentioned. Carbenes also add to aromatic systems, but the immediate products rearrange, usually with ring enlargement (see 5-50). Additions of carbenes to other double bonds, such as C=N (6-61 and 6-62), and to triple bonds have also been reported. 

All-carbon 2 + 3 cycloadditions 5-49 Dimerization of olefins 5-50 Addition of carbenes or carbenoids to olefins or alkynes 5-51 Tetramerization of alkynes... 

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