Electrophiles singlet carbene addition

Carbene protonation has been amply demonstrated by product studies, time-resolved spectroscopy, and kinetic measurements. The ability of singlet carbenes to accept a proton is not adequately described by the traditional scale of carbene philicities, which is based on addition reactions with alkenes. In particular, aryl- and diarylcarbenes excel as proton acceptors but would traditionally be classified as electrophiles. 

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 stability of azole carbenes can be attributed to electronic factors which operate in both the Tran d CT-frameworks (92JA5530). In the TT-framework, electron donation into the carbene out-of-plane p-orbital by the electron-rich system moderates the typical electrophilic reactivity of carbenes. In the o-framework, additional stability for the carbene electron pair may be gained from the o-electron-withdrawal effects on the carbene center by the more electronegative nitrogens, which moderates the carbene nucleophilic reactivity. The combination of these a- and TT-effects serves to increase the singlet-triplet gap and stabilize the singlet carbene over the more reactive triplet state. For carbenes with bulky substituents (tert-butyl, 1-adamantyl, etc.) steric effects provide additional stabilization. 

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 behaviour of carbenes depends upon whether they are in the singlet or triplet state. Additions of singlet carbenes to unsaturated systems are generally concerted and stereospecific. Additions of triplet carbenes are two-step processes they are non-stereospecific. The subject of addition of carbenes to alkynes fits into this chapter because most carbenes and carbenoids behave as electrophiles. 

A singlet carbene is inherently both an electrophile and a nucleophile, what is behaviorally decisive is whether, in the carbene/alkene addition transition state, it is the LUMO(carbene)/HOMO(alkene) or HOMO(carbene)/LUMO(alkene) interaction (cf., Fig. 5) which dominates and determines the electronic distribution. If the former interaction dominates, the carbene will exhibit electrophilic selectivity if the latter interaction is more important, nucleophilic selectivity will be observed. If both interactions are comparable, the carbene will display an ambiphilic selectivity pattern, in which it acts as an electrophile toward electron-rich alkenes, but as a nucleophile toward electron-poor alkenes. [8,69]... 

The reactivity of most singlet carbenes with alkenes is dominated by the electrophilic character of the carbene (the empty p orbital). Thus, the more electron rich the alkene, the faster the carbene addition. Increasing alkyl group substitution on alkenes increases the rate of addition. This trend parallels the reactivity for the addition of other electrophiles with alkenes, such as acids, Xj, and borane. Dialkylcarbenes are less selective than dihalocarbenes, whereas carbenes with neighboring O or N atoms are resonance stabilized (see margin) and are highly selective. This trend tracks the reactivity-selectivity principle (see Chapter 7), where the more stable carbenes are the more selective. 

C-Labelling showed that the thermal conversion of the phenyl trimethylsilyl diazo-compound (168) into (169) involved isomerization of a phenyl carbene into a cycloheptatrienylidene, silacyclopropanes were not involved. It was shown that the P-naphthyl carbene obtained by rearrangement of 4,5-benzocycloheptatrienylidene enters the singlet-triplet manifold as a singlet. Evidence has been reported that aryl carbenes can show both electrophilic and nucleophilic properties in their intramolecular ring expansion. Low-temperature photolysis of the sodium salt of the toluene-p-sulphonyl hydrazone (170) gave, by intramolecular carbene addition, the dibenzobicyclo[4,l,0]heptatriene (171) which was trapped by buta-l,3-diene. ... 

A rationalization of the cisUrans ratios of olefins produced in carbenic decomposition of diazo-compounds R CN2CH2R is based on competing electrostatic and steric effects in the intermediate singlet carbene. Heteroatom-containing substituents in the 3-position of cyclohexene can direct the addition of electrophilic species from a cis direction and rates of addition are sometimes accelerated. The addition of dichlorocarbeneto (284), however,... 

The radical versus electrophilic character of triplet and singlet carbenes, respectively, also shows up in relative reactivity patterns. The relative reactivity of singlet dibromocarbene toward selected olefins (Table 9.1) can be seen to be more in accord with the electrophilic models (bromination, epoxidation) than with the radical model (addition of CCls). Carbene reactivity is strongly affected by substituent groups. Various singlet carbenes have been characterized as nucleophilic. 

From the point of view of both synthetic and mechanistic interest, much attention has been focused on the addition reaction between carbenes and alkenes to give cyclopropanes. Characterization of the reactivity of substituted carbenes in addition reactions has emphasized stereochemistry and selectivity. The reactivities of singlet and triplet states are expected to be different. The triplet state is a diradical, and would be expected to exhibit a selectivity similar to free radicals and other species with unpaired electrons. The singlet state, with its unfilled p orbital, should be electrophilic and exhibit reactivity patterns similar to other electrophiles. Moreover, a triplet addition... 

Non-stereospecific cyclopropanation reactions of the diazafluorenylidene (10), generated by photolysis of the diazo compound, indicated a triplet carbene.18 Competition experiments suggested a singlet-triplet equilibrium at room temperature and a Hammett study of additions to substituted styrenes indicated that the carbene reacts as an electrophile (p = —0.65). 

An anomalous cycloaddition is the insertion of a carbene into an alkene. Some cheletropic reactions are straightforwardly allowed pericyclic reactions, which we can illustrate with the drawing 6.127 for the suprafacial addition of sulfur dioxide to a diene, and with the drawing 6.128 for the 8-electron antarafacial addition of sulfur dioxide to a triene. The problem comes with the insertion of a carbene into a double bond, which is well known to be stereospecifically suprafacial on the alkene with singlet electrophilic carbenes [see (Section 4.6.2) page 149]. This is clearly a forbidden pericyclic reaction if it takes place in the sense 6.129. 

Theoretical studies on the structure of the carbene cation and anion radical of the parent species (H2C and H2C , respectively) performed by MINDO/3, SCF/CI and ab initio (4-13G) calculations led to the conclusion that the structures H2C ( Ai) and H2C ( Bi) are similar to those of the neutral carbene in its lowest singlet ( Ai) and triplet ( Bi) state, respectively, if one compares CH bond lengths and HCH angles (see summary of Bethell and Parker, 1988 for neutral carbenes, see also Sect. 8.1 of this book). Removal of an electron from the neutral carbene is energetically much more expensive than addition of an electron. This theoretical result corresponds to conclusions that can be drawn from experimental experience concerning reaction products. They demonstrate that carbene cation radicals are very electrophilic. 

---