Allylic C-H cleavage

Thereafter, a serial of palladium-catalyzed intramolecular C-ff activation of allylic C-H bonds (Scheme 2.4) was reported by White [9], who first developed a novel route for accessing chiral syn-l,2-amino alcohols enabled through the discovery of a Pd/sulfoxide-catalyzed diastereoselective allylic C-H activation reaction of chiral homoallylic Af-tosyl carbamates. The author discovered that the addition of bis-sulfoxide ligands (L-1) to Pd(OAc)2 promoted allylic C-H cleavage of a-olefins versus oxypalladation in the presence of weak oxygen nucleophiles (i.e., carboxylic acids). Evidences presented that the mechanism proceeds via Pd(ll)/sulfoxide-promoted allylic C-H cleavage to furnish a n-allylic palladium intermediate followed by counterion-promoted functionalization with the tethered Af-tosyl carbamate nucleophile. 

In 2006, the White group further extended this chemistry to the construction of macrocyclic lactones [73]. In the presence of 10-20 mol% of Pd(OAc)2/phenyl bis-sulfoxide and 2 equiv. of BQ, a wide scope of 14-to 17-membered macrocyclic benzolactones could be afforded in moderate yields with outstanding regioselec-tivities under air. With the combination of Pd(0 Ac)2/bis-sulfoxide and phosphoric acid. White and coworkers achieved the synthesis of oxazolidinones from Af-Boc amines in good yields and excellent diastereoselectivities via allylic C-H cleavage (Scheme 3.40) [74]. 

The analogous experiment with 1-pentene also yields a single product as indicated in Table IV. In addition, the product, in which ethylene is eliminated, is different from that observed in the case of 2-pentene In this case, the process with the lowest activation energy does not correspond to the most exothermic process (Table IV). Again the process of lowest energy involves cleavage of the allylic C-C bond in preference to the allylic C-H... 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

A proposed mechanism for the isomerization is illustrated in Fig. 1. The ruthenium complex first coordinates to the olefin and transfers it from a terminal position to an internal position, providing an allyl alcohol [17,18]. The allyl alcohol is then converted to either another allyl alcohol through C-O cleavage (route a) or a ketone through C-H cleavage (route b). 

Azo-ene reactions. The ene reaction provides a powerful method for C-C bond formation with concomitant activation of an allylic C-H bond. A variety of functionalized carbon skeletons can be constructed due to the range of enophiles which can be used. For example, carbonyl compounds give homoallylic alcohols and imino derivatives of aldehydes afford homoallylic amines. The azo-ene reaction offers a method for effecting allylic amination by treatment of an alkene with an azo-diester to afford a diacyl hydrazine which upon N-N cleavage furnishes a carbamate. Subsequent hydrolysis of the carbamate provides an allylic amine. Use of chiral diazenedicarboxylates provides a method for effecting stereoselective electrophilic amination. 

With respect to the mechanistic pathway, as depicted in Scheme 36, after coordination of the allylic halide to Ru-complex 227, giving rise to Ru-complex 229, the C-S bond formation occurs via activation of the allylic C-H bond and subesquent C-H cleavage (step 229—>230). The rotation of the organic moiety around the C-S bond (step 231 —> 232) precedes HX elimination, which is the final step leading to Ru-complex 228. 

The preparative electrochemical oxidation of allylsilanes proceeds smoothly and the C-Si bond is cleaved selectively without affecting other allylic C-H bonds [110-113]. This selectivity is ascribed to the selective cleavage of the C-Si bond in the cation radical intermediate. The resulting allyl radical intermediate is further oxidized to give the allyl cation intermediate, which is trapped by nucleophiles such as alcohols, water, carbamates, and tosylamides to give the corresponding allylic substitution products as shown in Eq. (25). Usually, the nucleophiles are introduced to both ends of the allyl cation, and therefore a mixture of two regioisomeric products is formed. 

When CoCl2 is used as a co-catalyst, the CDC reactions of allylic sp3 C—H and 1,3-dicarbonyl compounds smoothly afford Trost—Tsuji-type products, which are traditionally prepared from allyl halides or acetates (Equation 11.4) [14]. Moreover, when NBS is used instead of TBHP, selective C—H cleavage of tertiary aliphatic amines is possible (Equation 11.5) [15]. For benzylic C—H bonds, direct C—C bond formation can be achieved using copper perchlorate (Equation 11.6) [16]. 

S.8.2.3.8.7t-Allyl Complexes from Allylic C—H Bond Cleavage... 

The cleavage of the allylic C—H bond by palladium is stereospecific e.g., specifically deuterated 4-cholestene (IV) loses a hydrogen or a deuterium syn to the incoming metal atom (Scheme 2) ... 

When the solution of the olefin complex XI is cooled below — 50°C, the hydrido-rt-allylnickel complex, XII, forms. The formation of a ii-allylnickel complex from allylic C—H bond cleavage is demonstrated by olefin XIII which with a dichloro complex of nickel affords the it-allylnickel complex, XIV... 

The metal inserts into the C—H bond and the reaction may be considered as a concerted oxidative addition. This is a rare example of an allyl-hydrido-metal complex being isolated from allylic C—H bond cleavage by the metal. Other examples are the reaction of Mo(0) and Ru(0) complexes with olefins where isolable hydrido-7t-allyl-metal(II) complexes are characterized (vide infra). 

Ill) Iron and Ruthenium. w-Allyliron complexes are not readily prepared directly from olefins by cleavage of an allylic C—H bond, but Jt-allylhydridoiron complexes are intermediates in olefin isomerization catalyzed by Fe(0) carbonyl complexes ". The intermediate 7i-allyliron hydride complexes formed are too reactive to be isolated and undergo a fast reductive elimination to give the isomerized or the starting olefin ... 

Unlike propane, propylene with the electron rich n bond can interact strongly with the acidic metal cations. Indeed, we find that (2+4) (via TSIO) can be an alternative pathway for the cleavage of the allylic C-H bond. TSIO is more favorable on the MoO c than on VO c, indicating that propylene conversion is facilitated by more acidic Mo oxides. Nevertheless, on both cases, the reaction barrier of propylene is 11.5-13.7 kcal/mol more favorable than propane such that it is difficult to stop the reaction at propylene. This finding provides a plausible reason why the yields of propylene are usually lower than 30% during ODHP [60], Furthermore, we also indicated that addition of some acidic oxides, such as MoO and TeO c, can boost the reaction to the a, p-unsaturated products, such as acrylic acid. This is consistent with the experimental observations that the Mo-V-Te-Nb-O system exhibits high reactivity as well as selectivity for the selective oxidation of propane to acrylic acid [24]. 

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