Examination scheme

Another point for structural diversification is the sulfonamide group. Imai had already shown that a wide variety of groups could be introduced at this position to optimize the reaction. Since a wide variety of sulfonyl chlorides are commercially available, a number of different types of groups could be examined (Scheme 3.34). Testing of a variety of aryl and alkyl groups on the 1,2-cyclohexanediamine backbone demonstrates that the simple methanesulfonamide 122 is clearly superior or equal to many other analogs in the cyclopropanation of cinnamyl alcohol (Table 3.11). Another concern which was directly addressed by this survey was the question of catalyst solubility. 

Since Pd complexes are well-known catalysts for enantioselective allylic substitution reactions, here the catalytic behaviour of palladium NPs for this reaction is examined (Scheme 1). One example involving a chiral phosphite with a carbohydrate backbone, able to coordinate firmly at the surface of NPs together with oxygen atoms capable to interact weakly with this surface, is presented. In particular. 

The detailed decomposition (P-H ehminahon) mechanism of the hydrido(alkoxo) complexes, mer-crs-[lr(H)(OR)Cl(PR 3)3] (R = Me, Et, Pr R = Me, Et H trans to Cl) (56, 58, 60), forming the dihydrides mer-cis-[lr H)2Cl PR )2] (57, 59) along with the corresponding aldehyde or ketone was examined (Scheme 6-8). The hydrido(ethoxo) as well as the hydrido(isopropoxo) complexes 60 could also be prepared by oxidative addition of ethanol and isopropanol to the phosphine complexes 39 [44]. In the initial stage of the P-H elimination, a pre-equiUbrium is assumed in which an unsaturated pentacoordinated product is generated by an alcohol-assisted dissociation of the chloride. From this intermediate the transition state is reached, and the rate-determining step is an irreversible scission of the P-C-H bond. This process has a low... 

Reactions of the hydrido(hydroxo) complex 2 with several substrates were examined (Scheme 6-14) [6]. The reactions are fairly complicated and several different types of reachons are observed depending on the substrate. Methyl acrylate and small Lewis bases such as CO, P(OMe)3, BuNC coordinate to the five-coordinated complex 2 affording the corresponding six-coordinate complexes. In reactions with the unsaturated bonds in dimethylacetylenedicarboxylate, carbon dioxide, phenylisocyanate indications for the addition across the O-H bond but not across the Os-OH bond were obtained. In reactions with olefins such as methyl vinyl ketone or allyl alcohol, elimination of a water molecule was observed to afford a hydrido metalla-cyclic compound or a hydrido (ethyl) complex. No OH insertion product was obtained. 

The reactivity of hydrido(ethoxo) complex 4 was examined (Scheme 6-15) [8]. Metatheses similar to those postulated for alcohol exchange (Eq. 6.5) occurred between HCl, LiCl, phenyl acetate or primary amines and yielded complexes 94. The reaction of 4 with cyclic anhydrides proceeded similarly to give iridium-assisted ring opening products 95. Heterocumulenes afforded the inserhon products 96 into the Ir-O bond. 

The reactions of a neutral 10 as well as a cationic dihydrido(acetato)osmium complex 12 with acetylenic compounds were examined (Scheme 6-17) [11-13]. A vinyU-dene 99, an osmacyclopropene 100, or a carbyne complex 101 were obtained, depending on the starting hydrido(acetato) complexes or the kind of acetylene used. In any case, the reaction proceeded by insertion of a C C triple bond into one of the two Os-H bonds, but the acetato ligands do not take part in the reaction and act as stabilizing ligands. 

The deleterious effects of catalyst poisoning when carrying out asymmetric hydrogenations at low catalyst loading caimot be overemphasised. In order to eliminate the possibility that the substrate synthesis introduced inhibitory impurities, an alternative synthetic protocol was examined (Scheme 7.4). The use of a brominating agent and an expensive palladium catalysed step in the initial route could limit the development of this as an economically favourable process and this was further motivation to examine alternative routes to the hydrogenation substrate. 

