Antiperiplanar transition state

An open-chain antiperiplanar transition state was initially proposed for this reaction74, although a synclinal alternative has since been suggested56. 

The stereoselectivity of these intermolecular reactions between 1-alkoxyallylstannanes and aldehydes induced by boron trifluoride-diethyl ether complex is consistent with an open-chain, antiperiplanar transition state. However, for intramolecular reactions, this transition state is inaccessible, and either (Z)-.yyn-products are formed, possibly from a synclinal process105, or 1,3-isomerization competes113. Remote substituents can influence the stereoselectivity of the intramolecular reaction114. 

The diastereoselectivity observed can be explained by a synclinal transition state, probably influenced by chelation and/or stereoelectronic effects of the developing cation38. The minor product is formed via an antiperiplanar transition state. All compounds obtained are useful precursors for several spirocyclic natural products, such as terpenes like lubimine or acoradi-ene. 

Here the P-hydrogen and the chlorine are both axial. This allow an antiperiplanar transition state. 

The stereoselectivity proved to be dependent on the nature of the halide ligand X and was found to be complete for X = Br. Reversal of the diastereoselectivity from anti to syn has been accomplished by adding Et2OBF3 [37]. The predominant syn selectivity in the presence of this Lewis acid is consistent with a non-cyclic antiperiplanar transition state. 

Chiral allenyltitanium reagents, prepared from propargylic phosphates as outlined, react with alkylidene malonates to afford 1,4-adducts with excellent anti dia-stereoselectivity (Table 9.26) [42]. The addition is presumed to take place through an open antiperiplanar transition state (Scheme 9.12). 

Darzens reaction of (-)-8-phenylmethyl a-chloroacetate (and a-bromoacetate) with various ketones (Scheme 2) yields ctT-glycidic esters (28) with high geometric and diastereofacial selectivity which can be explained in terms of both open-chain or non-chelated antiperiplanar transition state models for the initial aldol-type reaction the ketone approaches the Si-f ce of the Z-enolate such that the phenyl ring of the chiral auxiliary and the enolate portion are face-to-face. Aza-Darzens condensation reaction of iV-benzylideneaniline has also been studied. Kinetically controlled base-promoted lithiation of 3,3-diphenylpropiomesitylene results in Z enolate ratios in the range 94 6 (lithium diisopropylamide) to 50 50 (BuLi), depending on the choice of solvent and temperature. ... 

It appears likely that the reaction proceeds through the ene reaction pathway, although such an ene reaction pathway has not been previously recognized as a possible mechanism in the Mukaiyama aldol reaction. In general, an acyclic antiperiplanar transition-state model has been used to explain the formation of the syn-diastereomer from either ( )- or (Z)-silyl enol ethers [58]. However, the cyclic ene mechanism now provides another rationale for the. vyra-diastereose-lection regardless of the enol silyl ether geometiy (Figure 8C.7). 

It is suggested that steric effects tend to destabilize the antiperiplanar transition state normally associated with the formation of syn adducts in such reactions (Fig. 9). The alternative synclinal arrangement might benefit from favorable HOMO-LUMO interactions (see Fig. 3). 

A more recent study reached a similar conclusion [78]. It was found that cycliza-tions of (Z)- and ( )-3-phenyl-8-tributylstannyl-6-octenal were highly diastereoselec-tive (Fig. 17). The (Z) isomer yielded cis, fra/js-3-phenyl-2-vinylcyclohexanol as the major product (96 4) whereas the (E) isomer afforded the trans, trans isomer (95 5). A favorable HOMO-LUMO interaction was proposed as a decisive factor in stabilizing the favored synclinal transition states. This stabilization is lacking in the alternative synclinal and antiperiplanar transition states, neither of which has the correct geometry for orbital overlap. As in the previous study, the aldehyde and double bond substituents (asterisked carbons in Fig. 17) are unable to attain an anti orientation in the antiperiplanar transition states, as has been proposed for the intermolecular additions. 

