Olefins singlet carbenes

The stereospecificity of addition suggests a singlet(10) carbene although the ground state of cyclopentadienylidene is known to be a triplet. Attempts to produce a triplet species, which would be expected to react nonstereospecific-ally, in a 4-methyl-cw-2-pentene matrix at 77°K or by dilution of mixtures of the azo compound and olefin with hexafluorobenzene or octafluorocyclo-butane (inert diluents) were unsuccessful. It was concluded that the singlet carbene produced upon photolysis reacts more rapidly with the olefinic... 

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 ... 

These findings, and extensive subsequent work, are consistent with a singlet ground state for this species. The chemical reactivity of these carbenes towards olefins can be related empirically but quantitatively to the electronic properties of the substituents (Moss et al., 1977 Moss, 1980). An extreme example is dimethoxycarbene which does not exhibit at all the electrophilic properties normally associated with the vacant non-bonding orbital of a singlet carbene (Lemal et al., 1966). These findings are easily understood by... 

Much of the literature regarding dihalocarbenes is concerned with reactions of CX2 with olefinic substrates to give 1,1-dihalocyclopropane derivatives. These reactions occur with retention of stereospecificity, as expected for singlet carbenes. Dihalocarbenes also exhibit strong electrophilic behavior towards olefins, and will often not react with weakly nucleophilic species if stronger nucleophiles are present. 

Singlet carbenes add stereospecifically to olefins in a concerted reaction mechanism. 

Quantum chemical calculations show that this is exactly what happens. The bottom line is that singlet carbenes appear to behave as electrophiles insofar as their reactivity toward olefins. It should be possible to take advantage of this fact to steer selectivity. 

Product Studios. Alkenes are known as diagnostic reagents for spin state of reacting carbenes. Thus, the reaction of a singlet carbene with an olefin usually results in the formation of a cyclopropane through stereospecific addition to the C—C double bond, while a triplet carbene gives rise to a nonstereospecific addition product (see Section 7). 

Figure 14.10. Non-least-motion approach of a singlet carbene to an olefin showing the most favorable HOMO-LUMO interaction a) primary interaction (b) secondary interaction.

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... 

As recognized in the pioneering work of Meerwein, Ingold, and Whitmore,10 12 18 19 744 trivalent alkyl cations (C H2 +i+) play important roles in the acid-catalyzed transformations of hydrocarbons as well as various electrophilic and Friedel-Crafts-type reactions. Trivalent alkyl cations can directly be formed by the ionization of lone-pair (nonbonded electron pair) containing precursors (w-bases) such as alkyl halides, alcohols, thiols, and so on, or by protonation of singlet carbenes or olefins. 

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... 

The stereochemistry of these cycloadditions is so specific that Skell used it as a diagnostic test for distinguishing between singlet and triplet carbenes. According to Skell, the addition of singlet carbene to an olefin occurs in a concerted manner and is therefore stereospecific. However, in the case of triplet carbene, both the unpaired electrons cannot form a new covalent bond because of their parallel spins. Therefore, in the latter case the reaction will take place in two steps. In the first step a triplet diradical is formed, which undergoes spin inversion and then ring closure. For this the radical has to wait for the appropriate... 

Singlet carbenes add to olefins stereospecifically triplet carbenes add non-stereospecifically. 

However we rationalized that using silicon as a migrating group could result in a unique stabilization of the energy surface of olefin isomerization. This rationalization was based on silicon s well known ability to stabilize both a-carbanions and p-carbocations. Thus a hypothetical "dual-stabilized" zwitterion would be produced by a 90° twist of a vinyl silane, and a following 1,2-shift of Silicon would produce a singlet carbene possessed of the same hyperconjugative stabilization as in a P-silyl cation (Eq. 5). 

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