Carbene addition to olefins

Closs, G. L., Structures of Carbenes and the Stereochemistry of Carbene Additions to Olefins, 3, 193. 

A similar type of chiral rhodium porphyrin was found to be effective for the carbene-insertion reaction to olefins, where formation of the carbene complex takes place. Chiral rhodium complexes for catalytic stereoselective-carbene addition to olefins were prepared by condensation of a chiral aldehyde and pyrrole. Formation of the metal-carbene complex and substrate access to the catalytic center are crucial to the production of optically active cyclopropane derivatives. Optically active a-methoxy-a-(trifluoro-methyOphenylacetyl groups are linked witfi the amino groups of a,p,0L,p isomers of tetrakis-(2-aminophenyI)por-phyrin through amide bonds. Oxidation reactions of the... 

Carbene addition to olefinic linkages to form cyclopropanes also shows stereoselectivity (32). The cis compounds are the principal or exclusive adducts when X = alkyl, aryl, halo, alkylthio, or alkylseleno group, while the trans adducts are formed preferentially when X = trimethylsilyl, alkoxy-carbonyl, or aryloxy. It is clear that carbenes containing a soft substituent X prefer the reaction path leading to products in which X and alkyl R are closer, and the opposite is true when X is hard. 

MisceUaneous Polycyclic Systems.—Formation of cyclopropane rings by carbene addition to olefins, and subsequent elaboration by rearrangement or addition reactions, is an important method of generation of both polycyclic and bicyclic systems. Examples of carbene additions are shown in Scheme 43 296,298—301,304 Subsequent transformation by reduction or rearrangement as noted in Scheme 44 is possible. Not only does lithium aluminium... 

In dihalocarbene generation by phase-transfer catalysis the following steps seem to be involved (15) formation of CX anions dynamically anchored at the boundary reversible detachment with the help of the catalyst reversible carbene formation [Q+ CX3 ] [Q + X ] + CX2 addition to olefin. 

A singlet carbene was proposed to account for this stereoselectivity. Attempts to produce triplet carbene by collisional deactivation with octafluorocyclo-butane were unsuccessful and stereospecific addition to olefin still occurred. However, nonstereospecific addition to olefins and larger amounts of olefinic (insertion) products result from irradiation of the phenyldiazomethane in a frozen m-butene matrix at — 196°C ... 

The discovery of carbene and carbenoid additions to olefins was the major breakthrough that initiated the tapping of this structural resource for synthetic purposes. Even so, designed applications of cyclopropane chemistry in total syntheses remain limited. Most revolve around electrophilic type reactions such as acid induced ring opening or solvolysis of cyclopropyl carbinyl alcohol derivatives. One notable application apart from these electrophilic reactions is the excellent synthesis of allenes from dibromocyclopropanes 2). 

The linear polymerization of Scheme 7.15 represents an unusual case of diazoacetophenone oxidation. For instance, on the action of copper oxide, diazoacetophenone gives ketocarbene, which is involved in typical carbene reactions such as dimerization, addition to olefins, and insertion in the 0-H bonds of alcohols. If the amine cation-radical is used as an oxidant instead of copper oxide, only the polymer is formed. The ketocarbene was not observed despite careful searches (Jones 1981). 

A second characteristic reaction of carbenes is addition to olefins to yield cyclopropanes. Singlet carbenes might react as either nucleophiles or electrophiles triplets may be expected to behave like free radicals- The data in Table 5.9, showing the increase in rate of addition on substitution of electron-donating... 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefinic bond. If acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cyclopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdster, 1971 E.J. Corey, 197JB), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974). 

A more widely held view, argued persuasively by Gaspar and Hammond (1964), is that carbenes of different multiplicity react in ways which are quantitatively, and in some cases qualitatively, different, and that the spin state of the carbene must be taken into account in any discussion of reactivity. Since, for many carbenes, spectroscopic evidence (Section IIB) indicates that the ground state is a triplet, low reactivity and high selectivity is often regarded as being associated with this spin state. Further, the triplet state is associated with non-stereospecific addition to olefins, whereas stereospecific addition is usually taken to indicate reaction by way of the singlet state. 

Despite closely reasoned counter-arguments (DeMore and Benson, 1964), the commonly held view, due to Skell, is that singlet carbenes add to olefins in a stereospecific cia-manner, whereas attack by triplet carbenes leads to non-stereospecific addition (Skell and Woodworth, 1956). The rationale of this view is that a singlet carbene should react with the olefin to form a cyclopropane in a one-step, concerted process because in this way it could occur with conservation of spin (equation 23) the addition would thus be stereospecifically cis. On the other hand, a concerted addition of a triplet carbene would violate the rule of spin conservation in consequence, a multistep reaction, in which spin inversion of an intermediate 1,3-diradical constitutes a discrete process... 

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