Palladium azide

Methylcadmium azide, 0430 Nickel azide, 4771 Palladium ) azide, 4777... 

Complex palladium azides are obtained by dissolving palladium salts in excess azide, e.g., palladium nitrate and sodium azide (1 4) yield a triazido anion, [Pd (N3)3]" [167]. Similarly, a tetraazido anion, [Pd (N3)4] , is obtained from K2 [PdCU] by ligand exchange [162]. Another all-azido anion is the azide-bridged, binuclear complex [Pd2(N3)6] [168]. All of these have been isolated as salts of large organic cations. 

Figure 16. Palladium-azide bonding in [As(C6Hs)4]2lPd2(N3)6].

Palladium azide Pd(N3)2 with acids Crystalline solid explodes on heating sensitive to... 

On the other hand, sulphur reacts with the palladium azide complex to give a dimer which is linked by N atoms as indicated in (8.113). Unlike in the copper complex where all the N-N distances are equal, unequal distances in the palladium complex indicate the triple bonds as shown (8.97). Dinitrogen may act as a bridging group as in... 

Palladium-Catalyzed Decomposition of Azo Compounds, Azides, and Peroxides... 

A better method for preparing primary amines is to use the azide synthesis, in which azjde ion, N3, is used for SN2 reaction with a primary or secondary alkyl halide to give an alkyl azide, RN3. Because alkyl azides are not nucleophilic, overalkylation can t occur. Subsequent reduction of the alkyl azide, either by catalytic hydrogenation over a palladium catalyst or by reaction with LiAlK4. then leads to the desired primary amine. Although the method works well, low-molecular-weight alkyl azides are explosive and must be handled carefully. 

Sodium azide, reaction with l butyl chloroacetate, 46, 47 reaction with diazomum salt from o amino-f> -ni trobiphcny L, 46, 86 Sodium chlorodifluoroacetate, 47, SO reaction with tnphenylphosphme and benzaldehyde, 47, SO Sodium ethoxide, 46, 2S reaction with diethyl succinate, 46,2S Sodium formate as reducing agent in preparation of palladium catalyst, 46, 90... 

Acid derivatives that can be converted to amides include thiol acids (RCOSH), thiol esters (RCOSR), ° acyloxyboranes [RCOB(OR )2]. silicic esters [(RCOO)4Si], 1,1,1-trihalo ketones (RCOCXa), a-keto nitriles, acyl azides, and non-enolizable ketones (see the Haller-Bauer reaction 12-31). A polymer-bound acyl derivative was converted to an amide using tributylvinyl tin, trifluoroacetic acid, AsPh3, and a palladium catalyst. The source of amine in this reaction was the polymer itself, which was an amide resin. 

The NHCs have been used as ligands of different metal catalysts (i.e. copper, nickel, gold, cobalt, palladium, rhodium) in a wide range of cycloaddition reactions such as [4-1-2] (see Section 5.6), [3h-2], [2h-2h-2] and others. These NHC-metal catalysts have allowed reactions to occur at lower temperature and pressure. Furthermore, some NHC-TM catalysts even promote previously unknown reactions. One of the most popular reactions to generate 1,2,3-triazoles is the 1,3-dipolar Huisgen cycloaddition (reaction between azides and alkynes) [8]. Lately, this [3h-2] cycloaddition reaction has been aided by different [Cu(NHC)JX complexes [9]. The reactions between electron-rich, electron-poor and/or hindered alkynes 16 and azides 17 in the presence of low NHC-copper 18-20 loadings (in some cases even ppm amounts were used) afforded the 1,2,3-triazoles 21 regioselectively (Scheme 5.5 Table 5.2). 

Ene-nitrile oxidoisoquinolinium betaine 131 was readily prepared from vinyl triflate aldehyde 79 (Scheme 1.14). Palladium-catalyzed cyanation of vinyl triflate 79 with Zn(CN)2 in DMF at 60 °C produced ene-nitrile aldehyde 129 in 85 % yield [54]. Using the previously developed Staudinger-aza-Wittig reduction sequence, aldehyde 129 was coupled with cyclic ketal azide 121 to afford a 79 % yield of amine 130. The cyclic ketal amine 130 was then treated with 9 1 mixture of CH2CI2/TFA to provide ene-nitrile oxidoisoquinolinium betaine 131 in 93 % yield. 

