Photolysis carbene dimerizations

As the produced carbene dimerizes readily, the photolysis and collection of X-ray data were performed for a sample at 80 K. The crystal was irradiated at 365 nm for 2h using a high-pressure mercury lamp and a band-pass filter in combination. Although the crystal turned from yellow to red, there was no evidence for deterioration of the crystal quality. Figure 5-12 shows the molecular structures before and after irradiation. The dihedral angle between the two phenyl rings before irradiation is 70.2(1)°. 

Benzene is recognized as a very unreactive solvent, especially for triplet car-benes hence, the most reactive counterparts under these conditions must be the triplet carbenes themselves. However, the products obtained from the photolysis in benzene consist of a highly complex mixture containing small amounts of carbene dimers. It is then possible that the simple dimerization of 44 at the car-benic center must suffer from severe steric repulsion and, therefore, the carbene is forced to react at the other positions. The most probable reactive sites are the aromatic rings, where spin can be delocalized. 

Jones, [61] Moss, [62] and Goodman [63] recently showed that transient singlet carbenes (such as chlorocarbenes) interact weakly with aromatics, but not with simple olefins. For example, this interaction led to an extension of the benzylchlorocarbene lifetime from 23 ns (isooctane) to 285 ns (benzene). [62] Similarly, we found that photolysis of the diazo precursor in hex-l-ene at -60°C led to the carbene dimer 42, while in toluene we obtained the carbene 41, which is stable up to -30°C. 

By far the best source for 3a is (trimethylsilyl)diazomethane (19). It has already been mentioned that gas-phase pyrolysis of 19 alone yields products which are derived from intramolecular carbene reactions such as 1,3-C,H insertion and silylcarbene-to-silene rearrangement (see equation 20). Also, copyrolysis of 19 with alcohols or benzaldehyde allowed one to trap the silene but not the carbene 33 (see equation 5). Furthermore, solution photolysis of 19 in the presence of alcohols or amines did not give the X,H insertion products of the carbene but rather trapping products of the silene . On the other hand, photochemically generated carbene 3a did undergo some typical intermolec-ular carbene reactions such as cyclopropanation of alkenes (ethylene, frani-but-2-ene, but not 2,3-dimethylbut-2-ene, tetrafluoroethene and hexafluoropropene), and insertion into Si—H and methyl-C—H bonds (equation 39). The formal carbene dimer, trans-1,2-bis(trimethylsilyl)ethene, was a by-product in all photolyses in the presence of alkenes it is generally assumed that such carbene dimers result from reaction of the carbene with excess diazo compound. 

One of the first unequivocal examples of carbene dimerization in matrices seems to have been that of cyclopentadienylidene (44) (Scheme 11). ° Photolysis of diazocyclopentadiene (43) in Nj matrices produced the carbene 44, which could be detected directly through both its UV and IR absorptions, provided that very dilute matrices were employed. When matrices containing 44, but little or no 43, were warmed to 20-25 K, fulvalene (45) was produced, which was recognized by its highly structured and distinctive UV absorption spectrum. Several carbenes analogous to 44, e.g., tetrachloro- and tetrabro-mocyclopentadienylidene and fluorenylidene, - failed to undergo similar dimerizations even at 30-35 K. 

The comparatively small size of the simplest carbene (methylene) ensures that it has a definite mobility in frozen inert matrices, which leads to the formation of dimerization products under these conditions. It became possible only in 1981 to detect in the spectra of the diazomethane photolysis products bands at 1115 cm (Ar matrix) and 1109 cm (Xe matrix) which were attributed to the deformation vibration of methylene in its ground triplet state (Lee and Pimentel, 1981). 

The IR and UV spectra of the triplet cycloheptatrienylidene [71] were recorded after the UV photolysis (A>574 nm) of diazocycloheptatriene [72] in an argon matrix (McMahon and Chapman, 1986). This carbene interacts with the CO-doped matrix, forming the ketene [73], and it also dimerizes with formation of heptafulvalene [74]. Experiments have shown that [71] cannot be converted into the cycloheptatetraene [48] either photochemically... 

