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This report covers ten publications which appeared between September 2009 and September 2010.

1. Innermolecular Reactions of a Fluorophenylcarbene Inside a Hemicarcerand.

    Photolysis of fluorophenyldiazirine, incarcerated in a hemicarcerand, gave fluorophenylcarbene, which attacked an aryl unit of the host.  After rearrangement, a fluoromethoxy/phenyltropone derivative of the hemicarcerand was formed.  The incarcerated carbene was likely unstable at temperatures above 100 K.

2. Ultrafast Spectroscopy of Arylchlorodiazirines: Hammett Correlations of Excited State Lifetimes.

    Experimental and computational studies suggest that 375 nm excitation of arylchlorodiazirines furnishes their S1 excited states with lengthened C-N bonds, positive charge at the para and diazirine carbon atoms, and negative charge at the nitrogen atoms.  These structures rationalize the observed solvent and substituent effects on the excited state lifetimes.  The lifetimes are correlated by the Hammett substituent constants of the aryl substituents.

3. The Trichloromethide and Bromodichloromethide Carbanions.

    Reactions of photochemically generated dichlorocarbene with chloride or bromide ions afford trichloromethide (CCl3-) or bromodichloromethide (BrCCl2-) carbanions, the central intermediates in the classical base-catalyzed generation of dichlorocarbene from haloforms.  From kinetics measurements on the reaction of dichlorocarbene with bromide ion, and the additions of both the carbene and BrCCl2- to acrylonitrile, we extracted the equilibrium constant (K = 10 M-1) for the (reversible) reaction of CCl2 with Br- to form BrCCl2-.

4. Dihalodiazirines and the Photochemical Generation of Dihalocarbenes: New Light on
Old Problems.

    The Graham oxidation of amidines to halodiazirines, coupled with the diazirine exchange reaction, makes available many new diazirine precursors of carbenes.  Nitration of the phenoxy moieties of phenoxychloro- or phenoxyfluorodiazirine converts them to leaving groups, permitting the preparations of CCl2, CClF, and CF2.  Absolute rate constants and activation parameters are measured for the additions of these carbenes to several alkenes.  The comparative contributions of ΔH‡ and ΔS‡ to ΔG‡ are assessed for
these reactions, as are the dependences of ΔH‡ and ΔS‡ on the structures and reactivities of the carbenes and alkenes.

5. Solvation of Dichlorocarbene: Complexation with Aryl Ethers.

Dichlorocarbene (CCl2), generated by laser flash photolysis of dichlorodiazirine, formed p- and O-ylidic complexes with aromatic ethers such as anisole, 1,3-dimethoxy-benzene, 1,3,5-trimethoxybenzene, dibenzofuran, and dibenzo-18-crown-6, as well as with the aromatic ester, phenyl acetate.  These complexes were visualized by UV-Vis spectroscopy, and they retarded the addition of CCl2 to tetramethylethylene by factors of 18-152.  Computational studies based on density functional theory provided structures and energetics for the transient species and rationalized their absorption spectra. Complexes were not observed between CCl2 and simple, non-aromatic ethers such as THF, dioxane, or 18-crown-6, nor did these ethers much affect the addition rate of CCl2 to tetramethylethylene. Computations also suggested that p-complexes of CCl2 and (e.g.) mesitylene and durene were energetically reasonable transients.  Although these species were not detected spectroscopically, the aromatic compounds did slow the addition of CCl2 to tetramethylethylene by factors of 15 and 31, respectively.

6. Complexation of Dichlorocarbene by Methylanisoles.

    Dichlorocarbene generated by laser flash photolysis of dichlorodiazirine readily forms UV-vis active π- and O-ylidic complexes with methylanisole derivatives such as
4-methylanisole, 2,4-dimethylanisole, 2,4,6-trimethylanisole, and 2,3,5,6-tetrmethyl-anisole.  The complexes’ aborptions move to the red with each additional methyl substituent on the anisole moiety.  Complexation slows the addition of dichlorocarbene to (e.g.) tetramethylethylene, and the greater the degree of methyl substitution on the anisole ligand, the greater the deceleration of the carbene addition.  A deceleration factor of 96 is observed with 2,3,5,6-tetramethylanisole.

7. The Generation of Diazirinone: A Computational Study.

    Computational studies indicate that the reaction of p-nitrophenoxyfluorodiazirine
with fluoride ion should generate diazirinone (diazacyclopropenone).  However, fluoride ion is likely to catalyze the decomposition of diazirinone to carbon monoxide and nitrogen, so that diazirinone is likely to be unstable to the conditions used to generate it.

8. The Nucleophilic Reactivity of Fluoromethoxycarbene.

    Activation parameters are reported for the additions of fluoromethoxycarbene (FCOMe) to α-chloroacrylonitrile (Ea = 6.0 kcal/mol, ΔS* = -17 e.u., ΔG* = 10.5 kcal/mol); methyl acrylate (Ea = 9.7 kcal/mol, ΔS* = -14 e.u., ΔG* = 13.2 kcal/mol); and acrylonitrile (Ea = 11.1 kcal/mol, ΔS* = -8.8 e.u., ΔG* = 13.2 kcal/mol).  The reactivity of FCOMe is compared to that of dichlorocarbene and chloromethoxycarbene.  The comparison reveals a more pronounced nucleophilic character for FCOMe.  Computed transition state structural and charge parameters support this conclusion.

9. A Carbene – Carbene Complex Equilibrium

    Phenylchlorocarbene, generated by laser flash photolysis of phenylchloro-diazirine, formed highly stable π-type complexes with 1,3,5-trimethoxybenzene in pentane.  The carbene and carbene complexes were in equilibrium.  We measured the equilibrium constant (K = 1264 M-1 at 294 K) and, from its temperature dependence, extracted the associated thermodynamic parameters: ΔHo = -7.1 kcal/mol, ΔSo = -10.2 e.u., and ΔGo = -4.1 kcal/mol. The carbene complexes were characterized by UV-vis spectroscopy and computational analysis.

10. Carbon Dichloride: Dihalocarbenes Sixty Years After Hine.

    In this invited “Perspective” review of our recent work, we described new syntheses of dichlorodiazirine, difluorodiazirine, and chlorofluorodiazirine.  From these precursors, laser flash photolysis enabled the generation of CCl2, CF2, and CClF.  We described the formation and chemistry of bromodichloromethide carbanion from CCl2, the ambiphility of CCl2, the complexation of CCl2 by aromatic ethers, and the kinetics and activation parameters attending the additions of CCl2, CF2, and CClF to several alkenes.