Stereochemistry zwitterionic -cycloaddition

The stereochemistry of these reactions depends on the lifetime of the dipolar intermediate, which, in turn, is influenced by the polarity of the solvent. In the reactions of enol ethers with tetracyanoethylene, the stereochemistry of the enol ether is retained in nonpolar solvents. In polar solvents, cycloaddition is nonstereospecific, as a result of a longer lifetime for the zwitterionic intermediate.177... 

Reactions of 1 with epoxides involve some cycloaddition products, and thus will be treated here. Such reactions are quite complicated and have been studied in some depth.84,92 With cyclohexene oxide, 1 yields the disilaoxirane 48, cyclohexene, and the silyl enol ether 56 (Eq. 29). With ( )- and (Z)-stilbene oxides (Eq. 30) the products include 48, ( > and (Z)-stilbenes, the E- and Z-isomers of silyl enol ether 57, and only one (trans) stereoisomer of the five-membered ring compound 58. The products have been rationalized in terms of the mechanism detailed in Scheme 14, involving a ring-opened zwitterionic intermediate, allowing for carbon-carbon bond rotation and the observed stereochemistry. 

Also known are 2 + 2 cycloadditions proceeding by way of a bipolar ion, path (b) of Scheme l.28 These reactions occur in situations such as that depicted in Equation 12.14, where the intermediate zwitterion (10) is strongly stabilized. Tetracyanoethylene adds by this mechanism to /Mnethoxyphenyl-,29 alkoxyl-,30 and cyclopropyl-31 substituted olefins. The additions show large solvent effects.32 Partial loss of stereochemistry occurs as in the biradical cases, but it is much less pronounced. 

Abstract The main computational studies on the formation of (3-lactams through [2+2] cycloadditions published during 1992-2008 are reported with special emphasis on the mechanistic and selectivity aspects of these reactions. Disconnection of the N1-C2 and C3-C4 bonds of the azetidin-2-one ring leads to the reaction between ketenes and imines. Computational and experimental results point to a stepwise mechanism for this reaction. The first step consists of a nucleophilic attack of the iminic nitrogen on the sp-hybridized carbon atom of the ketene. The zwitterionic intermediate thus formed yields the corresponding (3-1 actant by means of a four-electron conrotatoty electrocyclization. The steroecontrol and the periselectivity of the reaction support this two-step mechanism. The [2+2] cycloaddition between isocyanates and alkenes takes place via a concerted (but asynchronous) mechanism that can be interpreted in terms of a [n2s + (n2s + n2s)] interaction between both reactants. Both the regio and the stereochemistry observed are compatible with this computational model. However, the combination of solvent and substituent effects can result in a stepwise mechanism. 

A number of 2-iminothietanes have been prepared from ketenimines and thio-benzophenone ° or thioxanthone. Addition of carbon oxysulfide or carbon disulfide to the phosphonium ylide of a ketenimine 312 gives an imino- 3-dithiolactone as previously noted.Arylsulfonylisothiocyanates undergo cycloaddition at 50°C to vinyl ethers to give 2-iminosulfonylthietanes in 50-76% yields.A ( 2g -I- 2 ) process involving zwitterions was suggested on the basis of the observed stereochemistry. 

The cycloaddition of A -sulfonyl amines (Af-sulfonyl imides) (e.g., 431 ) to alkenes gives 1,2-thiazetidine 1,1-dioxides. " The stereochemistry about the double bond of the alkene is preserved in the adduct, and a tight zwitterionic intermediate is favored. Considerable amounts of six-membered cyclic adducts also are formed in these additions. Yields vary from good to poor. Thermolysis of 432 yields sulfonyl amine 433 the higher reaction temperatures involving 433 enable less reactive alkenes such as cis-stilbene to be used successfully. The success of cycloadditions to enamines depends on the latter having no protons on the -alkene carbon or on an sp hybridized carbon attached to the a-position. ... 

Therefore, some conclusions have been generally accepted and have been summarized as follows the cycloaddition reaction is a stepwise reaction rather than a concerted one the reaction is initiated by nucleophihc attack of an imine to a ketene, giving rise to a zwitterionic intermediate a conrotatory eleclrocyclic ring-closure of the zwitterionic intermediate produces the final 2-azetidone product [85], As the stereochemistry of the structure of the P-lactams strongly affects their biological activity, the stereoselectivity of the process must be carefully considered. Uncatalysed as well as catalysed processes have been reported organometallic and organic catalysts have been utilized in procedures oriented to the syntheses of enantiopure P-lactams [90-92],... 

Careful application of the three simple postulates listed above can yield insight into the mechanism and stereochemistry of biradical reactions as complex as the thermal dimerization of cis, irons-1,5-cyclooctadiene [26] or the isomerization of allyl-substituted cyclopropanes via internal [2 + 2]-cycloaddition [27]. An attempt to do so here would take us too far afield, in view of the ease with which biradical intermediates interconvert. Instead let us move on to the considerably more stereoselective cycloaddition of reactant pairs with complementary polarity, that proceeds stepwise along a zwitterionic pathway. [23]... 

Meanwhile, however, experimental support for cycloaddition via a stepwise zwitterionic mechanism was being accumulated by Moore and his coworkers, [44] culminating in a study in which the zwitterionic intermediate was generated independently from a cyclic precursor and shown to produce a cyclobu-tanone with the same stereochemistry as the cycloaddition product of er -butyl cyanoketone (TBCK) and mw -trimethylsiloxypropene. [45]... 

---