X-Phos

Additional work on alkyl platinum reactions has been completed which mostly supports the generality of these observed insertion reactions. Treichel and Wagner 153) have varied the choice of phosphine ligand utilizing the complexes Pt(phos)2(CH3)X (phos = PEtj, X = Br, I and phos = PPhMe2, X = I) both ionic intermediate and inserted products were obtained. 

Brightener structures of only moderate molecular size are of interest for white grounds in the transfer printing of polyester fabrics. Derivatives of 6-acetamidoquinoxaline with an electron-donating substituent (X) in the 2-position (11.48) were prepared by converting quinoxalin-2-one to 2-chloro-6-nitroquinoxaline and condensation with amines (X = RNH), alcohols (X = RO) or phenols (X = PhO), followed by reduction and acetylation (Scheme 11.19). The nitro intermediates (11.49) are also of interest as low-energy disperse dyes for polyester [61]. 

Implementation of the C5, C5 -hydroxy lation protocol as described in Scheme 7.19 above (71/72 to 73/74) provided further efficiencies. The C5,C5 -chlorination proceeded uneventfully, but the chloro to alkoxy interchange was difficult and required optimization of the reaction conditions. The catalyst system derived from Pd2dba3 and the X-phos(t-Bu) ligand proved to be effective in the coupling with KOH to provide the desired bisphenol. The resulting product was highly unstable and decomposed under a one-pot alkylation protocol. Isolation of the bisphenol under carefully controlled conditions followed by immediate benzylation (BnBr, NaH, DMF) furnished key intermediate 79 in 70 % yield. 

The position of phosphorus with respect to nitrogen in the periodic table led to the expectation that it should be much easier to remove one electron from X -phos-phorins than from pyridines (see also p. 37). Indeed, soon after the synthesis of 2.4.6-triphenyl-X -phosphorin 20 by Markl, we discovered that addition of 2.4.6-triphenoxyl 57 in benzene induces oxidation to the very stable radical cation 55... 

Mach investigated hydrogen peroxide oxidation of 2.4.6-tri-tert-butyl-X -phos-phorin 24 in analogy to pyridine oxidation to pyridine-N-oxide. Whereas 2.4.6-tri-tert-butyl-pyridine could be recovered unchanged, 2.4.6-tri-tert-butyl-X -phos-phorin under the same conditions is irranediately oxidized to the 2-hydro-phos-phinic acid m.p. 203 °C (transformation at 170 °C) 86 (CH3 = H) which, accord-... 

Phosphorus H2O2, halogens, diazonium salts lead to 1.1-substituted X -phos-phorins or to rearranged 2-hydrophosphinic acids. 

Transformation of X -phosphorins to other X -phosphorin-derivatives. Methods 2) and 3) allow for the highest degree of synthetic variation. X -Phos-... 

The fact that X -phosphorins are not basic, as discussed above, matches the finding that phosphorinium ions 127, corresponding to the well-known pyridinium ions 126, caimot be isolated. However, they play a key role as interm iiates in many syntheses, since they are easily attacked by nucleophiles, forming X -phos-phorins 128. 

Markl, Lieb and Merz have described the carbanionic addition of lithium or magnesium organometallic compounds to the P atom of 2.4.6-triphenyl-X -phos-phorins, which form deep red s ts 143. These can be alkylated either at the P atom to form X -phosphorins 144 or at C—2 to yield 1,2-dihydro-X -phosphorins 145. Acylation with benzoylchloride affords the 1,4-dihydro-X -phosphorin derivatives 146. Addition of acids or water leads to the synthetically important intermediates 147 which can be reconverted to the X -phosphorin-salts 143 by 2 N NaOH. 

