Azines metallation

Sodium dichromate and various chromic salts are employed in the textile industry (195,196). The former is used as an oxidant and as a source of chromium, for example, to dye wool and synthetics with mordant acid dyes, oxidi2e vat dyes and indigosol dyes on wool, aftertreat direct dyes and sulfur dyes on cotton to improve washfastness, and oxidi2e dyed wool. Premera11i2ed dyes are also employed. These are hydroxya2o or a2omethine dyes in which chromium or other metals are combined in the dye (see Azine dyes DYES Azo dyes). 

Fig. 2. Phthalocyaiiine (22), and metallized fomiazan (23, 24) and azine (25) blue reactive dyes. Reactive Blue 15 [12225-39-7] (Cl 74459) (22) Reactive Blues (23) [78709-74-7] (8) and (24) (9) Reactive Blue 204 [109125-56-6] (25). Otlieiblue oxazine dyes aie other fluorotriazines [97140-65-3] (10,11), and a...

The factors in carboaromatic nucleophilic displacements summarized in this section are likely to be characteristic of heteroaromatic reactions and can be used to rationalize the behavior of azine derivatives. The effect of hydrogen bonding and of complexing with metal compounds in providing various degrees of electrophilic catalysis (cf. Section II, C) would be expected to be more extensive in heteroaromatics. 

As for cyclopropanation of alkenes with aryldiazomethanes, there seems to be only one report of a successful reaction with a group 9 transition metal catalyst Rh2(OAc)4 promotes phenylcyclopropane formation with phenyldiazomethane, but satisfactory yields are obtained only with vinyl ethers 4S) (Scheme 2). Cis- and trans-stilbene as well as benzalazine represent by-products of these reactions, and Rh2(OAc)4 has to be used in an unusually high concentration because the azine inhibits its catalytic activity. With most monosubstituted alkenes of Scheme 2, a preference for the Z-cyclopropane is observed similarly, -selectivity in cyclopropanation of cyclopentene is found. These selectivities are the exact opposite to those obtained in reactions of ethyl diazoacetate with the same olefins 45). Furthermore, they are temperature-dependent for example, the cisjtrcms ratio for l-ethoxy-2-phenylcyclopropane increases with decreasing temperature. 

In the analogous reaction of differently substituted azines RR C=NN=CRR , the products depend strongly on the metal used (Ti and Zr) as well as on the substituents R and R [43], With R = R = Me and M = Ti, substitution of the alkyne by the azine and subsequent CH activation of the complex 78 is observed. With R = Ph and R = H, the acetylene is also substituted and, through a reductive coupling of two azine molecules, the binudear Ti(III) complex 79 is formed. Using the zirconocene 2a, and with azine substituents R = Ph and R = H, no substitution of the alkyne is observed, but one of the C=N double bonds of the azine inserts into the Zr—C bond of the starting complex to yield complex 80. 

With R = R = Ph and using complexes 1 or 2a, the central N -N single bond of the azine is cleaved by both metals. In this case, the bis(imido) complexes 81 were formed, treatment of which with complexes such as CpCo(C2H2)2 can give heterobimetallic bis(alkylideneamido)-bridged complexes such as 82. Mach has used this concept for the reaction of methyl-substituted titanocenes with acetoneazine. With 3, monomeric Ti(III) complexes 83 and, after activation of the methyl groups, coupled products such as 84 could be obtained [44],... 

Terminal alkynes readily react with coordinatively unsaturated transition metal complexes to yield vinylidene complexes. If the vinylidene complex is sufficiently electrophilic, nucleophiles such as amides, alcohols or water can add to the a-carbon atom to yield heteroatom-substituted carbene complexes (Figure 2.10) [129 -135]. If the nucleophile is bound to the alkyne, intramolecular addition to the intermediate vinylidene will lead to the formation of heterocyclic carbene complexes [136-141]. Vinylidene complexes can further undergo [2 -i- 2] cycloadditions with imines, forming azetidin-2-ylidene complexes [142,143]. Cycloaddition to azines leads to the formation of pyrazolidin-3-ylidene complexes [143] (Table 2.7). 

The normal byproducts formed during the transition metal-catalyzed decomposition of diazoalkanes are carbene dimers and azines [496,1023,1329], These products result from the reaction of carbene complexes with the carbene precursor. Their formation can be suppressed by slow addition (e.g. with a syringe motor) of a dilute solution of the diazo compound to the mixture of substrate and catalyst. Carbene dimerization can, however, also be a synthetically useful process. If, e.g., diazoacetone is treated with 0.1% RuClCpIPPhjij at 65 °C in toluene, cw-3-hexene-2,5-dione is obtained in 81% yield with high stereoselectivity [1038]. 

The regiospecific synthesis of substituted azine derivatives by the use of heteroatom directed lithiation has also become a common occurrence in recent years, and this route has led to the successful synthesis of a wide variety of substituted derivatives [87MI1 88AHC(44)199 90CRV879 91AHC(52)187]. The activation and stabilization provided by the heteroatom substituent mean that deprotonation can occur at lower temperatures, and thus the problem of addition to the azomethine double bond is minimized. The utility of this method, and of the different substituent groups involved in the directed metalation process, is detailed in the following sections. 

As with their monocyclic analogs, lithiated derivatives of bicyclic di-azines can be prepared by halogen-metal exchange at low temperature, and examples include the 6-lithiopurine 130 (79JOC4612), and the 5-and 7-lithio-3//-l,2,3-triazolo[4,5-(f pyrimidines 131 and 132 (91CPB2793, 91CPB3037). 

The transition metal catalyzed carbon-carbon bond formation between organomagnesium reagents and aryl (vinyl) halides has been one of the pioneering entries into cross-coupling chemistry. The reaction has been widely utilized since than in azine chemistry,22 with the limitation that the functional group tolerance of Grignard reagents is only moderate. Here only some of the more recent developments will be mentioned. 

Finally, those organometallic derivatives of azines in which the metal is separated from the ring by one carbon atom are considered under the corresponding alkyl compound (Section 3.2.3.3). 

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