In the previous section we examined a particular canonical form for schemes, namely block structured schemes, in which the program is built in nodules connected in a tree-like fashion. In this section we carry this idea one step forward and examine schemes written in a "line-like" form - the so-called "WHILE schemes" or "GOTO-less schemes". 

Examine Scheme A in Example E - 2. Prove that it always halts and find the longest computation. Then construct the simplest strongly equivalent tree scheme you can find (its computation may or may not be shorter than those in the original scheme there is not necessarily a unique answer). 

Examine scheme A in Example E - 3. Show why scheme A is not tree-like. 

Thereafter, the reaction between the coordinated dienoate ligand in 106 and acrylonitrile was examined (Scheme 12). Surprisingly, this reaction is complete within 56 h (pseudo-first-order rate constant k — 1.4 x 10-5 s-1, ti/2 ca. 0.5 day) and affords only two products 107a and 107b in a ratio of 57 43 (or the correepsonding acids 103a,b via acid hydrolysis). Thus, in... 

The mechanistic photochemistry of cross-conjugated 2,5-cyclohexadienones has been comprehensively examined (Scheme 21). The photochemical isomerization of the... 

For the conversion of silyl-carbon bond in the cyclization products into hydroxy-carbon bond, several functionalized hydrosilanes were examined (Scheme 27).84,84a,84b Of the hydrosilanes examined, benzhydryldimethylsilane (HSiMe2CHPh2) was found to be most enantioselective in the reaction of diene 86a to give the cyclization product 87a with 93% ee. The second highest enantioselectivity (91% ee) was observed with hydrosiloxane HSiMe2OSiPh2Bu-/. The cyclization-hydrosilylation with the HSiMe2CHPh2 and catalyst 88 was very successful... 

In a recent article <2005JOC7947>, the products arising from the decomposition of the triazolopyridine 34 were examined (Scheme 10). The transient carbene 35 further led to 9-cyanofluorene 36 as the main product, along with... 

The binding mode of a vinylallene and a rhodium(I) complex depends greatly on the substitution patterns, probably for steric reasons. The reaction of a vinylallene lacking substituents at the vinylic terminus with RhCl(PPh3)3 provides a a2-bonded (vinylallene)rhodium complex having an essentially planar structure. Several stoichiometric reactions of the complex have been examined (Scheme 16.41) [41]. 

A closely related reaction having the phenol protected with a trimethylsilyl group was also examined (Scheme 31) [45]. Unlike the earlier examples, the cyclization reaction resulting from this substrate did not require the presence of a mild acid. The anodic oxidation in methanol solvent with no acetic acid led to a 73% yield of the tricyclic product. In a nearly identical reaction, an anodic oxidation of the trimethylsilyl-protected substrate in the presence of 2,6-lutidine led to the cyclized product in a 60% yield. The use of the silyl group expanded the utility of the anodic C-C bond-forming reaction being studied by allowing for the use of neutral and basic conditions. Hence, it would appear that the cyclization reactions are compatible with the presence of both base and acid sensitive functionality. 

Cyclization reactions utilizing a vinyl sulfide group were also examined (Scheme 34) [46], This substrate was chosen for study because, like the methyl substituents used earlier, the sulfide would have a favorable conformational effect on the substrate and would serve as an electron-donating group for making the olefin more nucleophilic. Unlike the methyl substituent, the use of the sulfide led to a carbonyl product that could then be used to further elaborate the cyclized product. Hence, the success of the vinyl sulfide-based cyclization reaction served to extend the synthetic scope of these reactions. 

Finally, the intramolecular coupling reaction between an olefin and a pyrrole ring has been examined (Scheme 40). In this example, a 66% isolated yield of the six-membered ring product was obtained. A vinyl sulfide moiety was used as the olefin participant and the nitrogen protected as the pivaloyl amide in order to minimize the competition between substrate and product oxidation. Unlike the furan cyclizations, the anodic oxidation of the pyrrole-based substrate led mainly to the desired aromatic product without the need for subsequent treatment with acid. 

