Additions to Electron-Deficient Alkenes

In addition to electron-deficient alkenes, under the catalysis of TiCLt, 1,2-allenylsi-lanes can react with aldehydes or N-acyliminium ion to afford five-membered vinylic silanes 71 and 72. Here the carbocations generated by a Lewis acid regiospecifically attack the C3 of the 1,2-allenylsilanes to produce a vinyl cation stabilized by hyper-... 

Asymmetric Nucleophilic Addition to Electron Deficient Alkenes... 

A number of techniques are now available allowing the preparation of enantiomerically pure (or at least enriched) compounds via asymmetric nucleophilic addition to electron-deficient alkenes. Some of these transformations have already been successfully applied in total synthesis. In most cases, the methods are based on diastereoselective reactions, employing chirally modified substrates or nucleophiles. There are only very few useful enantioselective procedures accessible so far. The search for efficient en-antioselective methods, especially for those which are catalytic and do not require the use of stoichiometric amounts of chiral auxiliaries, remains a challenging task for the future. 

The analysis of the syn-addition of molecular fluorine to ethylene at the MP2/6-31 +G level with IRC calculations indicates that F2 approaches the C=C bond vertically at the middle to form a perpendicular complex 38 as the intermediate. The latter complex then re-orientates to a rhombic-type transition state 39 to give the final syn-addition product 4084. This analysis rules out the involvement of the square-type complex 41 proposed earlier. However, these calculations do not clarify the F2 addition to electron-deficient alkenes, such as acrylonitrile84. 

Scheme 38 Asymmetric addition to electron-deficient alkenes.

Radicals with nucleophilic character add to electrophilic alkenes more rapidly than to nucleophilic alkenes whilst, conversely, the rate of addition of electrophilic radicals to electron-rich alkenes is greater than addition to electron-deficient alkenes. To be more sophisticated, we should refer to a high-SOMO (singly occupied molecular orbital nucleophilic) radical interacting favourably with a low-LUMO (lowest unoccupied molecular orbital electrophilic) alkene but, for simplicity and brevity, we will continue to use these short-hand terms [177]. For instance, compare rates of addition of perfluor-oalkyl radicals to ethene and various fluorinated derivatives with their rates of H-atom abstraction from heptane (Table 7.9) [186]. Broadly, reactivity of the alkene decreases with fluorine content, with trifluoromethyl having a large effect. 

An excess (5 1 mole ratio) of ethyl diazoacetate is used in these reactions to suppress cyclobutadimerization or Diels-Alder cyclodimerization. In difunctional molecules which have non-equivalent ionizable functionalities, cyclopropanation is highly selective for the more easily oxidized functionality. The latter selectivity is perhaps the most attractive aspect of the reaction. In contrast to transition metal (e.g. rhodium) catalyzed cyclopropanations, cation radical additions to electron deficient alkene moieties do not occur at all. The reaction is relatively sensitive to... 

Schmalz, H.-G. Asymmetric Nucleophilic Addition to Electron Deficient Alkenes. in Comp. Org. Synth, (eds. Trost, B. M.,Eleming, I.), 4, 199-236 (Pergamon, Oxford, 1991). 

Narasaka and Sakurai found that chromium carbene complexes, when exposed to a copper(II) reagent, generate acyl radicals by a one-electron oxidation, and these then undergo addition to electron-deficient alkenes (Scheme 4-27) [50J. The resulting copper(I) species reduces the resulting radical to an anion, and subsequent protonation leads to the addition product. This redox type acyl radical transfer reaction works particularly well for aromatic acyl radical systems, for which decarbonylation is not a problem. Related work has also recently appeared [51]. 

The acyl radical has a nucleophilic character in terms of rapid rates of addition to electron-deficient alkenes. The rate of addition of an acyl radical to acrylonitrile... 

PhotoadditioH of organohalides to alkenes. CuBr PBu is an effective catalyst for the regioselective photoinduced addition to electron-deficient alkenes. On reduction with zinc dust the overall result is the same as the conjugate addition using an organocuprate. However, in many cases this method is more convenient. 

Aryl radicals, addition to electron-deficient alkenes, 62, 67... 

Miyake Y, Nakajima K, Nishibayashi Y (2012) Visible-light-mediated utilization of a-aminoalkyl radicals addition to electron-deficient alkenes using photoredox catalysts. J Am Chem Soc 134 3338-3341... 

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