Keck reaction

The Keck reaction is the allylation of a carbonyl compound performed with an allylstannane and mediated by a Lewis acid. The reaction product is a homoallylic alcohol. 

Equilibria between the aldehyde and its trimer (trioxane) can further complicate the picture. The BFs-promoted Keck reaction is instantaneous at -80 °C with 4-f-BuC6H4CHO, and rapid with t-BuCHO, but does not take place with MeCHO at -80 °C. This is because MeCHO immediately trimerizes to para-acetaldehyde, which reacts only above -40 °C. Note that the desired reaction products tend to decompose if the reaction mixtures are warmed to room temperature. ... 

As will be discussed later, a useful feature of the Keck reaction is that both E and Z-crotylstannanes give predominantly syn adducts. 

The Keck reaction can be accelerated by using additives, which facilitate the equilibria leading to the regeneration of the catalyst 28. Walsh found that isopropanol increases the rate and the enantioselectivity of the allylation of... 

Yu has shown that i-PrSSiMes enhances the reaction rate of the asymmetric Keck reaction, and allows the catalyst loading of BINOL/Ti to be decreased to 1-2 mol%. i-PrSSiMes is prepared from (/-PrS)2Pb and TMSCl, followed by distillation, and is added over several hours to a solution of the aldehyde, stannane, and catalyst at -20 °C. The additive is postulated to aid in regenerating the catalyst through the favorable formation of Sn-S and Si-O bonds. ... 

Denmark has also applied his new method of Lewis base activation of Lewis acids to the Keck reaction. This approach uses pbosphoramide 96, which can be viewed as a chiral version of HMPA. Phosphoramide 96 is used to activate the weak Lewis acid SiCU and form the strong, cationic, chiral Lewis acid 97. One benefit of this method is that there is very little backgrormd reaction due to the achiral Lewis acid SiCL, because the... 

The fact that allylstannanes are highly reactive and react exclusively at the y-carbon, regardless of steric hindrance, makes the Keck reaction a useful tool to introduce quaternary centers. Rawal used this feature to prepare... 

Marko demonstrated the utility of stannane 107 in the synthesis of the lichen macrolide (+)-aspicilin. The Keck reaction of aldehyde 111 with stannane 107 provided allylsilane 112. Protecting group manipulations gave 113, and Sakurai reaction with orthoester 114 gave the ketal 115, which was converted to the desired material. 

An interesting way to derivatize a homoallylic alcohol, which is the product of a Keck reaction, is to force the olefin to migrate. Wipf and coworkers met with an imexpected difficulty in the synthesis of ( )-... 

In a straightforward application of enantioselective Keck reaction, Lee used x-oxo titanium catalyst 121 to convert aldehyde 122 into alcohol 123 to construct the left-hand side of the antitumor agent (-)-blepharocalyxin D. ... 

The methodology was adapted to the synthesis of a simplified fragment of bryostatin. As in the previous example, an aldehyde (130) was reacted with (2-chloromethyl)allylstannane 131 using BINOL/Ti(Oi-Pr)4 catalysis to obtain 132. In parallel, aldehyde 133 was subjected to a Keck reaction employing the silylated stannane 134. The resulting allylsilane 135... 

In conclusion, the Sakurai reaction is a powerful method to introduce a nucleophilic allyl synthon. Similar allylations include the Keck reaction of allylstannanes, and the Roush reaction of allylboranes. What sets the Sakurai reaction apart is the exceptional stability of allylsilanes, which are stable to air and water, and can easily be chromatographed, unlike most other allylmetal species. In addition, allylsilanes are easily accessible by a variety of reactions. These factors have allowed the Sakurai reaction to find widespread use, from basic methodology to total synthesis. 

Keck J 1960 Variational theory of chemical reaction rates applied to three-body recombinations J. Chem. Phys. 32 1035 Anderson J B 1973 Statistical theories of chemical reactions. Distributions in the transition region J. Chem. Phys. 58 4684... 

The assumptions of transition state theory allow for the derivation of a kinetic rate constant from equilibrium properties of the system. That seems almost too good to be true. In fact, it sometimes is [8,18-21]. Violations of the assumptions of TST do occur. In those cases, a more detailed description of the system dynamics is necessary for the accurate estimate of the kinetic rate constant. Keck [22] first demonstrated how molecular dynamics could be combined with transition state theory to evaluate the reaction rate constant (see also Ref. 17). In this section, an attempt is made to explain the essence of these dynamic corrections to TST. 

Keck et al. reported that a catalyst generated from (S)- or (l )-BINOL 12 and Ti(0-i-Pr)4 in a 2 1 ratio is more selective than the catalyst formed from a 1 1 mixture [19fj. The former catalyst was shown to catalyze the cycloaddition reaction of aldehydes 1 with Danishefsky s diene 2a affording the dihydropyrones 3 with moderate to excellent enantioselectivity (Scheme 4.12). The reaction proceeds well for different aldehydes with up to 97% ee and good yield of the cycloaddition products. 

Keck, J. C. (1978). Rate-controlled constrained equilibrium method for treating reactions in complex systems. In Maximum Entropy Formalism" (R. D. Levine and M. Tribus, eds). M.I.T. Press, Cambridge, MA. 

J. C. Keck, Variational theory of reaction rates, Adv. Chem. Phys. 13, 85 (1967). 

Keck A, Gilbert E, Koster R (2002) Influence of particles on sonochemical reactions in aqueous solutions. Ultrasonics 40(l-8) 661-665... 

Keck et al have obtained similar results in the reaction of (Z)- y-silyloxyallyl-tributyltin with a- and (3-alkoxyaldehydes (equation II). A single adduct is usually formed, that with a syn, vic-diol unit. 

A well-designed synthesis of the bottom portion of (+)-compactin (44) was reported by Keck and Kachensky using a vinylallene as the diene in an intramolecular Diels-Alder reaction (Scheme 19.10) [12]. This was done at a time when there was very little literature precedent on the use of vinylallenes as dienes. Based on examination of molecular models, it was reasoned that the transition state for the... 

This reaction is an extension of the Steelich esterification reaction. See E. P. Boden, G. E. Keck, Journal of Organic Chemistry 1985, 50, 2394. 

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