Center tertiary

There are many examples in the literature reporting the successful exchange of activated hydrogens in various enones (see compounds 19-25). Complications are encountered with 3/3-hydroxy-5oc-spirost-8(9)-en-ll-one since the exchange of the 14a-hydrogen is accompanied by inversion at this tertiary center (22). Another unusual case occurs during the attempted deutera-... 

The ether-forming step is an S -like reaction of the alkoxide ion on the silicon atom, with concurrent loss of the leaving chloride anion. Unlike most Sn2 reactions, though, this reaction takes place at a tertiary center—a trialJkyl-substituted silicon atom. The reaction occurs because silicon, a third-row atom, is larger than carbon and forms longer bonds. The three methyl substituents attached to silicon thus offer less steric hindrance to reaction than they do in the analogous ferf-butyl chloride. 

Optically active Art-butyl 2-(4-methylphenylsulfinyl)propanoate only reacted with aldehydes in the presence of e//-butylmagnesium bromide as a base36. The asymmetric induction for the formation of the hydroxylic center was higher in the case of aliphatic aldehydes (75-80%) than in the case of benzaldehyde (33%). The diastereoselectivity regarding the tertiary center to sulfur was not determined. 

Under some circumstances, acid-catalyzed ring opening of 2,2-disubstituted epoxides by sulfuric acid in dioxane goes with high inversion at the tertiary center.116... 

The trihuoromethylcycloalkane systems do provide, however, some examples of CF3 bound to a tertiary center, with 1-methyl-1-trihuoro-methylcyclohexane, cyclopentane, and cyclobutane all absorbing at higher fields (-81, -78, and -80ppm, respectively) than the secondary systems mentioned above, as would be expected based on the branching principle. 

The same iron (III) complexes also oxidize alkyl radicals, particularly those with secondary and tertiary centers, to the corresponding carbonium ions (7). 

Under basic conditions, obviously only one isomerization step takes place and thus a terminal alkyne will deliver 1,2-dienes selectively. With internal alkynes, on the other hand, selectivity can only be achieved when the alkyne is either symmetrical as in 14 [34] (Scheme 1.6) or has a tertiary center on one side as in 16 [35, 36] (Scheme 1.7). So, unlike potassium 3-aminopropylamide in 1,3-diaminopropane, where the Jt-bonds can migrate over a long distance by a sequence of deprotonations and reprotonations, here the stoichiometric deprotonation delivers one specific anion which is then reprotonated (in 16 after transmetalation). 

Reaction of 1-nitropropane with glutaraldehyde in aqueous ethanol in the presence of sodium hydroxide yields a mixture of two products, the major component of which, lr-ethyl-l-nitrocyclohexane-2c,6f-diol (98), can be isolated in 36% yield ). Acid-catalyzed acetylation converts (98) into the di-O-acetate, hydrogenation yields the corresponding amine, which has been characterized as the hydroacetate, N-acetate and triacetate. Configurational assignments followed from NMR data, which clearly showed the steric non-equivalence of the two hydroxyl groups vicinal to the tertiary center. 

As soon as carbon-bound substituents, with the exception of methyl (and perhaps ethyl) or tertiary centers arc admitted, the distinction between backbone and substituents becomes ambiguous. This problem arose in the 1950s, before the advent of the CIP system, in conjunction with early studies of C-C bond formation by addition to carbonyl groups. At that time,... 

When the tertiary center in 6 containing the chiral auxiliary is subjected to deprotonation/ benzylation the stereoselective formation of l-benzyl-l,2,3,4-tetrahydro-l-methylisoquinoline (97-98% ee) was reported which interestingly had the R configuration. Hence, the monoalky-lated intermediate related to 8 is alkylated from the bottom face, opposite to that found with unsubstituted 5. Obviously the existence of two diastereomeric lithiocompounds accounts for these results. Now the strong temperature dependence of the selectivity, which usually hints that an equilibrium is involved, can also be easily understood. 

Recently, M. Asami and T. Mukaiyama 124) synthesized ot-benzyloxyaldehydes (109) having a chiral tertiary center at the ot-carbon atom in high enantiomeric excess by successive treatment of the aminal (102) with diisobutylaluminium hydride (DIBAL-H) and Grignard reagents. The asymmetric reaction is applied to the total synthesis of exo-(+)-brevicomin (110), the principal aggregation pheromone in the frass of the female western pine beetle (Dendroctonus brevicomis). 

