Chlorocarbenes

Tlie existence of the ylide 19, which can formally be interpreted as the deprotonation product from the corresponding salt 7a, has been claimed by trapping chlorocarbene with pyridine during the laser-flash photolysis of e do-7-chlorodibenzo[n,c]bicyclo[4.1.0]heptane (18) (96JPC18426). Bromination of l-vinyl-2-pyridone (20) yields the bicyclic pyridinium bro-... 

Carbon tunneling in a second singlet chlorocarbene has also been proposed. It has proved impossible to observe noradamantylcarbene 73 spectroscopically, either by solution laser flash photolysis or with matrix isolation at low temperatures. It has been suggested that the carbene rearranges too rapidly, possibly via carbon tunneling, to adamantene (74). 

The reaction of 196 with phenyl chlorocarbene 198 illustrates the synthesis of indolizines by cyclization of pyridinium ylides (Scheme 7). Cyclization of ylide rotamer 199 generates the intermediate product 200, which undergoes elimination of chloride to provide compound 201 <2005EJ01532>. 

Moreover, ab initio calculations indicate that additions of chlorocarbenes to ethylene or TME lead to cyclopropanes either without the intermediacy of CACs,39 or via broad shallow wells for complexes that might occur in the reaction enthalpy profile but are not minima on the free energy surface.40 More recent calculations confirm these conclusions CACs are not predicted to be im-... 

The RIES mechanism provides a particularly good framework for several reaction systems. Examples include the C and H migrations of cyclobutyl-halocarbenes 17-F and 17-C1,28 the methyl migration and 1,3-CH insertion of t-butylchlorocarbene, 18,27 and the fragmentations of 2,2-dimethylcyclopropyl-chlorocarbene, 19,45 and tawis-f-butylcyclopropylcarbene, 20.46... 

The second of our principal concerns is the contribution of quantum mechanical tunneling (QMT) to singlet carbene 1,2-H shifts and related reactions. There is strong evidence that QMT is important in the low temperature matrix reactions of (e.g.) t-buty 1 chlorocarbene (18)58 and benzylchlorocarbene (10a).59... 

The intramolecular 1,2-H shifts of alkylchlorocarbenes are often very rapid making it difficult to relate structure with reactivity in terms of absolute rate constants. For example the ku values of Me2CHCCl, PhCHMeCCl, and EtCCl exceed 108 s 1 in hydrocarbon solvents at 25°C (Table 4).60 86 87 However, due to the stabilizing effect of the oxa spectator substituent, acetoxycarbenes react at much reduced rates relative to their chlorocarbene analogues,90,91 thus providing kinetically accessible results for a wide array of bystander-substituted alkylacetoxycarbenes.81 92... 

Nevertheless, acetoxycarbenes are less reactive than analogous chlorocarbenes, as shown by the data in Table 5, where we compare 1,2-H shift rate constants for phenoxymethyl-, cyclobutyl-, and isopropyl halo and acetoxy carbenes. Decreased rates of 1,2-H shifts are observed, due to the influence of the acetoxy spectator substituents, with the kinetic suppression reaching a factor > 900 for the comparison of cyclobutylacetoxycarbene with cyclobutylchlorocar-bene.81... 

This result can be rationalized, if, as mentioned earlier, the gain in covalency is taken to be roughly 30 kcal/mol. By approximating the intrinsic preference of both chlorocarbene subunits in 22 for a local configuration, as twice the A/Tst of phenyl chlorocarbene (about —7 kcal/mol. Table 1), the energetic price to be paid for the promotion of each divalent carbon center to a an/aTi configuration is about 14 kcal/mol, significandy less than the benefit due to the extra -ir bond that is formed in the (a ir/a-ir)- A state. 

When no electrophile is present, reduction of carbon tetrachloride leads to di-chlorocarbene by elimination of chloride ion from the trichloromethyl carbanion intermediate. Dichloromethane is the best solvent for this process [79], The car-bene is trapped by reaction with an alkene to form a dichlorocyclopropane (Table... 

Laser flash photolysis of 18 (Ar = -N02C6H4) led to the formation of the aryl chlorocarbene detected at 320 nm. In the presence of acetone, a new species was observed at X = 590 nm that was assigned the structure of carbonyl ylide 19. The ylide, formed by attack of acetone on the carbene, was shown to be irreversible, where the lifetime of the ylide (1.35 ps, fecyciization = 7.40 x 10 s ) was controlled by cyclization to the aryl epoxide 22. The rate constant for the cycloaddition of substituted benzaldehydes to produce dehydrodioxolane (21) was determined experimentally (e.g., p-ClPhCHO=6.16 x 10 M s ). 

Iron(II) porphyrins react readily with haloalkanes in the presence of reducing agents, e.g., excess iron powder, to give chlorocarbene complexes 15,32). With the insecticide DDT [2,2-bis(4-chlorophenyl)-1,1,1 -trichloro-ethane, (C1C6H4)2CHCC13], the reaction proceeds one step further, by elimination of HC1 from the carbene complex, to give diarylvinylidene complexes 33) ... 

Ab initio calculations on the structures of ethylchlorocarbene and chloromethyl-chlorocarbene show that the equilibrium between cis and trans conformers is shifted moderately to the trans isomer for the ethyl derivative and strongly to the cis for the chloromethyl (due to a stabilizing carbene n — rearrangement transition states are product-like and the Z-isomers are stabilized by a tiirough-space interaction of the p orbitals localized on the terminal atoms. 

Kinetic analysis of the rearrangement of benzylfluorocarbene, generated by laser flash photolysis of the corresponding diazirine, gave a rate constant of 9.2 x 106 s 1 at 26 °C with activation entropy —17.2 eu and activation energy 3.25 0.34 kcal mol-1, very similar to the values for the chlorocarbene.80 A product analysis study of the thermolysis and photolysis of the diazirine (73) in the presence of tetramethylethylene showed tiiat die ring-expanded cyclobutene and the cyclopropanation products do not arise via a common intermediate.81 The ring expansion was proposed to occur by loss of N2 from the diradical intermediate (74). 

The a-deuterium secondary KIE (k /k -)) for the rearrangements of (2,2-dideuteriocyclo-propyl)chlorocarbene to chlorocyclobutenes 390 and 391 have been... 

Phenyl chlorocarbene reacts with 2-pyridyl Schiff bases as the first step in the synthesis of imidazo[l,5-a]pyridiniums.103 The reaction mechanism is discussed in detail. The authors consider both attack of the pyridine nitrogen and attack by the imine nitrogen. 

The highly strained hydrocarbon, produced from dilithiopentalene and chlorocarbene, explodes violently at temperatures as low as —40°C when concentrated. Other isomers are probably similar. 

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