Cycloadditions of carbenes

Cycloaddition of Carbenes to conjugated Nitro Olefins (28). From the above it is evident that there is another synthetic route to nitronates (24 g) with the use of carbenium intermediates based on [1 + 2]-cycloaddition of carbenes RR C to conjugated nitro olefins (28) followed by isomerization of intermediate nitrocyclopropanes (23 g). However, this strategy was used only in one study (see Scheme 3.30). 

Hoffmann was the first to apply the concept of orbital symmetry to the cycloaddition of carbenes to olefins. This concept, which is based on EH-calculations was demonstrated for the [H-2]-cyclo-addition of triplet methylene to ethylene. 

Alternatively, [2 -1- 2] cycloaddition of carbene complexes to alkynes, followed by [2 -I- 2] cycloreversion can also lead to the formation of vinylcarbene complexes (Sections 3.2.5.6 and 2.2.4). 

The introduction of carbenes and carbenoids into synthetic organic chemistry revolutionized the synthesis of cyclopropane derivatives21. In particular, cyclopropanation of methylenecycloalkanes became a very useful method for the preparation of SPC. Moreover, since cycloaddition of carbenes to olefins involves a very fast concerted process (i.e. it eliminates any intermediates during the formation of the three-membered ring)21, the method is equally efficient for the preparation of both unstrained and highly strained compounds. 

Carbenes are much more reactive toward carbon-carbon double bonds than toward single bonds. Without doubt the most useful feature of a elimination is that it provides a practical route to cyclopropanes and cyclopropenes by [2 + 1 ] cycloaddition of carbenes to double or triple bonds. These additions are stereospecific suprafacial additions if they involve singlet carbenes, but can give mixtures with triplet carbenes ... 

The cycloaddition of carbenes to glycals is an effective method for the formation of cyclopropanated carbohydrates with high stereoselectivity,60... 

A series of fused pyrrole derivatives, such as 45, has been synthesized in moderate to good yields by means of [4+1] cycloaddition of carbenes, generated in situ from the precursors 46, and isocyanates, also formed in situ from the corresponding acyl azides <02OL4289>. 

In the case where free nitrenes are involved, both the singlet and the triplet nitrene will add to the double bond. On the basis of Skell s postulate which is now well established for the [1 +2]-cycloaddition of carbenes to double bonds 74>,... 

The cycloaddition of carbenes to olefins is a classical synthesis of cyclopropane derivatives. 

Of these three rules, the first one has been most widely used and is the one mainly applied to cycloalkenecarbenes. The well-known [1 -f2] cycloaddition of carbenes to olefins is shown in the following scheme. All the steps for the concerted singlet cycloaddition are evident. 

Compound 110 (obtained by reaction of D-glucal with propargylic alcohol) was subjected to cyclization with Co2(CO)g, which led to a fused tricyclic derivative 111 (O Scheme 48) [97]. [2+2] Cycloaddition of carbenes to glycals is an efficient route leading to cyclopropanated sugars with high stereoselectivity [1]. Such cyclopropanes can be transformed into a number... 

The [2+1] cycloaddition of carbenes and alkenes to give cyclopropanes was discussed in Chapter 2. Other cycloadditions are less common, although [4 + 3], [4 + 4], [6 + 4], [8 + 2] and many other cycloadditions are certainly known. The [4 + 3] cycloaddition in particular involves an allyl cation as the three-atom component and an electron-rich diene as the four-atom component. 

These reactions, and the [2+2] cycloadditions of carbenes and similar species, are sometimes called pseudopericyclic. 

Novikov, M.S. Khlebnikov, A.F. Besedina, O.V. Kostikov, R.R. The first example of intramolecular cycloaddition of carbene-derived azomethine ylides in a domino reaction of difluorocarbene with Schiff bases. Tetrahedron Lett. 2001, 42, 533-535. 

Catalytic cycloadditions of carbenes to alkenes is a straightforward method for synthesizing cyclopropanes [40]. In fact, cyclopropanes are present in a variety of natural products [41-43]. For example, they occur in some unusual amino acids, in natural phytotoxins such as coronatine, as well as in marine terpenes [44], sesquiterpenes [45], cyclosteroids [46-47] (as part of the A-cycle) or in the side chain of steroids [48]. 

Cycloadditions of Carbenes and Carbenoids to Olefins.—Base and Substituted Halo-genomethane. Dichlorocarbene adds to 1-aryl-1,3-dienes predominantly at the 3,4-double bond, the relative rate constants correlating with the constants of sub-... 

Cycloadditions of Carbenes and Ketens.—Carbenes in which the empty p-orbital is part of a Htickel aromatic system show nucleophilic properties and react with electron-poor alkenes. Diphenylcyclopropenylidene adds 1,4 to tetracyclone, giving, after CO loss, the spiro-system (315). The reaction of cycloheptatrienylidene is more complex. The initial adduct is not isolated but behaves as if it were (316) and loses CO to give an interconverting set of isomeric hydrocarbons which terminate in (317) and (318) in the ratio 1 2. The same set can be produced photochemically but not thermally from the spiro-hydrocarbon (319). ... 

Apart from the more general cyclopropanation methods, which involve the cycloaddition of carbene, carbenoid, or ylide to alkene, other methods for... 

The formation of substituted cycloheptatrienes by cycloaddition of carbenes to aromatic substrates has been widely studied, but yields are generally poor because of low selectivity. A Belgian group has now shown that catalytic quantities of rhodium salts of strong carboxylic acids are highly efficient in promoting the addition of carbenes to aromatic hydrocarbons, permitting ready... 

Cycloaddition of Carbenes and Carbenoids to Olefins.—The cycloaddition of carbenes or carbenoids, however prepared, to olefins is still the major method of generating a cyclopropane derivative, and theoretical investigations of the process have been published. ... 

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