Allyl carbonates tertiary

On the other hand, the expected alkene 598 was regioselectively formed from the allylic carbonate 597[388]. In these reactions, the hydride from formate preferentially attacks the tertiary carbon rather than the secondary carbon. 

Allylsilanes are available by treatment of allyl acetates and allyl carbonates with silyl cuprates17-18, with antarafacial stereochemistry being observed for displacement of tertiary allyl acetates19. This reaction provides a useful asymmetric synthesis of allylsilanes using esters and carbamates derived from optically active secondary alcohols antarafacial stereochemistry is observed for the esters, and suprafacial stereochemistry for the carbamates20,21. 

The intermediacy of a carbocation or complex-equivalent is attractive, if one considers that the nucleophilic ambident cyanide ion may be accomodated on secondary or tertiary cationic sites. Where exceptions (e.g., 125,126,134-136 cf. Sect. 4.3) exist, the cationic intermediate resides on a primary allylic carbon. The following skeletal types are examples of some biogenetic schemes offered in conjunction with the structural determination of isocyanoterpenoids ... 

Another Rh-catalyzed protocol that has potentially broad utility has come from the reactions of Cu(i) alkoxides with allylic carbonates.190,191 Under the action of Wilkinson s catalyst modified by P(OMe)3, a variety of primary, secondary, and even tertiary aliphatic alcohols undergo an allylic etherification process with a high degree of retention of regio- and stereochemistry, thus providing expeditious access to a and/or ct -stereogenic ether linkages (Scheme 5).192... 

Reactions with seven- to nine-membered cydic allylic carbonates or halides give the corresponding cydic allyltitanium compounds. These reagents add to aldehydes and imines with moderate to excellent diastereoselectivities [59]. The allyltitanium compound generated from 1-vinylcyclopropyl carbonate reacts regioselectively with aldehydes and ketones at the less substituted carbon atom to provide alkylidenecydopropane derivatives, as shown in Scheme 13.29 [60], The regiochemical outcome of the reaction can be rationalized by assuming an equilibrium between two allyltitanium spedes that favors the less strained tertiary structure. 

A diverse group of secondary and tertiary amines are readily synthesized from the reaction of primary and secondary amines with allylic carbonates in the presence of preformed iridium metalacycles, but the direct synthesis of primary amines via iridium-catalyzed allylic amination requires the use of ammonia as a nucleophile. The asymmetric allylation of ammonia had not been reported until very recently, and it is not a common reagent in other metal-catalyzed reactions. Nonetheless, Hartwig and coworkers developed the reactions of ammonia with allylic carbonates in the presence of la generated in situ [89]. Reactions conducted in the initial work led exclusively to the products from diallylation (Scheme 16). Further advances in... 

Rhodium-catalyzed allylic etherification could also be extended to the more challenging tertiary alcohols (Eq. 7). Although preliminary attempts revealed that the alkylation of the allylic carbonate 51 was feasible, the reaction required increased catalyst loading (20 mol%), affording the allylic ether 52 in 67% yield (2° 1°=47 1). 

Thus, a secondary carbon in a-position to oxygen is selectively attacked in the presence of a primary or a tertiary one. An allylic carbon in a-position to oxygen is methoxylated with total selectivity in the presence of a secondary carbon. Multiple methoxylation usually is not observed. Because of their low reactivity, primary and tertiary carbons as well as open-chain acetals are methoxylated with low current yields but often with high material yields. 

The reaction proceeds well with unhindered secondary amines as both nucleophiles and bases. The yield of allylic amine formed depends upon how easily palladium hydride elimination occurs from the intermediate. In cases such as the phenylation of 2,4-pentadienoic acid, elimination is very facile and no allylic amines are formed with secondary amine nucleophiles, while phenylation of isoprene in the presence of piperidine gives 29% phenylated diene and 69% phenylated allylic amine (equation 30).84 Arylation occurs at the least-substituted and least-hindered terminal diene carbon and the amine attacks the least-hindered terminal ir-allyl carbon. If one of the terminal ir-allyl carbons is substituted with two methyl groups, however, then amine substitution takes place at this carbon. The reasons for this unexpected result are not clear but perhaps the intermediate reacts in a a- rather than a ir-form and the tertiary center is more accessible to the nucleophile. Primary amines have been used in this reaction also, but yields are only low to moderate.85 A cyclic version occurs with o-iodoaniline and isoprene.85... 

As expected from the results obtained in the arylation of dienes with secondary amines (Section 4.4.5.l.2.iii), the amine attacks the least-substituted (hindered) end of the ir-allylic group. An exception to this behavior occurs in these reactions as it did in the diene arylation case where there are two methyl substituents on one terminal allylic carbon, in which case the amine attacks this tertiary carbon (equation 34).s7 If groups larger than methyls are present on one terminal allylic carbon, steric hindrance to attack at that carbon causes reaction at the other end of the allyl system. 