A variety of triazole-based monophosphines (ClickPhos) 141 have been prepared via efficient 1,3-dipolar cycloaddition of readily available azides and acetylenes and their palladium complexes provided excellent yields in the amination reactions and Suzuki-Miyaura coupling reactions of unactivated aryl chlorides <06JOC3928>. A novel P,N-type ligand family (ClickPhine) is easily accessible using the Cu(I)-catalyzed azide-alkyne cycloaddition reaction and was tested in palladium-catalyzed allylic alkylation reactions <06OL3227>. Novel chiral ligands, (S)-(+)-l-substituted aryl-4-(l-phenyl) ethylformamido-5-amino-1,2,3-triazoles 142,... 

Chang et al. reported a mild tandem intramolecular hydroamination of yne amines to form an endo-adduct intermediate, which reacts with electron-deficient azides to produce cyclic amidines <06JA12366>. Selected examples of an interesting synthetic route to tropene derivatives 165 via a dual hydroamination strategy is shown below. This one-step reaction makes use of a palladium catalyst and takes place by sequential intermolecular hydroamination of cycloheptatriene with aryl, heteroaryl, and primary alkyl amines to generate intermediate 166, followed by transannular intramolecular hydroamination <06JA8134>. 

Aryl and vinyl nitriles have been prepared very efficiently from the corresponding bromides by palladium-catalyzed reactions under microwaves. This energy source has been employed for the conversion of these nitriles into aryl and vinyl tetrazoles by cycloaddition reactions with sodium azide (Scheme 9.66). The direct transformation of aryl halides to the aryl tetrazoles in a one pot procedure could be accomplished both in solution and on a solid support [115], The reactions were complete in a few minutes, a reaction time considerably shorter than those previously reported for the thermal reactions. The cydoadditions were performed with sodium azide and ammonium chloride in DMF and, although no explosion occurred in the development of this work, the authors point out the necessity of taking adequate precautions against this eventuality. 

In the presence of copper and palladium catalysts, terminal alkynes 1222 react with trimethylsilyl azide and allyl methyl carbonate to provide 2,4-disubstituted 1,2,3-triazoles 1223 in moderate to good yield. Isomerization of the allyl substituent in the presence of a ruthenium catalyst gives 4-substituted 2-(l-propen-l-yl)-2//-l,2,3-triazoles 1224. 

Other routes to the key imine intermediate of the Pictet-Spengler reaction have also been reported. For example, Molina and coworkers [34] generated imine intermediate 34 by reaction of azide 32 with aldehyde 33 in the presence of tributylphosphine, via the corresponding iminophosphorane (Fig. 11). Use of triphenylphosphine was unsuccessful in the formation of an iminophosphorane. Without isolation, the intermediate was heated at 165 °C in a sealed tube with palladium on carbon, giving /J-carboline 35 in 45% yield. This com-... 

An interesting palladium-catalyzed allene/azide incorporation and intramolecular 1,3-dipolar cycloaddition cascade to tetrazolo[5,l-tf]isoquinoline has been published by Grigg et al. <2005TL5899>. In the first step of the events, 3-bromo-6-iodobenzonitrile 105 was reacted with the allene/trimethylsilylazide system in the presence of palladium(O) catalyst to yield a coupling product 106 which under the reaction conditions applied (DMF, 70 °C for 24 h) gave 107. 

Sato et al. carried out detailed studies on the possibilities of transformation of tetrazolo[l,5-tf]pyrazines 54 to 2-aminopyrazines 56 < 1994S931 >. These authors found that the generally used methods for this conversion fail because the starting compound exists in the stable bicyclic form 54, whereas partial formation of the azide valence bond isomer 55 would be necessary for the success of the transformation. Application of special reaction conditions succeeded, however hydrogenation over palladium catalyst in the presence of ammonium hydroxide or treatment with stannous chloride in a mixture of methanol and hydrochloric acid solved this problem. Thus, a great number of derivatives of 54 was reduced to the corresponding 2-aminopyrazine 56 in medium to high yields (45-100%). 

Another route involves a palladium-copper-catalyzed tandem carbon-carbon formation/cycloaddition sequence (Equation 12) <2005TL8531>. Notably, cycloadditions of azide to the internal alkynes failed under click chemistry reaction conditions <2003DDT1128>. Cyclization under oxidative conditions has been reported from dithioacetal 163 (Equation 13) <1996TL3925>. The formation of 164 as a single diastereoisomer has been explained by stereoelectronic effects. 

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