Mechanistic Studies on the Dual Reaction Pathways of Singlet Excited Dibenzoyl(methanato)boron Difluoride (DBMBF₂):  Reactions of the Excimer and Exciplexes

Abstract

The mechanism of the interaction of singlet excited dibenzoyl(methanato)boron difluoride (*DBMBF2 = *A) with olefins and substituted benzenes (SB) was investigated by the determination of effective quenching distances (R eff) from fluorescence monitors in acetonitrile as analyzed by the Smoluchowski−Collins−Kimball equation and by solvent effects on the dichotomy of the formation of cycloadducts and [4 + 2] dimers from 1,3-cyclohexadiene (CHD) and 1,3-cyclooctadiene (1,3-COD). The correlation of R eff with directly demonstrates that the reactants have to get closer to affect electron transfer from these substrates as the driving force become more positive and coalesce on exciplexes. Both photophysical and photochemical probes converge on the partition of encounter pairs to the SSRIP (the k et pathway which leads to radical ion reactions and nonfluorescent) and exciplexes (the k ex pathway which leads to cycloadducts and fluorescent species); the former pathway was promoted by low redox energies of donor−acceptor pairs and by high solvent polarity. The reaction with 1,3-cylcohexadiene gave high quantum yields of the [4 + 2] dimers in acetonitrile and of the cycloadduct in nonpolar solvents; the latter quantum yields were correlated with the solvent polarity parameter E T(30) in a smooth distribution regardless of pure non-SB or SB solvents or mixture solvents (including benzene), in spite of the fact that the reactive intermediates in the presence of SB are *DBMBF2−SB exciplexes. It is proposed that in the exciplex the locally excited state provides the driving force in the cycloaddition and the CT state regulates its regiospecificity. The quantum yield of the dimers from CHD was greatly enhanced, reaching their maximum at 12 mol % of p-xylene in acetonitrile; the enhancement was attenuated with the addition of toluene and benzene in that order. This shows that the p-xylene exciplex underwent extraordinary facile substitution with CHD by electron transfer in acetonitrile probably promoted by superexchange interactions. The *DBMBF2 excimer reacted with CHD and 1,3-COD to give higher yields of cycloadducts but not the dimers, which must mean that the excimer substitution with CHD proceeds to give the new exciplex but not the radical ion pair. The latter failure as well as that of the benzene exciplex was suggested to arise from the insignificant CT content of the excimer and exciplex. The frontier MO scheme was utilized to rationalize the enhanced reactivity of the exciplexes and excimer on the bases of increased electronic and orbital interactions, respectively.

NMR Studies on [2 + 3] Cycloaddition of Nitrile Oxides to Polyunsaturated Medium Size Rings

“…Facial selectivity was established by observation in the ROESY NMR spectrum of correlations between H13a,b at 3.32 and 3.19 ppm and H10a,b at 2.07 and 1.96 ppm, as well as between H13b and aryl H2'/H6' protons. The C4-C5 double bond was preserved as witnessed by observation in the 13 C NMR spectrum signals of the quaternary C5 at 133.8 ppm and C4 at 120.2 ppm coupled in the HSQC spectrum to H4 at 5.21 ppm.…”

“…Hydroximinoyl acid chlorides were prepared from the corresponding aryl aldehyde oximes and N-chlorosuccinimide (NCS) in N,N-dimethylformamide (DMF). 21 Spectra were recorded as follows: Fourier transform (FT) infrared (IR) spectra on a JASCO FTIR-420 spectrometer; 1 H, 13 C NMR, 2D correlation spectroscopy (COSY), 2D heteronuclear single quantum coherence (HSQC), 2D heteronuclear multiple bond correlation (HMBC) and 2D rotating frame nuclear Overhauser effect spectroscopy (ROESY) analyses on a Bruker AVANCE III 500 MHz and a Varian VNMRS 600 spectrometers in deuterated chloroform. The 1 H NMR spectra were recorded using single-pulse sequence and spectral width (SW) of 10000 Hz, 30° pulse width (pw) of 9.7 ms, an acquisition time (at) of ca.…”

“…The compounds described in this work are presented in Schemes 1-6 and Tables 1 and 2. 1 H and 13 C NMR chemical shifts of adducts 24-27 are shown, respectively, in Tables 3 and 4. The spectroscopic data for the other products are presented in Experimental.…”

“…Cycloaddition to the C 15 sesquiterpene trans,trans-8-methylene-1,4,4-trimethyl-1,5-cycloundecadiene (8) afforded a bis-adduct 9 in 55% yield (Scheme 2, Figure 1). A complete assignment of 1 H and 13 C NMR spectra of 9 was based on analysis of 1D and 2D 1 H and 13 C NMR spectra (COSY, HSQC and ROESY) and is presented in Experimental. Presence of HC=HC unit in E configuration (a large coupling constant of 15.5 Hz in 1 H NMR spectrum) confirmed by the 2D COSY spectrum and two olefinic H-C correlations in HSQC spectrum indicated that C4-C5 double bond was preserved.…”

“…12 Singlet excited dibenzoyl(methanato)boron difluoride underwent also cycloaddition to 1,3-cyclooctadiene. 13 Reaction of SO with 1,3-cyclooctadiene gave the corresponding [1 + 4] adduct in low 2% yield. 14 1,3-Cyclooctadiene was unreactive in titanium complex catalyzed [2 + 6] cycloaddition reaction with cycloheptatriene.…”