Carbanion capture process

Kagechika, K. and Shibasaki, M. (1991) Asymmetric Heck reaction a catalytic asymmetric synthesis of the key intermediate for A -capneUene-3i3,8j8,10o -triol and A -capnellene-3/S,8/3,10a,14-tetrol. J. Org. Chem., 56,4093 (b) Kagechika, K., Ohshima, T. and Shibasaki, M. (1993) Asymmetric Heck reaction-anion capture process. A catalytic asymmetric synthesis of the key intermediates for the capneUenols. Tetrahedron, 49, 1773-82 (c) Ohshima, T., Kagechika, K., Adachi, M., et al. (1996) Asymmetric Heck reaction-carbanion capture process. Catalytic as5mmetric total synthesis of (—)-A LcapneUene. J. Am. Chem. Soc., 118, 7108-16 (d) Itano, W., Ohshima, T. and Shibasaki, M. (2006) Synthesis of the tricyclic core of 5a-capneUenols using asymmetric Heck reaction-carbanion capture process. Synlett, 3053-6. 

Ohshima, T., Kagechika, K., Adachi, M. et al. (1996) Asymmetric Heck reaction-carbanion capture process. Catalytic asymmetric total synthesis of (—)-A -capnellene. J. Am. Chem. Soc., 118, 7108-16. 

Itano, W., Ohshima, T. and Shibasaki, M. (2006) Synthesis of the tricyclic core of 5a-capnellenols using asymmetric Heck reaction-carbanion capture process. Synlett, 3053-6. 

More general procedures for additions of halogen fluorides to highly fluori-nated olefins involve reactions with a source of nucleophilic fluoride ion, such as an alkali metal fluoride, in the presence of aposttive halogen donor [62 107, lOff, 109, 110, 111] (equations 11 and 12) These processes are likely to occur by the generation and capture of perfluorocarbamonic intermediates Tertiary fluormated carbanions can be isolated as cesium [112], silver [113], or tns(dimethylamino)sul-... 

Since the decarboxylation of 161 to 162 proceeds in a poor yield, it was suggested that formation of 162 in benzene occurs directly from 2-benzopyrylium salts 62 through primary nucleophilic attack by amine in position 3. In this case, an enamine fragment of pyruvic acid appears in the ring-opened intermediate 163, which undergoes easy decarboxylation (82TL459). The vinylic carbanion 164, formed by the loss of carbon dioxide, captures a proton by intra- or intermolecular process, then hetero-cyclization takes place. 

On the other hand, at high concentrations of acetaldehyde, when the intermediate enolate carbanion is rapidly captured by another molecule of aldehyde, reverse of the initial parallel proton-abstraction steps is prevented (k3[CH3CHO] k-1 and k 2[BH 1 ]), and the rate of the overall reaction is effectively limited by the initial proton abstractions these then constitute (parallel) rate-limiting steps. The overall process is now first order in acetaldehyde and shows general-base catalysis [5], i.e. the rate law is given by Equation 3.13 ... 

Gersmann et al.74 suggested another mode of initiation, which proceeds by deprotonation of RH to a carbanion that transfers one electron to an oxygen molecule capture of oxygen produces a peroxy radical that oxidizes another molecule of the initial carbanion, the last two steps constituting a chain-propagation process. In hydrocarbon oxidations, Russell73 also showed that reactions of carbanions with 02 proceed via a two-step one-electron transfer pathway [Eqs. (29)-(32)]. 

Deprotonation of the chiral 1,2-oxazine (34) by n-butyllithium proceeds with a high degree of stereoselectivity cis to the C-6 substituent subsequent capture of this carbanion with carbonyl compounds also proceeds syn to the C-6 substituent, so that the overall process occurs with retention of configuration at C-4 (Scheme 16). Although the related 1,2-oxazine (3 has not been condensed with carbonyl compounds, it is useful to note that the regioselectivity of its deprotonation can be easily controlled by the size of the base employed. Bulky amide bases preferentially abstract the proton at the exocyclic methyl group, whereas small amide bases such as lithium dimethyiamide preferentially abstract a proton at C-4. ... 

Another interesting synthetic application of the Birch reduction is the capture of the in itfM-generated carbanion with a carbon electrophile (usually an alkyl halide) forming a new carbon-carbon bond [9] this process has also been studied in an asymmetric manner [10]. 

Sodium is available as cast ingots that can be cut into small pieces with a knife. When more surface area is needed, it can be finely pulverized by stirring it in molten form in refluxing toluene, and then cooling. This was done for the process in Equation 9.14 [51], where the cooled toluene was replaced with ether once the sodium was prepared. Trimethylchlorosilane is slow to react with carbanions therefore, it can be present in a reaction mixture where it will capture oxyanions as they are formed. 

From a theoretical point of view, the key issue has been the basic nature of the metalation step, where the R groups moves from a R -H bond to a M-R bond. C-H activation is very common in organic chemistry as it allows the formation of functionalized hydrocarbons. Different mechanisms had been proposed for this metalation step, including electrophilic aromatic substitution, a-bond metathesis, oxidative addition/reductiveelimination and Heck-like insertion. Theoretical studies have facilitated narrowing the mechanistic possibilities to two main options oxidative addition/reductive elimination and proton abstraction by a base. In the oxidative addition/reductive elimination process the metal is inserted in the C-H bond with formal increase in the oxidation state of the metal, and the hydride leaves the metal coordination sphere of the metal afterwards. In the proton abstraction mechanism, the metal does not interact directly with the proton, which is captured by a base, with simultaneous formal creation of a carbanion that binds to the metal center. The mechanism of the reaction will depend on the presence of a base able to abstract the proton and of the existence of an energetically accessible oxidation state for the metal. 

---