Two-step sequences

Hydroboration-oxidation (Sections 6 11-6 13) This two step sequence achieves hydration of alkenes in a ste reospecific syn manner with a regiose lectivity opposite to Markovnikov s rule An organoborane is formed by electro philic addition of diborane to an alkene Oxidation of the organoborane inter mediate with hydrogen peroxide com pletes the process Rearrangements do not occur... 

Is the two step sequence depicted in the following equations con sistent with the second order kinetic behavior observed for the hydrolysis of methyl bromide ... 

Because acylation of an aromatic ring can be accomplished without rearrangement it is frequently used as the first step m a procedure for the alkylation of aromatic compounds by acylation-reduction As we saw m Section 12 6 Friedel-Crafts alkylation of ben zene with primary alkyl halides normally yields products having rearranged alkyl groups as substituents When a compound of the type ArCH2R is desired a two step sequence IS used m which the first step is a Friedel-Crafts acylation... 

Phosphorus ylides are prepared from alkyl halides by a two step sequence The first step is a nucleophilic substitution of the 8 2 type by triphenylphosphme on an alkyl halide to give an alkyltriphenylphosphonium salt... 

The structure of the bicychc monoterpene borneol is shown in Figure 26 7 Isoborneol a stereoisomer of borneol can be prepared in the labora tory by a two step sequence In the first step borneol is oxidized to camphor by treatment with chromic acid In the second step camphor is reduced with sodium borohydride to a mixture of 85% isoborneol and 15% borneol On the basis of these transformations deduce structural formulas for isoborneol and camphor... 

This two-step sequence is a valuable alternative to the direct double bond cleavage by ozonolysis. 

A two-step sequence of nitrile oxide-olehn cycloaddition and reduction of the resulting A -isoxazolines offers a unique and attractive alternative to the classical aldol reaction and its many variants (2J). The procedure bypasses traditional problems, including enolate equilibrium and cross condensation (20). 

Carboxylic acids can be prepared from nitriles by reaction with hot aqueous acid or base by a mechanism that we ll see in Section 20.9. Since nitriles themselves are usually made by Sts 2 reaction of a primary or secondary7 alkyl halide with CN , the two-step sequence of cyanide displacement followed by nitiile hydrolysis is a good way to make a carboxylic acid from an alkyl halide (RBr —> RC=N RC02H). 

Amino acids can also be prepared by a two-step sequence that involves Hell— Volhard-Zelinskii reaction of a carboxylic acid followed by treatment with ammonia. Show how you would prepare leucine, (C bCHCl-HClKNPfylCC H, and identify the mechanism of the second step. 

We ve already seen in Sections 20.7 and 21.7 how amines can be prepared by reduction of nitriles and amides with LiAlH4. The two-step sequence of 5 2 displacement with C followed by reduction thus converts an alkyl halide into a primary alkylamine having one more carbon atom. Amide reduction converts carboxylic acids and their derivatives into amines with the same number of carbon atoms. 

The cydization to structurally defined, soluble LPPP then takes place in a two-step sequence, consisting of reduction of the keto group followed by ring closure of the secondary alcohol groups of 14 in a Friedcl-Crafts-type alkylation. 

The final two stages are very straightforward. Oxidative scission of the C3-C5 double bond in 6 with ozone provides triketone 5 which, without purification, is subjected to a base-induced intramolecular aldol/dehydration reaction. The crystalline product obtained from this two-step sequence (45 % overall yield) was actually an 85 15 mixture of ( )-progesterone and a diastereomeric substance, epimeric at C-17. Two recrystallizations afforded racemic progesterone [( )-(1)] in diastereomerically pure form. 

What is the second and final step What is the numerical value of k l Devise another two-step sequence that would also be consistent with the kinetic data. Give the value of the rate constant associated with the first step. 

A process consisting of a sequence of n reactions proceeds by way of n transition states. Consider the general two-step sequences for the following net reactions ... 

Substrate titration. Devise two schemes for the conversion of substrate to product that match the pH profile in Fig. 6-1 b. These two are to feature substrate titration. Devise a third in which a steady-state intermediate intervenes in a two-step sequence. 

An alternative reported in the same publication involves the in situ conversion of the carboxylic acids to the corresponding acyl chlorides using PS-PPh3 and CCI3CN (THE, 10 °C, 5 min) before treatment with the amidoxime in the presence of DIEA (THE, 150 °C, 15 min). The resin-bound phosphine not interfering with the second step, and THF being the best solvent for both steps, the two-steps sequence could be performed one-pot with yields comparable to those obtained using the HBTU/PS-BEMP combination (Scheme 12). 

Catalytic Cycles. In the unpolluted troposphere, the two step sequence Rl, R2 was early recognized as the dominant source of HO (20,91,92) and HO as the dominant reactant for CO removal (93,94). 

The effect of structure of the alkyl group on the stability of monoalkyl-thallium(III) compounds can best be understood by reference to the different mechanisms by which these compounds undergo decomposition. A number of authors have attributed the instability of monoalkylthallium(III) compounds to facile C—T1 bond heterolysis and formation of carbonium ions [Eq. (25)] (52, 66, 79). This explanation is, however, somewhat suspect in cases where primary carbonium ions would be involved and either the two-step sequence shown in Eqs. (26), (27), or the fully synchronous 8 2 displacement shown in Eq. (28), is more compatible with the known facts. Examination of the oxythallation reactions that have been described reveals that Eq. (27) [or, for concerted reactions, Eq. (28)] can be elaborated, and that five major types of decomposition can be recognized for RTlXj compounds. These are outlined in Scheme 8, where Y, the nucleophile... 

In a termolecular reaction, three chemical species collide simultaneously. Termolecular reactions are rare because they require a collision of three species at the same time and in exactly the right orientation to form products. The odds against such a simultaneous three-body collision are high. Instead, processes involving three species usually occur in two-step sequences. In the first step, two molecules collide and form a collision complex. In a second step, a third molecule collides with the complex before it breaks apart. Most chemical reactions, including all those introduced in this book, can be described at the molecular level as sequences of bimolecular and unimolecular elementary reactions. 

The linear appearance of the plot shows that this reaction obeys a first-order rate law. Additional mechanistic studies suggest that alkene formation proceeds in a two-step sequence. In the first step, which is rate-determining, the C — Br bond breaks to generate a bromide anion and an unstable cationic intermediate, hi the second step, the intermediate transfers a proton to a water molecule, forming the alkene and H3 ... 

An example of the application of process profile analysis was provided by Berkoff and coworkers (Berkoff et al., 1986). It concerned the synthesis of 7,8-dichloro-l,2,3,4-tetrahydrowoquinoline (1) according to the two step sequence shown in Fig. 2.6. 

If necessary for further applications, the 2-pyridyl group can be exchanged by alkyl in a two-step sequence that takes advantage of the enhanced leaving-group ability of the 2-pyridyl group. 

This reaction is one of the steps in cycle 2 (Figure 5.1), when starting with the initial HC of the feed. The two-step sequence is ... 

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