Benzoquinones reactions with phosphines

To our knowledge, the first examples of asymmetrically substituted monocyclic phosphoranes are 60 and 61, described by Moriarty et al.135 and involving the reaction of a substituted o-benzoquinone136,137 (Scheme 6) on an aminophosphine (59), itself obtained by alcoholysis of 58 with l-( — )-menthol. In contrast to the amino phosphine 53 (Scheme 5), 59 is a mixture of the diastereoisomers 59a and b, and its reaction with 3,5-di-tert-butyl-l,2-benzoquinone yields two diastereoisomeric phosphoranes, 60a and b. Finally, alcoholysis of the P(V)—NR2 bond138 in 60a and b leads to 61a and b or 62. 

The phosphine bleaching agent THP (see the Coniferaldehyde and Ferulic Acid section) also reacts with quinones. When methoxy-p-benzoquinone is reacted with THP, a zwitterionic phosphonium ion (Figure 3.7) is formed [49]. The broad, 368 nm absorption of the quinone is replaced with a much sharper absorption band at 305 nm. This differs from the reaction with sodium hydrosullite, which reduces quinones to the corresponding hydroquinones. 

In the group of Backvall a method was developed involving palladium and benzoquinone as cocatalyst (Fig. 4.42) [103]. The difficulty of the catalytic reaction lies in the problematic reoxidation of Pd(0) which cannot be achieved by dioxygen directly (see also Wacker process). To overcome this a number of electron mediators have been developed, such as benzoquinone in combination with metal macrocycles, heteropolyacids or other metal salts (see Fig. 4.42). Alternatively a bimetallic palladium(II) air oxidation system, involving bridging phosphines, can be used which does not require additional mediators [115]. This approach would also allow the development of asymmetric Pd-catalyzed allylic oxidation. 

Aliphatic and aromatic thiols have been added to (5) -methylphenylvinylphosphine oxide (84) to provide optically active P-alkylthio- and p-arylthioethylphenylphosphine oxides (85).The reaction of the phosphine sulphide (86) with tetrachloro-o-benzoquinone gives (87) which undergoes further rearrangement. ... 

Reaction of p-benzoquinone with fluorodisilane provides p-bis(siloxy)benzene 81 in 41% yield in the presence of the phosphine-palladium catalyst (Eq.40) [55]. 

The choice of the most effective catalyst system is highly dependent on the type of olefin under consideration. Polymerization of CO and aliphatic a-olefines is most suitably carried out employing a catalytic system modified with a symmetrical, Cs-bridged aryldiphosphine ligand (2,71). However, these systems are not suitable for copolymerization of CO and styrene (41). For this reaction palladium(II)-based catalysts modified with a conjugated diimine (39,41), a bisoxazoline (43,44), a phosphine-phosphite (43), or a phosphine-imine ligand (43) have been employed, in, under chain-transfer conditions, combination with an oxidant promotor, such as 1,4-benzoquinone or 1,4-naphthoquinone (39-47,72), or a polar, acidic type of solvent (73,74). 

Especially water is prohlematic for the reaction as it can lead to the decomposition of the palladium compound, for example, in the reaaion of Pd(OAc)2 with a phosphine hy partial oxidation to the phosphine oxide R3P=0. On the other hand, it can act in an analogous manner as methanol as an activator for the copolymerization reaction. Intermediary [Pd-H] catalytic species, which can he formed, for example, by water gas shift reactions, are also highly reactive and can decompose to Pd (0). This reaction can be prevented by the addition of oxidants such as benzoquinone, which reoxidizes decomposed Pd (0) to active Pd(ll). However, this compound is also involved in the formation of ester-terminated oligomers or polymers, mainly [E-E], by oxidation of the growing polymer chain with methanol. 

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