Combination w Disproportionation

Hensley ei al.,n reported the only direct experimental observation of head-to-head linkages in PS by 2D INADEQUATE NMR on, 3C-enriched PS. The method did not enable these groups to be quantified with sufficient precision for evaluation of W tc- Zammit el a/.130 studied chain distribution of low molecular weight PS prepared with AIBN initiator by MALDI-TOF. Separate distributions of chains formed by combination and disproportionation were observed. They estimated kJkK. at 90 °C to be 0.057. 

Falconer and Cvetanovic (40) attempted to obtain a more quantitative value for the fraction of nonterminal addition in the case of propylene. They produced hydrogen atoms by mercury photosensitized decomposition of H2, using at least 100 times as much H2 as C3H6 and total pressures of 40 and of 250 mm. Under these conditions the reactions of importance were the combination and disproportionation of the iso- and n-propyl radicals and their cross reactions, the combination of the two radicals with H atoms (assumed to be equally probable), and a very small amount of decomposition of hot n-propyl radicals. Disproportionation to combination ratios were taken as 1.64 for two iso-propyl, 1.14 for two w-propyl, and hence 1.39 was taken as the mean of the two values for one iso- and one n-propyl radical. Using these values and the analysis of the products, the nonterminal addition of H atoms to C3H6 and C3D6 was found to amount to 6 1%. 

Cypionka H, Smock A, BOttcher MA (1998) A combined pathway of sulfur compound disproportionation in Desulfovibrio desulfuricans. FEMS Microbiol Lett 166 181-186 Czamanske GK, Rye RO (1974) Experimentally determined sulfur isotope fractionations between sphalerite and galena in the temperature range 600°C to 275°C. Econ Geol 69 17-25 Dansgaard W (1964) Stable isotope in precipitation. Tellus 16 436 68... 

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

In the absence of substrate, the combination of Fe° and HOOH in dry MeCN results in the latter s rapid disproportionation to O2 and H2O, but the Fe remains unoxidized. Similar combinations of Fe and t-BuOOH or w-ClC6H4C(0)00H do not promote their rapid disproportionation, but facifitate a slow hydrolysis process (ROOH + H2O - ROH + HOOH, assayed by ROH). Because all of the HOOH is consumed in the experiments of Table 10, the reaction efficiency for the Fe -HOOH system represents a cmde measure of the relative rate of reaction for the adduct with substrate and HOOH. Scheme 3 outlines an activation cycle that is based on the argument that within an aprotic matrix the iron(II) center is a strong Lewis acid that weakens the O bond of HOOH. 

Another explanation follows from the above discussion on stereoelectronic factors.If overlap between the semi-occupied orbital and the breaking C-H bond favors disproportionation, then substituents which delocalize the free spin will serve to reduce this interaction and disfavor disproportionation. A proposal along these lines was made originally by Nelson and Bartlett w ho also noted that diminishment of the spin density at C could retard combination. However, it is not necessary that the two effects should cancel one another. 

When [WClgj and Sn(CH3)4 or LiCH3 are used for disproportionation of 2-alkenes at low olefin/W ratios, significant amounts of 1-alkenes corresponding to the combination of CH2 with the appropriate alkylidene derivative of the olefin are obtained, together with large amounts of methane and ethane. 

---