Electrostatic activation, directe

Miscellaneous Iminium Catalyzed Transformations The enantioselective construction of three-membered hetero- or carbocyclic ring systems is an important objective for practitioners of chemical synthesis in academic and industrial settings. To date, important advances have been made in the iminium activation realm, which enable asymmetric entry to a-formyl cyclopropanes and epoxides. In terms of cyclopropane synthesis, a new class of iminium catalyst has been introduced, providing the enantioselective stepwise [2 + 1] union of sulfonium ylides and ot,p-unsaturated aldehydes.As shown in Scheme 11.6a, the zwitterionic hydro-indoline-derived catalyst (19) enables both iminium geometry control and directed electrostatic activation of sulfonium ylides in proximity to the incipient iminium reaction partner. This combination of geometric and stereoelectronic effects has been proposed as being essential for enantio- and diastereocontrol in forming two of the three cyclopropyl bonds. 

Enantioselective organocatalytic cyclopropanations have been performed using directed electrostatic activation conditions.164 Using a new class of iminium catalysts, cyclopropanation has been conducted with enals but not electron deficient alkenes, such as unsaturated nitrile, nitro, or alkylidene malonate systems. 

A novel method for enantioselective organocatalytic cyclopropanation has been developed, using a new class of iminium intermediates and based on the concept of directed electrostatic activation (DEA). This novel organocatalytic mechanism exploits dual activation of ylide (153) and enal (152) substrates through the formation of the iminium intermediate (155) and electrostatic activation (156). The resulting (g) trisubstituted cyclopropanes (157) were obtained with high levels of enantio- and diastereo-control.180... 

In an effort to explain these atypical reactivity patterns, a mechanistic postulate based on direct electrostatic activation (DEA) was proposed (Scheme 3.8). Indeed, the zwitterionic iminium ions derived from catalyst 10 and a,/i-un saturated aldehydes enable both iminium geometry control and direct electrostatic activation of the approaching sulfonium ylides. The combination of geometric and electronic control seems to be essential for enantio- and diastereocontrol in the formation of the desired cyclopropyl compound. 

Kunz RK, MacMillan DWC (2005) Enantioselective organocatalytic cyclopro-panations. The identification of a new class of iminium catalyst based upon directed electrostatic activation. J Am Chem Soc 127 3240-3241 Lacour J, Hebbe-Viton V (2003) Recent developments in chiral anion mediated asymmetric chemistry. Chem Soc Rev 32 373-382 Li X, List B (2007) Catalytic asymmetric hydrogenation of aldehydes. Chem Commun 17 1739-1741... 

An alternative to the continuum model is to model the protein electrostatic field directly. This is normally achieved by carrying out a hybrid quantum mechanics/molecular mechanics (QM/MM) treatment in which the active site model is treated with DFT while the rest of the protein is modelled via classical molecular mechanics (MM) [44]. The MM charges then implicitly define the dielectric inside the protein. In either event, it appears that excessive charges are to be avoided. That is, the quantum mechanical part of the model system will normally not have an overall charge greater in magnitude than one. 

In 2005 Kunz and MacMillan reported the use of 2-carbo gflic acid dihydroindole 38 as a directed electrostatic activation (DEA) catalyst for the cyclopropanation reaction of a,(3,-unsaturated aldehydes with stabilised sulfur ylides (Scheme 11.40). ... 

Scheme 11.40 As3mmetric q clopropanation reaction of a,p,-unsaturated aldehydes with sulfur ylides catalysed by the directed electrostatic activation catalyst 38.

Indoline-2-carboxylic acid /9S767-06-7/M 163.2, m 177"(dec), [a] +11.0" (c 0.3, MeOH)- Purify it as for the S-enantiomer below. The R-(+)-hydrochloride has [172152-19-1]. It is a proline based oiganocatalyst which promotes the enantioseleetive formation of cyclopropanes by reaction between 2-(dimethyl-X -sulfanylidene)-l-phenyl-ethanone (for stable sulfonium ylides see Ratts Yao J Org Chem 31 1185 1966) and but-2-enals in high yields and very high stereoselectivity involving Direct Electrostatic Activation (DEA) [Kunz Mac Millan JA/w Chem Soc 127 3240 2005.]... 

Scheme 1.4 MacMillan s directed electrostatic activation in the cyclopropanation of an a, P-unsaturated aldehyde...

Sequential Iminium-Enamine Catalysis. Directed Electrostatic Activation. A comparison of the standard catalytic cycles for enamine activation (Scheme 2.1) and for iminium ion activation (Scheme 2.12) show that iminium catalysis proceeds, after the addition of the nucleophile, via an ( )-enamine. In the presence of a suitable electrophile, this enamine gives rise to an iminium ion that after hydrolysis can give rise to an a,p-diftmctionalyzed carbonyl (Scheme 2.13) [85]. Scheme 2.13 also shows that when using a chiral 2-substituted pyrrohdine or an imidazolidinone as the catalyst, the sequential apphcation of the steric model for Michael addition to iminium ions (Figure 2.15) and of the steric model for electrophilic attack to enamines (Figure 2.IB) predicts the absolute stereochemistry of the major isomer obtained in the reaction. 

FIGURE 2.21. Transition state models for directed electrostatic activation (DEA). 

R. K. Kunz, D. W. C. MacMillan, J. Am. Chem. Soc. 2005, 127, 3240-3241. Enantioselective organocatalytic cyclo-propanations. The identification of a new class of imininm catalyst based upon directed electrostatic activation. 

A second-generation catalyst in which the carboxylic acid of (S)-(-)-indoline-2-carboxylic acid was replaced by tetrazolic acid was used to improve enantioselectiv-ity as a consequence of increased steric bulk while retaining important structural functionahty associated with the proposed directed electrostatic activation mode [101], Combination of the iminium catalysis with arsonium yhdes also provided access to cyclopropanes with high enantioselectivity [102]. 

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