Diazene conversion

Chlorodifluoroacetonitrile and silver(II) fluoride give104 the azo compound bis(2-chloro-l,l,2,2-tetrafluoroethyl)diazene in 45% conversion in a reaction reminiscent of those of per-fluoro nitriles. 

Photochemically induced epoxidation of tetrafluoroethene by oxygen with improved yields (71-76%, conversion 21-62%) is achieved in the presence of radical generators such as tri-bromofluoromethane, 1,2-dibromotctrafluorocthane, ethyl nitrite or 1 H, /F,5//-octafluoropen-tyl nitrite.36 The oxidation of tetrafluoroethene with oxygen can also be catalyzed with bis(trifluoromethyl)diazene an undistillable viscous oil with peroxide composition is formed initially which can be quantitatively converted into carbonyl fluoride when heated.37-38... 

The oxidation of phenylhydrazine and 1,2-disubstituted hydrazines to hydrazones and diazenes by CI2C proceeds via formation of unstable azomethine imines.95 The conversion of alcohols into alkyl halides is achieved by reaction with CCI4 (or CBr4) in DMF under electrochemical reduction.96 The reaction of dihalocarbene X2C with DMF to form a Vilsmaier reagent (93) is proposed as the key process. The reaction of simple isocyanates (RNCO) with dimethoxycarbene normally gives hydantoin-type products. In the reaction with vinyhsocyanates such as (94), however, hydroindoles (95) are formed in good yields.97... 

Although trans-diazene is thermodynamically unstable with respect to its decomposition, the rate of this reaction is surprisingly slow. This is because of orbital symmetry and the sterically unfavorable trans configuration. Thermal conversion to the more reactive cis isomer is necessary for the reaction to occur. It has also been noted that trans-diazene reacts surprisingly slowly with HC1 in the gas phase (167). 

The synthesis of diazene complexes has been described (Sections V,B, C, and D). It is worth reiterating that no diazene complex has been prepared via reactions of coordinated dinitrogen and no stoichiometric conversion to hydrazine or ammonia has been demonstrated. 

Fig. 17.69. Two-step sequence for the conversion of a ketone into the homologous nitrile ("reductive cyanation of a carbonyl compound"). In the second step of the reaction the diazene anion G is generated and decomposes in a similar way as the diazene anion D in the Wolff-Kishner reduction of Figure 17.67 and the diazene anion G of the semicarbazone reduction in Figure 17.68.

The Rh-induced conversion of co-carbonyl diazo ketones 34 generates carbonyl ylides 35, and the interception of these transient 1,3-dipolar species with dipolarophiles gives rise to [3-1-2] cycloadducts 36 (Scheme 7, upper part) (96CRV223). The carbonyl group is isoelectronic with the diazene functionality of azo compounds. This prompted the investigation of co-diazenyl a -diazo ketones under similar conditions (Scheme 7, lower part). 

When the 2,3-diazabicyclo[2.2.2]-2-octene bears an endo-phenyl substituent, only the exo-face of the protonated diazene undergoes the cycloaddition of cyclohexadicnc affording the anti-isomer126. Conversely, an equimolar mixture of the anti- and syw-isomers is produced from the exo-phenyl-substituted diazene. 

It is one example of an important group of reactions between triazolediones and polycyclic alkenes. Related syntheses in this group are summarized in Table 6. In each case, carbenium ion rearrangements, like that for benzvalene, lead to urazoles which on conversion to the five-membered ring diazene and deazetization generated a three-membered ring. 

Thermal and photochemical decompositions of cyclic diazenes usually proceed via diradicals as intermediates. Good synthetic routes to polycyclic diazenes are available, and consequently many interesting members of this class of compounds, containing three-membered rings, have been prepared and their decompositions studied under a variety of conditions. The thermal conversion of exo-6,7-diazatricyclo[3.2.1.0 ]oct-6-ene to cyclohexa-1,4-diene and nitrogen (Table 18, entry 1) was so much more rapid than the analogous decompositions of model cyclic diazenes that it probably occurred by a synchronous reaction, i.e. a diradical intermediate... 

Photochemically induced nitrogen-halogen bond cleavage is responsible for the conversion of N,N-dihalo-1,1-difluoroamines into the corresponding diazenes, and for the formation of the 1,1-adduct (147) and the 1,2-adduct (148) obtained on photodecomposition of 1,4-dibromo-2,5-piperazinedione (149) in the presence of 3,4-dihydro-2H-pyran. The photolysis of hypoiodites constitutes... 

