Pericyclic Additions

Fig. 15. Topology of the orbital interactions for the cyclo-addition pericyclic transition state of C2(CN)4 + [Fe(CO)3(l,4-i -C7H8)].

The local softness has been applied with much success in interpreting and predicting the regio-selectivities of different types of organic reactions including radical additions , nucleophilic additions " , pericyclic [2 + i]330-333 2 + 2] " and [3 + 2] " " additions, hydrogen shifts and internal rotations. . ... 

Until recently, N-sulfonyl imines had found only limited and sporadic use in organic synthesis. During the past decade, however, it has become increasingly clear that these species are valuable synthons and are capable of undergoing many unique transformations. A comprehensive review of the chemistry of these compounds is presented here with particular emphasis on their applications in stereoselective processes. Methods for preparing N -sulfonyl imines are outlined, along with a survey of their uses in a wide range of addition, pericyclic and cycloaddition reactions. 

Example Anoth er example of Iron tier orbital theory uses the reaction ol phenyl-butadiene with ph en ylethylene. This reaction is a [4-1-21 pericyclic addition to form a six-membered ring. It could proceed with the two phenyl rings close to each other (head to head) or further away front each other (head to tail). 

Mechanistically the observed stereospecificity can be rationalized by a concerted, pericyclic reaction. In a one-step cycloaddition reaction the dienophile 8 adds 1,4 to the diene 7 via a six-membered cyclic, aromatic transition state 9, where three r-bonds are broken and one jr- and two cr-bonds are formed. The arrangement of the substituents relative to each other at the stereogenic centers of the reactants is retained in the product 10, as a result of the stereospecific y -addition. 

In addition to polar and radical reactions, there is a third, less commonly encountered process called a pericyclic reaction. Rather than explain pericyclic reactions now, though, we ll look at them more carefully in Chapter 30. 

In a more general view, the most promising choices in this selection of pericyclic carbonyl additions are allylboronates [M = B(OR)2, Section D.1.3.3.3.3.], allyltitanates [M = Ti(OR)3,... 

Since the double-bond configuration is established in the final elimination step from a /t-silicon-(or tin-) substituted carbenium ion in a conformation of lowest energy, often high E selectivity is observed. In reactions of allylstannanes, catalyzed by tin(TV) chloride or titanium(IV) chloride, occasionally a metal exchange occurs, followed by the pericyclic addition pathway leading to the iwti-diastereomers17 19. A more detailed discussion is given in Section D.1.3.3.3.5. 

As a consequence of the pericyclic reaction path, the addition of a-stereogenic allylmctals to carbonyl compounds is accompanied by an effective 1,3-chirality transfer in the allylic moiety combined with 1,4-chira induction at the prostereogenic carbonyl group3032. The scheme also demonstrates the importance of the orientation of the substituent X in the six-membered transition state. By changing from a pseudo-axial to a pseudo-equatorial position, the cation-induced sy/i-attack addresses opposite faces of both prostereogenic moieties, leading to a Z-and an -isomer, these being enantiomeric in respect to the chiral moiety. 

Diisopropylamino(dimethyl)silyl]-2-propenyl]lithium adds to aromatic and x-branched aldehydes in the presence of anhydrous zinc chloride with essentially complete anti stereoselectiv-ity3s. as expected from the chair-like pericyclic transition state formed by the ( -intermediate. The addition products are not isolated, but after O-silylation, oxidative desilylation with retention of configuration forms the rmft-diols. 

In spite of the steric and electronic differences, the nucleophilic addition of enolates to imines (aza aldoi reaction) proceeds probably through similar pericyclic transition states as the carbonyl compounds do. 

Jones and coworkers200 found that a variety of sulphenic acids may be generated by thermolysis of the readily available /J-cyanosulphoxides (equation 81) and observed their highly regiospecific addition also to non-conjugated alkynes (Table 12). As expected for a pericyclic mechanism, the reaction afforded the product of a stereospecific cis-addition. However, the regioselectivity of the addition suggests that the partial carbon-sulphur bond in the transition state 148 is polarized in such a way that the carbon atom has some cationic character (equation 82). 

The HX compounds are electrophilic reagents, and many polyhalo and polycyano alkenes, (e.g., Cl2C=CHCl) do not react with them at all in the absence of free-radical conditions. When such reactions do occur, however, they take place by a nucleophilic addition mechanism, (i.e., initial attack is by X ). This type of mechanism also occurs with Michael-type substrates C=C—Z, where the orientation is always such that the halogen goes to the carbon that does not bear the Z, so the product is of the form X—C—CH—Z, even in the presence of free-radical initiators. Hydrogen iodide adds 1,4 to conjugated dienes in the gas phase by a pericyclic mechanism ... 

The novel [6+2] annulation approach developed by the Takeda group has also been included in a threefold anionic/pericyclic process (Scheme 2.149) [340]. The reaction leads to functionalized eight-membered rings 2-659 in a highly stereoselective manner, starting from acylsilanes 2-656 and 3-(trimethylsilyl)vinyl-lithium (2-657). After 1,2-addition and 1,2-Brook rearrangement, the cyclobutane 2-... 

The combination of an anionic and a pericyclic process has found broad application in synthetic organic chemistry. In these transformations, a primary addition is frequently followed by an elimination to give a reactive intermediate, which is then... 

The reported proposed sequence also offers two additional alternative mechanisms for the cyclodimerization of BCP (3), involving either intermediate 463 or 464 [6a, 13b]. However, they appear less likely, requiring successive three-membered ring fissions and formations. Alternatively, a thermally allowed concerted [jt2s + rt2a -I- pericyclic reaction involving the Walsh type molecular orbital of cyclopropane [124] has been proposed (Fig. 4) [13b]. 

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