UV photolysis (Chapman et al., 1976 Chedekel et al., 1976) and vacuum pyrolysis (Mal tsev et al., 1980) of trimethylsilyldiazomethane [122]. The silene formation occurred as a result of fast isomerization of the primary reaction product, excited singlet trimethylsilylcarbene [123] (the ground state of this carbene is triplet). When the gas-phase reaction mixture was diluted with inert gas (helium) singlet-triplet conversion took place due to intermolecular collisions and loss of excitation. As a result the final products [124] of formal dimerization of the triplet carbene [123] were obtained. 

Photolysis of acyldisilanes at A > 360 nm (103,104) was shown, based on trapping experiments, to yield both silenes 22 and the isomeric siloxy-carbenes 23, but with polysilylacylsilanes only silenes 24 are formed, as shown by trapping experiments and NMR spectroscopy (104,122-124) (see Scheme 4). These silenes react conventionally with alcohols, 2,3-dimethylbutadiene (with one or two giving some evidence of minor amounts of ene-like products), and in a [2 + 2] manner with phenyl-propyne. Ketones, however, do not react cleanly. Perhaps the most unusual behavior of this family of silenes is their exclusive head-to-head dimerization as described in Section V. More recently it has been found that these silenes undergo thermal [2 + 2] reactions with butadiene itself (with minor amounts of the [2 + 4] adduct) and with styrene and vinyl-naphthalene. Also, it has been found that a dimethylsilylene precursor will... 

Photolysis of DAX in a methylcyclohexane glass at 77 K creates a metastable species detected by its optical absorption spectrum (Table 4). This solution does not exhibit an epr spectrum characteristic of a triplet carbene, nor is one observed when DAX is irradiated in Fluorolube (where triplet carbenes are generally more stable). Warming the frozen solution causes the reaction of the metastable transient and the formation of dimeric xanthone azine. 

Flash photolysis of 5-diazo-lO, ll-dihydro-dibenzo[a, d]cycloheptene (75) — which can be regarded as a bridged diphenyl-carbene — at room temperature in hquid paraffin first produced the spectrum of the triplet carbene 16, which then disappeared to give the electronic spectrum of the radical 17. The latter finally gave the dimer 5,5 -bi (10, ll-dihydrodibenzo[a, djcycloheptenyl) 18 2). 

A series of four-membered Si- or P-containing heterocycles was produced in various yields by electrocyclization or [2-t-2]-dimerization of compounds with multiple carbon-silicon or carbon-phosphorus bonds. These unsaturated species are formed under photolysis of diazo compounds containing a P or Si atom in an a position with respect to the diazo group, following a [l,2]-sigmatropic shift in the intermediate carbene. 

The results are in accord with the proposed supramolecular structures of diazirine 45 and its hosts (vide supra). No azine, di(3-nortricyclanylidene)hydrazine (53), was formed upon photolysis of the dimeric 45 (oc-CyD)2 complex because of a complete encapsulation of the reactive guest. Bimolecular reaction of the guests within the (45 /J-CyD)2 dimer should, of course, produce azine 53 by virtue of their geminated assemblage. Yet, the affinity of carbene 46 for the O H bonds present in both CyDs predominated, giving D-substituted CyDs (55a, 55b, iso-55a, and /.w-55b), via in-nermolecular reactions,21 that were detected using FAB MS analysis (Table 5).133... 

In a laser flash-photolysis study, 2-phenyladamantene was generated in benzene at room temperature from 3-noradamantyl(phenyl)diazomethane. This strained cycloalkene decays with second-order kinetics to give a dimer, and reacts much faster with O2 and Bu3SnH than with methanol, thus revealing a substantial radical character. Diphenyldiazomethanes possessing stable /er -butylaminoxyl and Ullman s nitronyl nitroxide radicals, e.g. (25), have been prepared by photolysis of the parent diazomethanes. Analysis of ESR fine structures showed that the carbene and radical centres couple ferromagnetically in these molecules, as expected. 

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