The ability to access bridged-ring systems is exemplified by the efficient conversion of diazene 59 to cycloadduct 60. It is noteworthy that the conversion can routinely be carried out on a 20 g scale [25]. This chemistry was explored in conjunction with the development of a route to a paclitaxel analog (61) (Scheme 8), and is currently being examined as a key step in the development of a synthesis of aphidicolin ]26]. 

The opportunity to conduct a diyl trapping reaction depends upon the availability of bicyclic azo compounds to serve as the diyl precursor. A basic approach to these systems consists of fulvene formation [5], Diels-Alder cycloaddition, selective saturation of the A-5,6 pi bond, and conversion of the biscarbamate to the diazene linkage,[6] as depicted in Fig. (3). 

Diazene 19 was synthesized in the manner portrayed below. Thus, treatment of anhydride 20 with sodium borohydride selectively reduces one carbonyl to a methylene unit. Reduction of the resulting lactone with DIBAL followed by a Wittig reaction and oxidation with PCC afforded aldehyde 22. When treated with cyclopentadiene in the presence of diethylamine in methanol, 22 undergoes a smooth and efficient conversion to fulvene 23. Diels-Alder cycloaddition to the azodicarboxylate 24 proceeded rapidly, a characteristic of reactions with this electron deficient chlorinated dienophile [8]. Selective reduction of the endocyclic n bond using diimide generated in situ, followed by the electrochemical reductive cleavage of the biscarbamate led to diazene 19 [6]. 

Commercially available dihydro-5-(hydroxymethyl)-4,4-dimethyl-2(3H)-furanone (42) was chosen as the starting material since it incorporated the essential structural features that are present in the acyclic chain of diazene 41. To synthesize both coriolin (3) and hypnophilin (4), enone 39 was selected as a common intermediate. While 39 had previously been converted to coriolin (3), its conversion to hypnophilin (4) had not been accomplished. 

Key to the successful implementation of our plan was the construction of aldehyde 97 and its subsequent conversion to the diazene 96 on a multigram scale see Scheme 6. Scale, and the ability to scale up, are clearly important issues as they relate to the ability to utilize adaptations of this chemistry to construct useful amounts of bioactive materials. As indicated earlier in this chapter, the dioxolane subunit appended to the diylophile is critical to guarantee formation of the bridged cycloadduct, and is designed to serve as a synthon for the Ci hydroxyl group of the natural products [35]. Conversion of 95 to the target structure 92 was predicated upon the successful isomerization of the C-C Tt-bond to the... 

Schreiber et al. have been able to apply their enediyne intramolecular Diels-Alder approach to the synthesis of dynemicin model systems [268-270], culminating in the total synthesis of di- and tri-O-methyl dynemicin A methyl esters 388 and 389 (Scheme 7-78) [271], derivatives of the natural product itself. Highlights of this synthetic approach include (a) intramolecular lactonization and concomitant Diels-Alder cyclization (380- 381) (b) allylic hydroxylation followed by an allylic diazene rearrangement in order to regiospecifically isomerize a double bond (381 - 382) (c) a-hydroxylation of the lactone 381 and subsequent conversion to the P-ketoester 383 (d) annelation of the anthraquinone unit (383- 384- 385- 386) (e) mild base-induced P-elimination of the N-protecting group of 386 to give the free amine 387 and (f) a final oxidation to complete the anthraquinone (387 - 388). 

In contrast to their Fe(ii) congeners, Fe(i) or Fe(0) do activate N2 toward protonation. A first example was the so-called Leigh cycle, which involves the conversion of N2 to NH3 mediated by iron(O) phosphine complexes. In this context, Tyler and coworkers synthesized the complex [Fe(N2) (DMeO-PrPE)2] supported by the DMeOPrPE = l,2-bis[bis(methoxypropyl)phos-phinojethane ligand, which renders the iron-N2 complex water soluble. Based on this work, the reduction and protonation of N2 coordinated to the simpler Fe(dmpe)2 complex has been treated by DFT. In this work, three plausible mechanistic pathways were evaluated (1) symmetric protonation, where protons are added to each nitrogen in an alternating sequence proceeding through diazene and hydrazine intermediates (2) asymmetric... 

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