Reaction of crotylchromium with a-methyl-p,7-unsaturated aldehyde (87) afforded (88) as the major diastereomer. The other Cram product (89), which is expected to arise from an antiperiplanar transition state (46 Scheme 3), is obtained from a BFs-catalyzed tributylcrotylstaimane addition. The remaining members of the stereo triad can be accessed by inversion of the C-2 hydroxy (i.e. 88 to 91 and 89 to 90)... 

The stereochemical outcome of these reactions has been interpreted on the basis of two types of transition state models. These models use very different orientations of the reacting double bonds to explain the stereoselectivities. The first is referr to as an open or extended antiperiplanar transition state... 

Denmark studied the intramolecular allylation reaction of allylstannane 15 in order to differentiate between the syn and anti S g transition states (only anti S h is shown below) as well as to differentiate between the synclinal and antiperiplanar transition states 19 and 20, which are analogous to transition states 11 and 14, respectively (Eq. (11.1)) [51]. Denmark found that the major product of the BF3 OEt2-promoted reaction of 15 was adduct 16, which must arise from the synclinal, anti S e transition state 19. The minor adduct 17 must ari.se through the antiperiplanar transition state 20. 

Denmark argued that the synclinal transition state 19 may be favored due to stabilization by stereoelectronic effects such as secondary orbital overlap or minimization of charge separation. The allylstannane HOMO and the aldehyde LUMO could participate in. secondary orbital overlap in transition state 19, with specific-interactions between the allylstannane a-carbon and the aldehyde oxygen [50, 55]. Alternatively, the preference for the synclinal transition state 19 can also be attributed to minimization of charge separation in the transition state, compared to the situation in the antiperiplanar transition state 20 [50, 56],... 

Throughout this review, we will generally invoke the Denmark-Keck synclinal transition state 11 rather than the Yamamoto antiperiplanar transition state 12 in our analysis of Type II allylation reactions, in spite of the fact that many research groups favor the antiperiplanar transition state 12 when rationalizing their results (see below). As previously noted by Keck, subtle changes in electronics, steric interactions of the reacting partners, and the Lewis acid that promotes the reaction ultimately determine whether one of the transition states, 11 or 12, is favored over the other [53]. 

The analysis of open transition states in the chelate-controlled allylation reactions of a- and ff-alkoxy aldehydes with Type II allylmetal reagents is much simpler (Fig. 11-5). In these cases, only the synclinal transition state 21 and the antiperiplanar transition state 22 are considered as viable possibilities. Other possible transition states have been eliminated because of the perceived requirement that... 

The stereochemistry of adduct 106 is consistent with formation through either the Felkin, synclinal transition state 109 or the Felkin, antiperiplanar transition state 110 (Fig. 11-11). A discussion of the merits of these two transition states appears earlier in this chapter (Section 11.1.2). 

The observed selectivity for the 5,6-syn adduct 348 can be explained by either the anti Sg synclinal transition state 350 or the anti Se antiperiplanar transition state 351 (Fig. 11-33). It is generally accepted that the silicon substituent adopts a position anti to the incoming electrophile anti Sy so as to minimize steric interactions and to maximize the stereoelectronic effects (3d 2p donation by the silicon substituent) [9, 52]. When explaining the selectivity for products 348, Panek... 

As shown in Table 11-20, unbranched aldehydes (e.g. CH3CHO) react much less selectively with chiral crotylsilanes, where the anti adduct 349 is the minor product. This observation can be explained by an increase in product formation through the antiperiplanar transition state 352, which places the ) -methyl group of the crotylsilane in a position gauche to the aldehyde R group. Thus, as the size of the aldehyde R group decreases, transition state 352 becomes more viable. 

