Some Synthetic Applications

Decarbonylation of aldehydes is frequently used in synthetic organic chemistry [19]. Aromatic aldehydes and enals, but also saturated aldehydes, have been shortened by one C atom with this transformation. In most cases, rhodium complexes were used in a stoichiometric reaction, but also catalytic transformations have been described [20]. It was found that RhCl3-3H20 modified with dppp was less air-sensitive than [Rh(COD)Cl]2 (COD = 1,5-cyclooctadiene) modified with dppp or the Wilkinson complex and therefore better suited for lab-scale experiments [21]. While using the Wilkinson complex, strictly oxygen-free conditions were essential for the success. Besides homogeneous rhodium catalysts, also supported complexes were suggested recently [22]. The reaction in ionic liquids is a possibility to recycle the precious rhodium complex [23]. 

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The preparation and some synthetic applications of lithium dialkylcuprates were described earlier (Section 14 11) The most prominent feature of these reagents is then-capacity to undergo conjugate addition to a p unsaturated aldehydes and ketones... 

The [2 + 2] photodimerization of a, j8-unsaturated sulfones is correctly viewed as a photoreaction of alkenes, rather than the sulfone group, and this aspect has been reviewed recently by Reid, as part of a wider survey of the photoreaction of O- and S-heterocycles. The topic continues to attract considerable interest and a few recent examples, as well as some synthetic applications, will be discussed here. Much of the photodimerization work has been carried out on the benzo[fc]thiophene (thianaphthene) 1,1-dioxide system. For example. Porter and coworkers have shown that both 3-carboxybenzo[i]thiophene 1,1-dioxide (65) and its methyl ester give only the head-to-head (hth), anti dimer (66) on irradiation in ethanol. In a rather unusual finding for such systems, the same dimer was obtained on thermal dimerization of 65. Similar findings for a much wider variety of 3-substituted benzo[fi]thiophene 1,1-dioxides have been reported more recently by Geneste and coworkers . In the 2-substituted analogs, the hth dimer is accompanied by some of the head-to-tail (htt), anti dimer. The formation of the major dimer appears to proceed by way of an excited triplet and the regiochemistry observed is in accord with frontier MO theory. 

Several radical reaction have some synthetic application, including radical substitution (Section 11.4.1) and the SflJvl reaction (Section 11.4.2). 

The first of the few low-temperature methods for the formation of an o-QM was a method developed by Rokita.5 It is principally used for reversible DNA alkylation. However, it has recently begun to find its way into some synthetic applications. It utilizes a silylated phenol, which proves vastly more manageable as an o-QM precursor than the corresponding o-hydroxyl benzyl halide (Fig. 4.6). In this kinetically controlled process, expulsion of a benzylic leaving group is triggered at low temperature by treatment with a fluoride ion, which causes a (3-elimination. 

In this section a brief review of the various reactions arising from type I cleavage and some synthetic applications of these reactions will be presented. 

The reaction of carbonyl compounds to olefins often yields products difficult to obtain synthetically by other routes. The excellent yields obtainable under proper conditions make this reaction of definite preparative interest. Examples of some synthetic applications of oxetane formation follow ... 

Besides the above electrophiles, the acetylene—titanium complexes react regioselectively with other acetylenes providing the corresponding titanacyclopentadienes. An example of a homo-coupling reaction is shown in Eq. 9.11 [30], which also displays some synthetic applications [30,31]. Especially noteworthy is the highly regioselective cross-coupling reaction of unsymmetrical internal and terminal acetylenes, which is illustrated in Eq. 9.12... 

Figure 2.51 reports some synthetic applications of a-allyl- and a-vinyltrifluoro-... 

Figure 5.13 Production of cii-diols using P. putida F39/D and some synthetic applications.

Some synthetic applications involving an intramolecular VNS reaction have been developed.116-118... 

Transamination reactions of this type have found some synthetic application. The synthesis of the nickel(n) complex of a macrocycle indicated in Fig. 5-58 clearly involves... 

Since the rearrangement of silicon azides is known, we looked for some synthetic applications in organometallic chemistry. For example, ring extension reactions involving sila.imines intermediates can be described ... 

Some synthetic applications of this reaction are summarized in Table 7. 

There are a number of examples of photochemical transformations of DAMN which may have some synthetic applications. At first, irradiation isomerizes the cir-dinitrile to the rrani -dinitrile, which then forms 5-amino-4-cyanoimidazole (18). Both azctidine or azirine intermediates have been postulated for these cyclizations [54, 55]. Although yields are good the photolytic conditions require dilute solutions ( lO moll solutions of the enaminonitriles) and may not prove practicable for the synthesis of larger quantities of imidazoles. On a small scale, however, the method merits consideration particularly since the process works for a wide variety of enaminonitriles (Scheme 2.1.9) [56-58]. 

Lipases, and many other enzymes as well, have been shown to be active in organic solvents with low water content [54] (< 1% v/v of water). From this finding some synthetic applications are derived [55], the most important are the following ... 

In these fields, as elsewhere, dithiolium cations are useful synthetic intermediates. In the case of l,6,6aA -trithiapentaIenes, some synthetic applications of 1,2-dithiolium cations were reviewed by one of us in this series in 1971. For these particular topics we shall not refer to papers already considered in that review (the corresponding information will be found in Ref. 2 pp, 173-177, 181, 187-190, 193, 197, 201-202, 217, and 223). 

Disubstituted cyclopropanes possessing at least one electron-withdrawing substituent can undergo base-induced cisjtrans isomerization. This transformation leads to the thermodynamically more stable isomers, probably by opening and reclosure of the three-membered ring, " and the trani-isomer is overwhelmingly preferred. Some synthetic applications are summarized in the table below. 

Reduction by hydrides of two types of organometallic compounds has received some synthetic applications so only these cases will be discussed. 

Some synthetic applications are shown in Schemes 2.39-2.41. 1,1-Addition of perfluoroalkyl iodides to isonitriles (26) provides perfluoroalkyl imidoyl iodides (27) [24], which have been employed as useful reactive intermediates for trifluoromethylated amino acids and heterocycles [25].Stereo-controlledperfluoroalkylation [26] andbromodifluoromethy-lation [27] of the enolates (28) and (30) give diastereomerically enriched fluoro-building blocks (29) and (31). 

Etiolates formed by organocopper conjugate addition may be acylated cleanly by acid chlorides to give 3-diketones. Although 0- and C-acylation are both possible, the latter is favored by the use of acid chlorides rather than anhydrides and by the use of diethyl ether as solvent, rather than DME. Good yields of 3-diketones have been obtained by acylation of the anions derived from both acyclic and cyclic unsaturated ketones with cuprates, or in copper-catalyzed Grignard reactions. Some synthetic applications are given in Scheme 54. 

The DMDO oxidation of iodocyclohexane affords lran5-2-iodocyclohexanol as the final product via intermediate formation of iodosylcyclohexane followed by elimination of hypoiodous acid, which then adds to the alkene generated in the elimination step [103]. The oxidative deiodination of iodoalkanes via conversion into iodosylalkanes followed by nucleophilic substitution of the iodosyl group has found some synthetic application, particularly in the synthesis of steroidal products (Section 3.1.19) [107]. 

Several examples of alkenyliodonium salts have been known for more than a hundred years, but it was only in the 1990s that these compounds become readily available and found some synthetic application. The chemistry of alkenyl(aryl)iodonium salts has been covered in several reviews by Ochiai [431], Okuyama [432-434] and Zefirov and coauthors [435]. 

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