Aldehydes competitive allylation reaction

Scheme 13.12 Allylation of imines catalyzed by lanthanide triflate. Table 13.12 Competitive allylation reaction of aldehydes and imines...

A second problem with Sarett oxidation is the difficulty in isolating the products from a pyridine solution. An advantage of the technique, as mentioned above, is that alkenes, ketals, sulfides, and tetrahydropyranyl ethers are oxidized much slower than alcohols and rarely give competitive side reactions. Oxidation of secondary alcohols proceeds in good yield, but oxidation of primary aliphatic alcohols often gives low yields of the aldehyde. 2 Benzylic and allylic alcohols give good yields, however. 

Recently, nanometer tin-mediated allylation of aldehydes or ketones in distilled or tap water gave rise to homoallyl alcohols in high yield without any other assistance such as heat or supersonic or acidic media (Eq. 8.41). Allylation of P-keto aldehydes and functionalized imines by diallyltin dibromide was carried out to generate skipped and conjugated dienes. Aldehydes are allylated with CH2=CHCH2SnBu3 using Sn catalysts in acidic aqueous media. Exclusive aldehyde selectivity was observed for competitive reactions of aldehydes and ketones in the presence of 5 mol% of (CH2=CHCH2)4Sn or SnCU in a mixture of aqueous HCl and THE (Eq. 8.42). ... 

Evidence of variables that influence the relative rates of reaction of olefins and alcohols was obtained from experiments with compounds that have both olefinic and alcoholic functions and by the competitive oxidation of mixtures of olefins and alcohols. The data of Table VI show that when the double bond has no substituents, as in allyl alcohol, but-3-en-l-ol, or 2-methylbut-3-en-l-ol, only the epoxide is formed but when the double bond has substituents, the epoxida-tion rate is decreased and ketone and aldehyde products from the oxidation of the OH group are formed. This effect is more pronounced with a greater degree of substitution. Since the double bond and the OH group are part of the same molecule, the difference must arise from the different abilities of the reactants to coordinate and react at the titanium center restricted transition-state shape selectivity is a possibility. The terminal double bond, sterically less hindered, interacts strongly with titanium, preventing coordination of the competing OH... 

A parallel trend is observed for MgBr2-promoted additions of cis- and trans-crotyl tributylstannanes to a-benzyloxy aldehydes but the effect is much smaller (Table 9) [18], In such reactions the orientation of the allylic stannane and the chelated aldehyde is governed by steric effects in which the vinylic y-hydrogen orients over the five-membered chelate (Fig. 4). Support for this picture is provided by competition experiments in which y3,)8-dimethylallyl tributyltin was found to be markedly slower than the crotyl or allyl derivatives in additions to a-benzyloxypropanal. The observed rate decrease was attributed to the disfavored relationship of a vinylic methyl substituent with the chelate ring resulting in unfavorable steric interactions. 

In most cases, carbon-carbon bond formation occurs at the least substituted terminus of the allylic unit. A wide range of aldehydes and ketones can be utilized in the reaction, and one cyclization process hasten reported (equation 16). Aromatic and a,p-unsaturated substrates cannot be used owing to competitive pinacolic coupling reactions promoted by Sml2. 

Denmark has spectroscopically examined the reaction of both allyl- and 2-bute-nylstannanes with aldehydes using the Lewis acids SnCU and BF3-OEt2 [73, 82]. First, the metathesis of both allyltributylstannane and tetraallyltin with SnCl4 was determined (by C NMR spectroscopy) to be instantaneous at -80 °C. The reaction of allyltributylstannane with a complexed aldehyde was detemiined to be significantly more complicated. When a molar equivalent of SnCU per aldehyde was employed, metathesis was determined to be the preferred pathway for aldehydes. When one half a molar equivalent of SnC per aldehyde is used, the reaction pathways and product distribution become very sensitive to both the aldehyde structure and addition order. A spectrum of mechanistic pathways was documented ranging from direct addition (acetaldehyde) to complete metathesis (pivalalde-hyde) to a competitive addition and metathesis (4-t-butylbenzaldehyde). The results obtained with a molar equivalent of SnCl4 are most relevant, as this reagent stoichiometry is most commonly used in the addition reactions. 

Another elegant example of a multicomponent aza-Sakurai reaction was reported by Jarvo in 2009 [99]. In this case, Pd(OAc)j was used as catalyst both for the generation in situ of the aUylsilane from allyl trifluoroacetate and HMDS and also for the allylation of the in situ formed imine fi om the corresponding aldehydes and anisidine (H NPMP) (Scheme 12.22). This was concluded after a study base on competition experiments. This procedure allows the synthesis of a variety of homoallylic amines in good to excellent yields (67-96%), starting from different substrates including electron-rich aromatic and aliphatic aldehydes. 

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