The reason for the selective formation of the highly hindered N-tertiary alkyl amine isomer appears to be that the tertiary 7r-allylic carbon is more susceptible to nucleophilic attack by the amine than the secondary carbon, presumably because of the weaker palladium-tertiary carbon bond. Similar results have been observed in several related reactions. 

The principal exception to this statement involves tertiary carbon, allylic carbon, and benzyl carbon atoms atl ached to X. In such cases it is often found that the second-order reaction with solvate ion such as RO (in ROH solvent) is much slower than apparent first-order reaction with solvent. These systems may be recognized as those giving quite stable carbonium ions, and the solvolysis has b( en ascribed to an Sifl mechanism. 

In secondary steps, radical species formed in BR photooxidation are able to initiate the oxidation of the neighbouring styrenic moieties the photooxidation of the styrenic phase is enhanced by the presence of polybutadiene in ABS, compared with PS homopolymer. Butadiene grafting sites, containing tertiary allylic carbon atoms (A), are preferentially oxidized in tertiary hydroperoxides in the first stages of ABS photooxidation rather than in secondary allylic carbon atoms (B) ... 

The evolution of HBr in the bromination reactions and the uptake of one bromine atom per ring indicate substitution at a secondary allylic carbon atom. The ease of oxidation and cross linking of the polymers and the presence of hydroxyl groups imply the intermediate formation of hydroperoxides on allylic carbon atoms. Treatment of the hydroxyl-containing polymer with benzoyl chloride indicates that the bulk of the hydroxyl groups are on secondary carbon atoms, since tertiary hydroxyl groups would tend to be replaced by chlorine. Although these results do not permit the elimination of structure B, it appears that the bulk of the structural units in the polycyclopentadiene corresponds to 1,2- addition (A). 

If the conjugated diene is not symmetrical, the major products of the reaction are those obtained by adding the electrophile to whichever terminal sp carbon results in formation of the more stable carbocation. For example, in the reaction of 2-methyl-l,3-butadiene with HBr, the proton adds preferentially to C-1 because the positive charge on the resulting carbocation is shared by a tertiary allylic and a primary allylic carbon. Adding the proton to C-4 would form a carbocation with the positive charge shared by a secondary allylic and a primary allylic carbon. Because addition to C-1 forms the more stable carbocation, 3-bromo-3-methyl-l-butene and l-bromo-3-methyl-2-butene are the major products of the reaction. 

SOLUTION TO 11a First we need to determine which of the terminal sp carbons of the conjugated system is going to be the C-1 carbon. The proton will be more apt to add to the indicated sp carbon because the carbocation that is formed shares its positive charge with a tertiary allylic and a secondary allylic carbon. If the proton were to add to the sp carbon at the other end of the conjugated system, the carbocation that would be formed would be less stable because its positive charge would be shared by a primary allylic and a secondary allylic carbon. Therefore, 3-chloro-3-methylcyclohexene is the 1,2-addition product and 3-chloro-l-methylcyclohexene is the 1,4-addition product. 3-Chloro-3-methylcyclohexene is the kinetic product because of the chloride ion s proximity to C-2, and 3-chloro-l-methylcyclohexene is the thermodynamic product because its more highly substituted double bond makes it more stable. 

Allylic substitutions. Allylic carbonates are converted to the corresponding ethers, amines, sulfides on exposure to various nucleophiles in the presence of Pd2(dba)j, and a tertiary phosphine. 

SnI reactivity e (allylic and tertiary) > a (allylic and secondary) > d (forms satne cation as e. but requires iot. . >.iion at t-niiiary carbon, so will be slower) > c > b > f (these follow cation stability order)... 

Carbon-Oxygen Bond Formation. CAN is an efficient reagent for the conversion of epoxides into /3-nitrato alcohols. 1,2-cA-Diols can be prepared from alkenes by reaction with CAN/I2 followed by hydrolysis with KOH. Of particular interest is the high-yield synthesis of various a-hydroxy ketones and a-amino ketones from oxiranes and aziridines, respectively. The reactions are operated under mild conditions with the use of NBS and a catalytic amount of CAN as the reagents (eq 25). In another case, N-(silylmethyl)amides can be converted to A-(methoxymethyl)amides by CAN in methanol (eq 26). This chemistry has found application in the removal of electroauxiliaries from peptide substrates. Other CAN-mediated C-0 bondforming reactions include the oxidative rearrangement of aryl cyclobutanes and oxetanes, the conversion of allylic and tertiary benzylic alcohols into their corresponding ethers, and the alkoxylation of cephem sulfoxides at the position a to the ester moiety. 

A radical mechanism is proposed for this process, because the C-H bond at the allylic carbon, benzylic carbon, or tertiary carbon is relatively weaker than other C-H bonds, which can homolytically cleave and form radicals, as shown below. 

Mechanism 10.1 applies the S l mechanism to this hydrolysis. Its key feature is step 2 in which the nucleophile (water) attacks the allylic carbocation. Attack occurs at both allylic carbons, but at different rates. The major product, a tertiary alcohol, results from attack by water at the tertiary carbon. The minor product, a primary alcohol, comes from attack by water at the primary carbon. 

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