Reaction paths for addition of dichlorocarbene to 1,2-disubstituted cyclopropenes were calculated using hybrid density functional theory (B3LYP/6-31G) in the gas phase and in the presence of a continuum solvation model corresponding to acetonitrile. In both the gas phase and acetonitrile, :CCl2-cyclopropene addition follows an asymmetric, non-least-motion approach. Barriers to addition range from 0 to 2 kcal/mol. The reactions proceed in concerted fashion in both the gas phase and solution to yield 1,3-dienes or bicyclobutanes. The reaction pathway on this complex potential energy surface of this reaction appears to bifurcate, and the product distribution is believed to be controlled by reaction dynamics. At the present level of theory, there appears to be no minimum on the potential energy surface corresponding to a dipolar intermediate.

Fragmentation reactions of n-butoxychlorocarbene (9), isobutoxychlorocarbene (10), and benzyloxychlorocarbene (2) were studied by product analysis and by laser flash photolysis (LFP). The carbenes were generated photochemically from 3-alkyl-3-chlorodiazirines in (1) MeCN solution; (2) 5.77 M pyridine in MeCN; (3) 0.504 M tetrabutylammonium chloride (Bu4NCl) in MeCN; or (4) 5.77 M pyridine + 0.504 M Bu4NCl in MeCN. In MeCN, 9 gave mainly HCl-capture product, n-butyl dichloromethyl ether (45.1%), carbene dimer (9.8%), and azine (14.6%). Fragmentation products 1-butene (14.4%), 2-butyl chloride (6.0%), and 1-butyl chloride (10.2%) were limited. With added pyridine, HCl was scavenged, the dichloromethyl ether was suppressed, and 1-butene (42.1%), 2-butyl chloride (7.3%), and 1-butyl chloride (24.1%) were dominant. With added Bu4NCl, 1-butyl chloride increased to 46.4%. With both pyridine and Bu4NCl, 1-butyl choride was 63% of the product, with 1-butene at 22.8%. A similar pattern was observed with carbene 10. Products in MeCN included isobutene (23%), 1- and 2-butene (8−9%), tert-butyl chloride (1.7%), 2-butyl chloride (5.8%), isobutyl chloride (0.4%), isobutyl dichloromethyl ether (53%), and carbene dimer (8%). The isobutyl chloride fragmentation product increased from 0.4% in MeCN, to 7.3% with pyridine, to 32% with Bu4NCl, and to 38% with both addends. With 2, benzyl chloride increased from 83% in MeCN to 91% with added pyridine and Bu4NCl. The increases in chloride displacement products are attributed to bimolecular attacks of chloride ions at the α-carbon atoms of the carbenes, particularly 9 and 10. LFP kinetic studies show that the rate constants for fragmentations of these carbenes increase linearly with the concentration of added Bu4NCl in pyridine−MeCN. Second-order rate constants (k 2) as a function of [Cl-] (M-1 s-1, 24 °C) for the fragmentations are 8.2 × 106 (9), 2.7 × 106 (10), and 2.2 × 106 (2). The decrease in k 2 as R in ROCCl changes from n-butyl to isobutyl to benzyl is in accord with a SN2-like mechanism for the carbene fragmentations in which the benzyl case involves the competitive incursion of SN1-like fragmentation.

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Benzylchlorocarbene (1, BCC) was generated photochemically from benzylchlorodiazirine (2) in isooctane, methylcyclohexane (MCH), and tetrachloroethane (TCE) at temperatures from ∼30 to −75 °C. At −70 °C in isooctane, the identified products included Z/E-β-chlorostyrenes 4 (46.6%), α-chlorostyrene 5 (2.4%), 1,1-dichloro-2-phenylethane 6 (1.9%), a BCC-isooctane insertion product 8 (5.5%), carbene dimers 9 (3.8%), and azine 3 (30%). The significant incursion of intermolecular products 3, 8, and 9 implies that laser flash photolytic (LFP) kinetic data for the decay of BCC obtained at low temperature is biased and should not be employed in Arrhenius analyses. Accordingly, previously obtained curved Arrhenius correlations for BCC do not necessarily implicate quantum mechanical tunneling (QMT) in the 1,2-H shift rearrangement of BCC to 4. Similarly in MCH, where BCC affords a solvent insertion product in ∼44−53% yield, the curved Arrhenius correlation (Figure ) cannot be readily interpreted. In polar solvents such as TCE, clean H-shift reactions of BCC are obtained even at −71 °C; an Arrhenius correlation of LFP kinetic data is linear from 3 to −71 °C (Figure ), affording E a = 3.2 kcal mol-1 and log A = 10.0 s-1. Therefore, QMT does not appear to play a major role in the 1,2-H shift rearrangement of BCC at ambient or near ambient temperature in solution.