C-Cleavage

At temperatures >300° C, substituted pyrroHdines can be obtained by reaction of substituted a2iridines (R = CH, C2H ) and conjugated olefins X = CN, CO2CH2, CH=CH2) with C—C cleavage in the three-membered ring (214—216). 

Oxirane on thermolysis or photolysis suffers C—O homolysis to give a plethora of products (Scheme 2). Substituted oxiranes behave similarly on thermolysis although some C—C cleavage is observed (Scheme 3). Cyclopentene and cyclohexene oxides undergo only C—O cleavage (Scheme 4). 

Jeger, Wolf and coworkers extensively studied the photochemistry of butadienyloxiranes (81HCA198) and oxiranyl enones (81HCA224). Butadienyloxiranes give both C—O and C—C cleavage (Scheme 13) the paths open to oxiranyl enones have been systematically analyzed... 

The most important connective transforms in retrosynthetic analysis are the family of C=C cleavage transforms, including one-step (e.g. ozonolytic) or two-step (e.g. OSO4 followed by Pb(OAc)4) (Chart 25). There are many elegant syntheses of challenging molecules which depend on such processes. Two examples will provide an idea of the underlying retrosynthetic approach. 

C. Cleavage of Bonds Attaching Atoms or Groups to the a-Carbon of Ketones... 

The cycle shown in Figure 20.4 at first appears to be a complicated way to oxidize acetate units to COg, but there is a chemical basis for the apparent complexity. Oxidation of an acetyl group to a pair of COg molecules requires C—C cleavage ... 

In many instances, C—C cleavage reactions in biological systems occur between carbon atoms a- and /3- to a carbonyl group ... 

Another common type of C—C cleavage is a-cleavage of an a-hydroxy-ketone ... 

It is worth noting that the carbon-carbon bond cleaved in the TCA pathway entered as an acetate unit in the previous turn of the cycle. Thus, the oxidative decarboxylations that cleave this bond are just a cleverly disguised acetate C—C cleavage and oxidation. 

Eaborn, C., Cleavages of aryl-silicon and related bonds by electrophiles, <7. Organomet. Chem. 100, 43 (1975). 

Alkoxy (R0 ) radicals react at near diffusion controlled rates with trialkyl phosphites to give phosphoranyl radicals [ROP(OR )3] that typically undergo very fast -scission to generate alkyl radicals (R ) and phosphates [OP(OR )3]. In a mechanistic study, trimethyl phosphite, P(OMe)3, has been used as an efficient and selective trap in oxiranylcarbinyl radical systems formed from haloepoxides under thermal AIBN/n-Bu3SnH conditions at about 80 °C (Scheme 27) [64]. The formation of alkenes resulting from the capture of allyloxy radicals by P(OMe)3 fulfils a prior prediction that, under conditions close to kinetic control, products of C-0 cleavage (path a. Scheme 27), not just those of C-C cleavage (path b. Scheme 27) may result. 

C-C formation. C-C cleavage CH + 2 PhCsCl h CPhCHCPhCPh - CPh (Mo Coi/(<-CPhH/(2-) -CPhCHCPhCPh)iCOi i e-CsHs)2l M02C0 face ss... 

Strategy XII Reconnections Synthesis of 1,2- and 1,4-Difunctionalised Compounds by C=C Cleavage... 

The oxidations of these by Ce(IV), Mn(III) and V(V) have been studied extensively . Kinetic data are summarised in Table 13. The main tasks are those of (/ ) discriminating between C-C cleavage and oxidation of >CHOH groups to >C-0 and (//) assessing the role of cyclic complexes. 

The acidity dependences of V(V) oxidations are significant. That of pinacol , which undergoes 100% C-C cleavage, is a+bh ). The first (acid-independent) term is rare in V(V) oxidations and implies that V02 is the active oxidant the second term implies, on the basis of the Zucker-Hammett hypothesis, that the transition state has the structure (J5), the mechanism being... 

Evidence concerning the relative extents of C-C and C-H fission is less well defined for Ce(IV) and Mn(III) as compared with V(V). Pinacol is cleaved to acetone in all cases, but while Mn(IlI) pyrophosphate [like V(V)] oxidises pinacol much faster than butane-2 3-diol, the rate ratio with Ce(IV) is only approximately 3 and the production of acetaldehyde from butane-2 3-diol by Ce(IV) oxidation demonstrates C-C cleavage . It is probable, therefore, that Mn(III) oxidises the disecondary glycol by C-H fission. 

This is an typical example of a dicarboxylic acid in that C-C cleavage is the only route for oxidation. No study of the Co(III) oxidation has been made although it is highly probable that reaction would proceed through an oxalate complex. The thermal decomposition of Co(Ox)3 has been shown to be a first-order process and probably involves an internal redox reaction, viz. 

The target of vanadate-catalyzed photolysis is presumably an amino acid near the catalytic site of the Ca " -ATPase. The vanadate-catalyzed photocleavage at the V cleavage site in the absence of Ca " is 500 amino acids away from the C cleavage site which is attacked in the presence of Ca. Both sites are probably adjacent in the native structure to the catalytic site of the Ca -ATPase. 

Several supported metalhc catalysts were evalrrated for the selective hydrogenolysis of glycerol. Initially, the reactions were performed tmder acidic conditions in order to promote the formation of 1,3-PDO. Rutheniirm-based catalysts were found to be the most active catalysts but significant amount of tmdesired products resulted from C-C cleavages were detected. On the contrary, Rh/C catalysts were found selective to C-O cleavages. As far as the selectivity to 1,3-PDO was concerned, we previously reported that the addition of iron salts in the medium improved the l,3-PDO/l,2-PDO selectivity (11). A systematic study on the influence of additives was therefore carried out in the present investigation. Mineral and organic acids were evaluated for this purpose (Table 35.1). 

However, there are several experimental observations which are against an interpretation of the reaction as being a one-step C—C cleavage process which would give rise to the formation of the relatively unstable a-oxocarbenium ion 28. 

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