Dimesityl 1,2 —► 1,3 isomerization

The UV-irradiation of dimesityl(mesitylethynyl)borane in donor solvents induces isomerization to the crystalline trimesitylborirene (87-JA2526) Bulky substituents prevent subsequent dimerization into derivatives of l,4-diboracyclohexa-2,5-diene, which was the chief obstacle to obtaining borirene. According to ab initio calculations (86JA3960), the... 

One of the most important features of the ozonolysis reaction of alkenes is one in which ozone adds to the C=C bond to form a primary ozonide (1,2,3-trioxolane). The Criegee mechanism suggests that this unstable intermediate decomposes into a carbonyl compound and a carbonyl oxide that recombine to form a final isomeric ozonide (1,2,4-trioxolane). Direct spectroscopic evidence for a substituted carbonyl oxide has only recently been reported by Sander and coworkers for the NMR characterization of dimesityl carbonyl oxide. Kraka and coworkers have theoretically modeled dimesityl carbonyl oxide and confirmed the structural aspects reported by Sander and coworkers on the basis of NMR data. 

Trimesitylmethane is an example of isomerism and isomerization in ArgZX systems with C3 symmetry. Pioneering studies on related triaryl-methanes provided valuable observations concerning structure and mechanism of stereoisomerization. 39-41) jn particular, an X-ray diffraction study 41> of dimesityl-(2,4,6-trimethoxyphenyl)methane showed that this compound adopts a propeller conformation in the solid state. A similar conformation was found for triphenylmethane in the gas phase by electron diffraction.42) Nmr evidence is also consistent with such a conformation for triarylmethanes in solution. 39-41) jn the following, we shall briefly describe our experience with trimesitylmethane (7). 43)... 

The evidence that ,Z-isomerization of 92-95 proceeds by Si=Si bond rotation and not a mechanism involving silylene intermediates, produced by cleavage of the Si=Si bond followed by recombination, rests upon the fact that no trapping products consistent with the intermediacy of the corresponding diarylsilylenes could be detected upon heating the disilenes in the presence of known silylene traps such as methanol, triethylsilane or 2,3-dimethyl-l,3-butadiene. In fact, one tetraaryldisilene has been shown to isomerize by this mechanism, the 1,2-dimesityl-l,2-bis(2,4,6-tris[bis(trimethylsilyl)methyl]phenyl derivatives (E)- and (Z)-97a (equation 70)142,143. Arrhenius parameters for the thermal dissociation of (E)- and (Z)-97a to diarylsilylene 98 are listed in equation 70. 

Several silacyclopropenes 62a-c were synthesized by photochemical isomerization of ethynyldisilanes (Scheme 28), while l,l-dimesityl-2,3-diphenylsilacyclopropene 62d was obtained by the reaction of the photoche-mically generated dimesitylsilylene with diphenylacetylene (Equation 23). All silacyclopropenes are air stable and were characterized by H- and C-NMR spectroscopy <2003OM2436>. 

As an afterthought, it might be noted that the transition structure in Fig. 9.11 appears to be on the pathway for thermal isomerization of 1,2-dioxetane to 1,3-dioxetane this reaction is not known to occur, presumably because spin-non-conservative fragmentation is a more efficient process. It is suggestive, however, that the analogous isomerization of 1,2-dimesityl-1,2-di-t-butyl-disiladioxetane to its 1,3 isomer, has been observed both in solution and in the solid state [63]. 

Synthesis of AU Iidene 1,2-Oxasiletanes. The regiospecific oxidation of siliranes to 1,2-oxasiletanes can be accomplished in moderate yield with diphenyl sulfoxide. When 1,1-dimesityl-2Z-neopentylidenesilirane was heated with diphenyl sulfoxide in benzene, 2,2-dimesityl-4Z-neopentylidene-l,2-oxasiletane was formed in 70% yield (eq 11). In contrast, use of cyclo-hexyldimethylamine A-oxide instead of diphenyl sulfoxide gave the isomeric 1,2-oxasiletane in 44% yield along with mesitol in 40% yield (eq 12). 

The heat of formation of 2,8,9-trioxa-5-aza-l-boratricyclo[3,3,3,0 > ]undecane in the gas phase has been determined (191.5 0.4 kcalmol- ) as has the bond energy of the N B dative bond. A study describing the recrystallization of triarylboranes has appeared as has a report concerning the solvent isotope effect on the rate of methoxyl exchange between methyl borate and methanol. 2-[l-(o-dihydroxyborylphenyl)-2-phenylethyl]-2-imidazoline has been used for the selective determination of tartaric acid in aqueous solution and the absorption and emission properties of a series of phenyl-substituted dimesityl-boranes have been described. Synchronous spin- and stereo-isomerization of the complex cw-Fe(phen) a(NCBPh 3) 2 have been reported, The kinetics and mechanism of the protodeboronation of ferroceneboronic acid in moderately concentrated HaSO have been studied. ... 

A stereospecific synthesis of B-( )-l-alkenyl-9-BBN derivatives that are not available by hydroboration reactions has been developed. The 9-BBN dimer has featured in thermal isomerization and thermolysis studies and similar studies on dimesityl-3-hexyl-borane show that, in comparison to the diphenyl compound, it is quite resistant to thermal isomerization and an explanation is offered. ... 

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