Z-selective generation

Fig. 13.15. Highly Z-selective generation of a ketone enolate. The transition state A is destabilized so strongly by 1,2-inter-actions that deprotonation occurs exclusively via transition state B. - Note Don t let yourself be misled if it seems as though this is a deprotonation of H atoms lying almost in the plane of the C=0 double bond. According to the discussion of Figure 13.12, this would be quite unfavorable due to the lack of ff( i Ar 0 interaction. This is not true, though. For, if the transition states A and B displayed the same dihedral angle as a cyclohexane chair, the dihedral angle between the crucial C-H bond and the C=0 bond in the transition states A and B, respectively, would amount to 60° each.

Fig. 10.12. Highly Z-selective generation of a ketone enolate. The transition state A is destabilized so strongly by 1,2-interactions that deprotonation occurs exclusively via the transition state B.

Fig. 10.14. Highly Z -selective generation of ester enolates in a THF/DMPU solvent mixture. DMPU, N,N -dimethylpropyleneurea.

In the rearrangement of allyl fluoroacetates, trialkylsilyl triflates have been introduced as a new reagent for the Z-selective generation of silyl ketene acetals485. Thus, when (T)-crotyl fluoroacetates are treated at ambient temperatures with a trialkylsilyl triflate in the presence of a tertiary amine, rearranged products with a svn relationship are preferentially obtained. The ketene acetal intermediates cannot be isolated and the geometry has been deduced from the stereochemistry of the products. The selectivity of this process improves in the order triisopropyl > ferf-butyldimethylsilyl > rerf-hexyldimethylsilyl > trimethylsilyl a triethylsilyl (see Table 11). 

Bent ansa-metallocenes of early transition metals (especially Ti, Zr, Hf) have attracted considerable interest due to their catalytic activity in the polymerization of a-olefins. Ruthenium-catalyzed olefin metathesis has been used to connect two Cp substituents coordinated to the same metal [120c, 121a] by RCM or to connect two bent metallocenes by cross metathesis [121b]. A remarkable influence of the catalyst on E/Z selectivity was described for the latter case while first-generation catalyst 9 yields a 1 1 mixture of E- and Z-dimer 127, -127 is the only product formed with 56d (Eq. 19). 

The selective generation of syn-isomers can plausibly be explained by the formation of (Z)-enolates 2-73 via 2-72 in the addition step and by a six-membered transition state 2-75 in the subsequent aldol reaction. 

E.Z-Selectivity in the insertion by unsymmetrical carbenoid 24, is specifically indicative of the transition state of the stepwise mechanism. Based on the evidence that carbenoid 24, which is generated from 42 or 43 (E Z = 84 16), exists nearly exclusively in the -configuration under the equilibrium even at —95°C,29 the observed stereoselectivity for E-isomers in the insertion products verifies that hydride abstraction takes place via an Sn2-like transition state 52 with inversion of configuration at the carbenoid carbon, followed by the recombination of menthone 40 and carbanion 53 (Scheme 19). 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

As described above in Eq. 43, simple allylboronates can be transformed into more elaborated ones using olefin cross-metathesis. " Treatment of pinacol allylboronate 31 with a variety of olefin partners in the presence of Grubbs second-generation catalyst 142 smoothly leads to formation of 3-substituted allylboronates 143 as cross-metathesis products (Eq. 104). Unfortunately, these new allylic boronates are formed as mixtures of geometrical isomers with modest E/Z selectivity. They are not isolated but rather are treated directly with benzaldehyde to give the corresponding homoallylic alcohol products in good yields (Table A). 

When, and only when, all reaction constituents are combined, a vinylidene complex (49) forms. The observed enyne product is generated by coupling of the a-acetylide of 49 with its vinylidene moiety, followed by protodemetallation. The integrity of kinetically controlled product ratios is safeguarded by the avoidance of Rh-H intermediates. (Z)-Selectivity was rationalized on the basis of minimizing... 

Sigmatropic rearrangements proceed via closed transition states in the Claisen-Ireland variation a silyl-enol ester, 27 or 28, is used, which may be selectively generated in (Z) or (E) configuration12 (Section A. 1.6.3.1.). As shown in the transition states 32 and 29, this results in the formation of 30/31 and 33/34. 