Maruoka has found that simple alcohols can also be used in the oxy-Michael reaction [107], Using the axially chiral biaryl catalyst 67 (1 mol%) the conjugate addition of methanol, ethanol and aUyl alcohol to a, 3-unsaturated aldehydes was examined (Scheme 29). Despite moderate yields (55-83%) and enantioselectivities (16-53% ee), the high activity of this catalyst suggests that further optimisation... 

A potential versatile route into a-amino acids and their derivatives is via a combination of (i) nitrile hydratase/amidase-mediated conversion of substituted malo-nonitriles to the corresponding amide/acid followed by (ii) stereospecific Hofmann rearrangement of the amide group to the corresponding amine. Using a series of a,a-disubstituted malononitriles 14, cyanocarboxamides 15 and bis-carboxamides 16, the substrate specificity of the nitrile hydratase and amidase from Rhodococcus rhodochrous IF015564 was initially examined (Scheme 2.7). The amidase hydrolyzed the diamide 16 to produce (R)-17 with 95% conversion and 98%e.e. Amide 17 was then chemically converted to a precursor of (S)-a-methyldopa. It was found... 

To verify this mechanistic possibility, the solvent dependence of the ene products derived from the photooxygenation of the isomeric a,-unsaturated esters 93-E and 93-Z was examined (Scheme 23). For 93-E, the two ene products are formed from two different perepoxides. When the oxygen atom of the perepoxide intermediate is placed syn to the ester group, 93b is produced, whereas 93a is formed from the opposite case. For isomer 93-E, the expected solvent effect was found (93a/93b = 85/15 in CCI4 or benzene, and 70/30 in DMSO). On the other hand, for the isomer 93-Z, both products are formed from the same intermediate (the perepoxide oxygen is placed anti to the ester functionality), and no solvent dependence on the ene products was found (93a/93b = 95/5 in CCI4 and 93/7 in DMSO). 

The magnesium ion-mediated nitrone cycloadditions of an a,y-disubstituted allyl alcohol are stereoselective, and show opposite regioselectivity to that observed when zinc-mediated reactions are examined (Scheme 11.48) (167). The exo-syn-isomer of the isoxazolidine-5-alcohol regioisomer and the exo-syn-isomer of the isoxazolidine-4-alcohol regioisomer are the exclusive cycloadducts in the magnesium- and zinc-mediated reactions, respectively. 

A few years ago an old reaction, that of the thermal decomposition of arylsulfonyl azides in pyridines, was re-examined (Scheme 42) (72JOC2022). Among the products identified was the 3-phenylsulfonamido derivative (96), which was proposed to arise by an electrophilic attack by phenylsulfonylnitrene. 

The search for stable diaminocarbenes dates back to the early 1960s and is associated with the name of Wanziick." At that time the preparation of the 1,3-diphenyl imidazolidin-2-ylidenes (Ilia) was first examined (Scheme 8.4). Precursors of Ilia included the dimeric and electron-rich olefin IIIa 2 and the chloroform adduct 3a of the desired carbene. By means of cross-coupling experiments, however, it was shown that IIIa 2 was not in equilibrium with the two carbene units. On the other... 

In spite of its long-assumed intermediacy in several reactions, no carbon-substituted silylene was directly observed for many years. In 1979, however, Drahnak et al. detected a broad ultraviolet (UV) absorption band (3-niax = 450 nm) after the photolysis of dodecamethylcyclohexasilane (6) in 3-methylpentane. This band was assigned to dimethylsilylene (5). Many different approaches to this intermediate, either photochemicaUy or thermally, were examined (Scheme 14.6). ... 

The intramolecular behavior of a range of annulated a,/3 y,8-unsaturated carbonyl ylides has also been examined (Scheme 42). 

Recently [89IZV(ip)] the effect of the cycloalkanone oxime ring size on their pyrrolization rate under the action of acetylene in the KOH/DMSO system at 86°C and atmospheric pressure has been examined (Scheme 19). 

The effects of different substituents (R) (both steric and electronic) on the 1,5-ipso substitution reactions of radicals of the type (73) to give (74) and finally biaryls (75) after loss of SO2 have been examined (Scheme 31).154,155 The results indicated that the introduction of either an electron-donating or -releasing group in the ortho position... 

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