Carbenes to alkenes

Thomson1 iOv Click Organic Interactive to use a web-based palette to predict products of the addition of various carbenes to alkenes. 

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. 

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. 

Ab initio and RRKM calculations indicate that the reactions of C, CH, and (H2C ) with acetylene occur with no barrier." Laser flash photolysis of the cyclopropanes (69) and (70) was used to generate the corresponding dihalocarbenes. The absolute rate constant for the formation of a pyridine ylide from Br2C was (4-11) x 10 lmoP s. The rates of additions of these carbenes to alkenes were measured by competition with pyridine ylide formation and the reactivity of BrClC was found to resemble that of Br2C rather than CI2C . 

Fig. 10.1. Mechanisms for addition of singlet and triplet carbenes to alkenes.

From the relative reactivities in Table 7.3, we conclude that 7a is dominant for CH3CCI and CCI2, which exhibit electrophilic selectivity toward the alkenes. On the other hand, CH3OCCH3 displays strongly nucleophilic selectivity toward the electron-poor alkenes. Resonance donation from the methoxy oxygen atom to the carbene Ip orbital (8, Fig. 7.7) is sufficiently strong to render CH3OCCH3 a nucleophilic carbene for which a 71 carbene to alkene electron donation is dominant in the transition state (viz., 7b). 

F. Mendez and M. A. Garcia-Garibay, A Hard-Soft Acid-Base and DFT Analysis of Singlet-Triplet Gaps and the Addition of Singlet Carbenes to Alkenes, J. Org. Chem. 1999, 64, 7061. 

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). 

Problem 9.29 (a) Use the following observations to discuss stereospecificity of carbene addition, (b) Suggest mechanisms for addition of (i) singlet and (ii) triplet carbenes to alkenes. 

Mechanism 8-8 Formation of Halohydrins 352 8-10 Catalytic Hydrogenation of Alkenes 355 8-11 Addition of Carbenes to Alkenes 358 8-12 Epoxidation of Alkenes 360... 

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. 

The synthesis of fluorinated cyclopropanes has been achieved via the addition of fluorinated carbenes to alkenes and via the addition of nonfluorinated carbenes to fluorinated alkenes. 

Table 14. [2 + 1J Cycloadditions of (Trifluoromethyl)carbene to Alkenes and Alkynes...

Addition of Water to Alkenes Oxymercuration 239 Addition of Water to Alkenes Hydroboration 242 Addition of Carbenes to Alkenes Cyclopropane Synthesis Reduction of Alkenes Hydrogenation 249... 

The application of the Woodward-Hoffmann rules to cheletropic reactions is not straightforward. In the [2+1] cycloaddition of singlet carbenes to alkenes, the stereochemistry of the alkene is preserved in the product, so the alkene must react suprafacially. The Woodward-Hoffmann rules suggest that the carbene component of this thermal, four-electron reaction must react antarafacially. However, what this means for a species lacking a 77 system is difficult to interpret. 

Copper-catalyzed 124a-c) or light-induced l24c d) decompositions of diazoalkanes in the presence of allenes result in the formation of spiropentanes as the main products. Some reports have been published concerning the addition of unsaturated carbenes to alkenes to give methylenecyclopropanes (Route b)125). Unfortunately, this method seems not to be practicable for preparations on a somewhat larger scale. 

The cycloaddition of free carbenes to alkenes to give cyclopropanes is well known (see equation 10.2). Transition metal-carbene complexes, acting either stoichiometrically or catalytically, promote cyclopropanation, sometimes in a synthetically-useful manner. Several mechanistic pathways have been observed, some of which involve mid-transition metal alkylidenes. Two of the most common of these are outlined in Scheme 10.6.67... 

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