Carbo-phosphorins are relatively basic. They are protonated by aqueous hydrogen chloride, thereby losing their color. According to Markl, the UV spectrum of the salt is similar to that of 1,2-dihydro-X -phosphorins (Fig. 25). However, it cannot be concluded on the basis of the UV spectrum alone that protonation takes place at position C—2. The C-4 position can also be protonated, as has been shown by NMR spectroscopy to be the case for 1, l-dimethoxy-2.4.6-triphenyl-X -phos-phorin. The influence of acid addition on the UV spectra is similar in both classes of compounds (compare Fig. 25 with Fig. 26). However, it should be noted that aqueous acids cannot be used to protonate l,l-dimethoxy-2.4.6-triphenyl-X -phos-... 

Most of the light yellow to yellowish-green colored 2.4.6-aryl-substituted X -phos-phorins fluoresce strongly. This phenomenon is an excellent aid in tracing X-phosphorins in preparative work. l.l-Bis-[4-nitro-phenoxy-] 2.4.6-triphenyl-XS-phosphorin and the l.l-dialkylthio-2.4.6-triphenyl-X -phosphorins do not fluorescence. 

Indeed, thermolysis of l.l-dimethylthio-2.4.6-triphenyl-X -phosphorin at 180 °C, neat or in toluene, affords preparative amounts of 2.4.6-triphenyl-X -phos-phorin in yields of 87% and 66%, respectively. 

Three independent X-ray structure determinations of X -phosphorins, i. e. 1.1-dimethyl-2.4.6-tripheny 1-X -phosphorin, 1.1 -dim ethoxy-2.4.6 -triphenyl-X -phos-phorin and l.l-bis-dimethylamino-2.4.6-triphenyl-X -phosphorin, have been made The result of one of these is shown in Fig. 36. All X-ray struc-... 

Table 31. 7T-AO coefficients of the phoq>honis and the C-2 and C-6 atoms in the TT-HOMO of X -phosphorins and X -pho horins...

These results show that the 3pzAO of phosphorus contributes considerably to ring conjugation in X -phosphorins The determining factor is that the highest occupied molecular orbital is of n type in both phosphorin systems. In X -phos-phorin the next lower MO is localized at the P atom to the extent of 60% (as an n MO). In the X -phosphorin system this is not possible, which is in accordance with the observed PE spectral intensities of Fig. 37, p. 115. The very different electron distribution of both X - and X -phosphorins in comparison to that of pyridine is in full accord with the chemistry of these classes of compounds ... 

According to calculations by Schweig and coworkers X -phosphorins should be oxidized to the radical cation at a lower oxidation potential than the X -phosphorins. Careful experiments on l.l-dimethoxy-2.4.6-triphenyl-X -phos-phorin by Weber confirmed these predictions. Accordingly, the 6jt system of X -phosphorin loses an electron more readily than that of X-phosphorin it is thus not the lone-electron pair at phosphorus, but rather the 6jr-electron system which is responsible for the easy oxidation of the phosphorins to radical cations. 

FIGURE 8. X-phos, a derivative of ligand 11 in Figure 5, synthesized for amination of unactivated aryl tosylates... 

Reaction of diarylamines with aryl nonaflates as part of the synthesis of discrete oligomeric triarylamines has also been reported104. Only one paper reports the palladium-catalyzed reactions of diarylamines with aryl tosylates this reaction was conducted with X-phos 31 (Figure 8) as the ligand in mixed solvents of 5 1 toluene t-butanol98. 

Hi. Reactions with aryl tosylates. The reaction of aniline with an activated aryl tosylate has been catalyzed by a combination of palladium and DfBPF 19 (Figure 6) to produce a high yield of diarylamine under relatively mild conditions79. The reaction of unactivated aryl tosylates with anilines has been more recently reported using catalysts containing X-phos 31 (Figure 8) in the presence of 5% phenylboronic acid98. 

The less reactive chlorobenzene and arene sulfonates coupled with phenylindole in the presence of a palladium catalyst generated from X-phos (31, Figure 8). For example, 4-tezt-butylphenyl tosylate reacted to give quantitative yields of ZV-(4-tezt-butylphenyl)indole in the presence of 2 mol% Pd, 5 mol% X-phos and K3PO4 as the base98. 

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