Carbenes cycloadditions

Cyclopropane-fused chlorins derived from tetraphenylporphyrins can be prepared by the aforementioned carbene cycloaddition route, e.g. conjugative addition of nialonate to nickel(II) nitrotetraphenylporphyrin 14 (M = Ni) yields the cyclopropane-fused chlorin 15.22... 

Alkenes are scavengers that are able to differentiate between carbenes (cycloaddition) and carbocations (electrophilic addition). The reactions of phenyl-carbene (117) with equimolar mixtures of methanol and alkenes afforded phenylcyclopropanes (120) and benzyl methyl ether (121) as the major products (Scheme 24).51 Electrophilic addition of the benzyl cation (118) to alkenes, leading to 122 and 123 by way of 119, was a minor route (ca. 6%). Isobutene and enol ethers gave similar results. The overall contribution of 118 must be more than 6% as (part of) the ether 121 also originates from 118. Alcohols and enol ethers react with diarylcarbenium ions at about the same rates (ca. 109 M-1 s-1), somewhat faster than alkenes (ca. 108 M-1 s-1).52 By extrapolation, diffusion-controlled rates and indiscriminate reactions are expected for the free (solvated) benzyl cation (118). In support of this notion, the product distributions in Scheme 24 only respond slightly to the nature of the n bond (alkene vs. enol ether). The formation of free benzyl cations from phenylcarbene and methanol is thus estimated to be in the range of 10-15%. However, the major route to the benzyl ether 121, whether by ion-pair collapse or by way of an ylide, cannot be identified. 

Scheme 25. Carbene cycloaddition onto sterically congested bicyclopropylidenes 24c, 62 [ 117,...

Scheme 27. Preparation of branched triangulanes 128,129 via carbene cycloadditions [33,65,105]...

The transition state of singlet carbene cycloaddition to alkenes involves an electrophilic approach of the vacant p orbital to the n bond of alkenes. By contrast, the first step of the triplet addition process may involve the in-plane a orbital of the carbene. As in the case of C—H insertion (see Section 5.1), the difference in the transition structure between the singlet and triplet cycloaddition becomes important in the intramolecular process, especially when approach to a double bond is restricted by ring strain. Direct photolysis of ( )-2-(2-butenyl)phenyldiazomethane (99) in the presence of methanol gives l-ethenyl-l,la,6,6fl-tetrahydrocycloprop [fljindene [100, 29%, (E/Z)= 10 1] and l-(2-butenyl)-2-(methoxymethyl)benzene (101, 67%). Triplet-sensitized photolysis results in a marked increase in the indene (52%, EjZ) = 1.3.T) at the expense of the ether formation (4%) (Scheme 9.30). On the other hand, direct photolysis of phenyldiazomethane in an equimolar mixture of... 

The intramolecular diene-carbene cycloaddition equivalence and an enantioselective Birch reduction-alkylation by the chiral auxiliary approach. Total synthesis of ( )- and (—)-longi-folene. Journal of Organic Chemistry,... 

For easier comparison the result of the thermal reaction is included for compounds 46 and 47. As can be seen in the reaction scheme above direct photolysis of the pyrazolines 46 and 47 proceeds mainly with retention of the original stereochemistry in the cyclopropanes isolated. 48,49 and 50 however lead mainly to the inverted stereochemistry in the cyclopropanes. The singlet biradical 51 formed from 46—49 is therefore clearly not on the same energy surface as a, . possible singlet diradical in the carbene cycloaddition. However one knows today that singlet carbene cycloaddition is a concerted process, so no such diradical should be detectable. 

Ring expansion of dichlorocarbene cycloadducts may be followed by further carbene cycloaddition to give 70 and 71 (R2N = morphcjJino) (Scheme 36). In the case of the pyrrolidine enamine, the reaction stops at the eniminium salt stage (69) to give structures such as 72 and 73 on hydrolysis59,60. 

The spin state of the carbene is an essential feature for the stereoselectivity of carbene cycloaddition to double bonds and for the regioselectivity for insertion reactions, the singlet being stereoselective in cycloadditions. Typical carbene reactions are ... 

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