In this case the Zs/Z-selectivity does not matter because a terminal double bond is generated. In principle the Zs/Z-selectivity depends on the stability of the ylide employed (see chapter 9). The most valuable feature of the Wittig procedure is that, in contrast to elimination and pyrolytic reactions, it gives rise to alkenes with unambiguous position of the double bond. 

The selective generation of (poly)sulfide-linked molecules from the resin macromolecules at lower pyrolysis temperatures is exemplified b / Figure 10, which shows mass chromatograms of m/z 57 of the 358 and 610°C pyrolysates. At 358°C, phytenes and phytane dominate this mass... 

The ketone enolate A of Figure 13.47 is generated in a Z-selective fashion (as we saw in Figure 13.15). The bulky and branched enolate substituent destabilizes the Zimmerman-Traxler transition state C by way of the discussed 1,3-diaxial interaction, while the transition state structure B is not affected. Hence, the aldol addition of enolate A occurs almost exclusively via transition state B, and the -configured aldol adducts D (Figure 13.47) are formed with a near-perfect simple diastereoselectivity. The acidic workup converts the initially formed trimethysilyloxy-substituted aldol adducts into the hydroxylated aldol adducts. 

Z)-Allyltrimethylsilanes.1 Reaction of aldehydes with 2-trimethylsilylethy-lidinetriphenylphosphorane has been used to convert aldehydes into allyltrime-thylsilanes as a mixture of (E)- and (Z)-isomers (9,492). Replacement of triphen-ylphosphine by tris(2-methylphenyl)phosphine enhances (Z)-selectivity, particularly in reactions with aliphatic saturated aldehydes. These aldehydes can be converted into the (Z)-allylsilanes (Z/E > 94%) with 1, and with use of BuLi at 0° for generation of the phosphorane. 

Because the geometry of the 9-double bond was not clear at that time, Corey et al. 75> tried to prepare the (Z)-9-isomer as well as the ( )-9-isomer of leukotriene-A (78 and 86). In the synthesis of the former isomer the tribenzoyl derivative of D-(—)-ribose (79) was converted in 8 stepy into the optically active epoxyaldehyde 71 and the latter to 72. 72 was olefmated with ylide 82, generated by treatment of the corresponding phosphonium mesylate with lithium diisopropylamide in THF/ HMPA75) (Scheme 15). In the first olefination step 72+82- 78, however, similar to the first method, a A9-isomer mixture was formed. The loss of (Z)-selectivity of the Wittig reaction is due to the use of conjugated unsaturated, i.e. moderate ylides of type 82, and had to be expected because of the mechanism of the Wittig reaction (see Sect. 2). 

In this case, the Wittig reaction is not entirely Z-selective, and it generates some E-isomer. Lindlar-catalysed reduction, on the other hand, generates pure Z-alkene. 

When Z selectively reacts with either R, or M, the ESR spectrum of the reaction mixture can reveal the structure of the originally generated R and/or M. 2-Methyl-2-nitrosopropane, Me3C—N=0 [131-133], and derivatives of nitrosocompounds [133] are used as spin-trapping agents. 

In contrast to the metal-catalyzed reactions, photochemically generated methoxycarbonyl-trimethylsilylcarbene cyclopropanates styrene and 1-hexene with a slight Z-selectivity. While the preference for the sterically more congested cyclopropane is somewhat surprising, the rather... 

The reaction of p-chlorobenzaldehyde with phenyldiazomethane in the presence of (MeOlsP and catalytic amounts of meso-tetraphenylporphyrin iron chloride (ClFeTPP) resulted in the formation of the corresponding alkenes with an /Z-selectivity of 86 14, but the yield was low (30%). When phenyldiazomethane is generated in situ from the corresponding potassium tosylhydrazone salt, the olefin yield increases to 92% with E/Z-selectivity of 97 3. Thus, high levels of -selectivity are obtained with semistabilized ylides by this method . This process is applied to a wide range of aldehydes and is practical as compared to standard Wittig reaction, and therefore finds applications in industry, o... 

SCHEME 10.68 Silyl enol-ethers bearing sugar-derived auxiliaries can be generated with high E selectivity using LTMP as a base where no E/Z selectivity is realized using LHMDS. 

Disubstituted alkenes with a trans double bond can be generated routinely from Wittig rearrangements in the case of ester enolate Wittig rearrangements, cis selectivity can be achieved. For trisub-stituted alkenes, ( )- and (Z)-selective syntheses are known. 

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