It was then found that a tertiary center is not required at C-5, and that tetra-0-acetyl-/ -D-xylopyranose (27), treated with radicals derived from N-bromosuccinimide, affords a mixture of (55)- and (5R)-tetra-O-acety 1-5-bromo-/ -D-xylopyranose (28 and 29 see Scheme 6), which is in accord with, but does not necessarily follow from, the finding that 27 exists in solution in both chair conformations, each of which is subject to axial hydrogen abstraction and subsequent axial bromination27 35 (see Section III,2). In this... 

Saunders and Stofko have observed 1,3- 1,4-, and 1,5-intramolecular shifts from tertiary to tertiary center in superacid and have calculated the activation barriers to be 8.5, 12-13, and 6-7 kcal mole-1, respectively. (M. Saunders and J. J. Stofko, Jr., J. Amer. Chem. Soc., 95, 252 (1973)). 

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... 

Similar adducts can be obtained from C6H,C=CCI and C6H5SC=CC1. Reaction with enolates not at tertiary centers results in isomerization of the primary adducts to allenes. 

The future challenges in this area will be the development of catalyst sytems that promote the formation of higher molecular weight material and that also have the ability to control the microstructure of the polymer. Other features for new catalyst systems would involve the ability to distinguish between primary, secondary, and tertiary centers and couple both alkylsilanes and arylsilanes efficiently. To achieve these goals will require a better understanding of the mechanistic pathways for dehydrocoupling than is currently available. 

A severe limitation of this method, however, is the failure of the ylide 103 to yield oxaspiropentanes vide supra) from a,p-unsaturated ketones and the poor yields of vinylcyclopropanes obtained from its reactions with hindered ketones or with con-formationally rigid six-membered rings. Moreover, attempts to extend the oxaspiro-pentane ring opening to compounds containing an adjacent tertiary center have failed thus, oxaspiropentane 110 did not lead to 111, Eq. (33) 57). 

When the electron-withdrawing group is too close to the tertiary CH bond, the electrophilic attack is prevented and the starting material may be recovered. The presence of an aromatic ring should be avoided since, when not strongly deactivated, it reacts with the fluorine faster than any tertiary center (equation 166). 

While the example illustrated in Scheme 5.8 shows equilibrium between two chemically identical carbocations, there are factors influencing the direction of these transformations when applied to more complex systems. If we consider Scheme 5.9, we notice that the positive charge migrates exclusively to the tertiary center, reflecting the increased stability of tertiary carbocations over primary carbocations. In general, where 1,2-hydride shifts are possible, rearrangement of less stable carbocations to more stable carbocations is expected. 

While the hydride shift illustrated in Scheme 5.12 cannot occur as a part of the pinacol rearrangement, the intermediate carbocation is subject to alkyl migrations. As shown in Scheme 5.13, a 1,2-alkyl shift results in transfer of the cation from a tertiary center to a center adjacent to a heteroatom. As the oxygen heteroatom possesses lone electron pairs, these lone pairs serve to stabilize the cation. Thus, the illustrated 1,2-alkyl shift transforms a carbocation into a more stable carbocation. 

This is an Sk2 displacement of a trifluoroacetoxy anion by a fluoride anion. The related Sk2 mechanism is not favored because of steric factors. Specifically, the trifluoroacetate resides at a tertiary center. Please note that the fluoride anion is... 

Because tertiary centers are not susceptible to SN2 reactions, this reaction will proceed via an S l mechanism. 

This reaction will show competition between SN1 and SN2 mechanisms due to the fact that this center is less hindered than a tertiary center but more hindered than a primary center. An SN1 mechanism will be favored using highly polar, aprotic solvents to stabilize the forming carbocation. An SN2 mechanism will be favored when nonpolar solvents are used. 

Allylic alkylation. In general, allylic alkviation catalyzed by transition metals results from attack at the less substituted carbon atom of the ir-allyl intermediate. Deviation from this pattern is observed with some nucleo[ihilcs when Mo(CO)h is used as catalyst. For example, the anion of dimethyl malonate genei ated with 0,N-bis(trimethylsilyl)acctamidc (BSA) reacts with the allylic acetate 1 mainly by attack at the tertiary center to give 2. 

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