Reaction of the same aldehyde 97c with enantiomeric crotylsilanes (5)-217 (R=Me, Et) results in preferential formation of the syn,nnft-dipropionates 359. These adducts can arise either through the Felkin synclinal transition state 360 or the Felkin antiperiplanar transition state 361 (Eq. (11.28)). In the reactions of aldehyde 97c with both the (R)- and (S)-crotylsilane reagent 217, the major products result from crotylsilane addition to the aldehyde via the normally favored Felkin orientation in the transition state. The chirality of the crotylsilane and the stereo-electronic preference for anti S e addition then dictate the facial selectivity of the crotylsilane reagent, which is translated into the stereochemistry of the C(5) methyl substituent of the product. 

Reaction of the /y-benzyloxy-o-methyl chiral aldehyde 97a with (/ )-crolylsi-lanes 217 (R = H, Et) under catalysis by TiC affords the ann,antt-dipropionate adduct 362 (Eq. (11.29)). The diastereoselectivity in this reaction is best explained by anti S e addition of the chiral crotylsilane to the least hindered face of the fi-alkoxy aldehyde chelate, as shown in the synclinal transition state 363. Finally, the anri.syn-dipropionate 364 may be obtained as the major adduct when aldehyde 97a is treated under the same conditions with the enantiomeric crotylsilane reagents (5)-217 (Eq. (11.30), R=Me, Et). This adduct should arise from the antiperiplanar transition state 365, where the anti S e facial selectivity of the crotylsilane reagent and the facial bias of the chiral aldehyde are maintained. In these cases, the factors that dictate the utilization of the synclinal vs the antiperiplanar transition states are (1) the requirement that a small substituent (H) occupy the position over the chelate ring, (2) that C-C bond formation occurs anti to the sterically demanding a-methyl group of the aldehyde and (3) the requirement for an anti Se mechanism, which dictates the stereochemistry of C(5) of the adducts 362 and 364. 

The syn stereochemistry of adducts 383, the major products of the BF3-OEt2-catalyzed reactions of allenylstannane (5)-218, indicates that these reactions proceed preferentially either through the synclinal transition state 385 or the antiperiplanar transition state 386 (Scheme 11-27). However, when the aldehyde R substituent is unbranched (e.g. -hexCHO, Table 11-21), the antiperiplanar transition state 387, which leads to the anti adduct 384, becomes more favorable, presumably due to a diminished steric interaction between the aldehyde R sub.stituent and the allenylstannane Me substituent. 

Using the enantiomeric allenylstannane (S)-218a with MgBf2 as the promoter, the anft>yn-dipropionate 398 is obtained in high yield and diastereoselectivity, where the diastereoselection is consistent with reaction occurring through the chelate antiperiplanar transition state 399 (Eq. (11.32)). 

Figure 4.20 Views of the antiperiplanar transition state for the E2 elimination.

Scheme 5.2.28). Anticipated production of oxonium 133 leads to the C-1 alkylation products 134 and 135 in a 3 1 ratio, which can be rationalized via synclinal 136 and antiperiplanar 137, respectively. The lack of facial basis in the oxocarbenium 133 leads equally to attack from above and below the plane of the conjugated cation, yielding an additional pair of syn and anti adducts analogous to 134/135 in similar ratio (dr 4 1). In related fashion, Rychnovsky has described the allylation reactions of 4-acetoxy-l,3-dioxanes.43 The presence of the 5-methyl substituent in 138 requires treatment with reactive -2-butenyl tri-n-butylstannane, affording 139 as the major product (dr 4 1) via the antiperiplanar transition state arrangement analogous to 137. 

The authors surmise that the trichlorostannyl intermediate 218 directs a chelation-controlled addition via 220, which may involve pseudo-axial complexation of the carbamate carbonyl. The stereoselectivity of the allylation is significantly altered by the use of 2.0 equivalents of SnCU to produce the corresponding i yn.i yn-isomer of 219 via the antiperiplanar transition state derived from the a-chelation model for addition of y-alkoxyallylstannanes. 

---