Herrmann’s catalyst

Other than the aforementioned ynones, P-iodo-p/y-enone 132 was also converted into a 2,5-disubstituted furan, 133, under Pd-catalyzed cyclization conditions using Herrmann s catalyst, palladacycle catalyst 101 [105]. 

Herrmann s catalyst 40 can effect the intramolecular twefo-cyclization of the vinyl bromide 41 to give the corresponding cyclohexyl-fused 277-chromene (Equation 20). The same catalyst is also effective during the intramolecular coupling of phenols with aryl halides to afford 67/-benzo[7]chromen-l-ols 42 (Equation 21) <1997TL6379, 1997JOC2>. 

McQuade and coworkers reported the use of Pd nanoparticles encapsulated in cross-Hnked reverse micelles in the Heck reaction between aryl iodides and methyl acrylate (Fig. 10.10) [35]. The catalyst showed high activity, comparable to that of Herrmann s catalyst however activity remained limited to aryl iodides. Interestingly, low Pd loadings exhibited higher activities, a result consistent with catalysis by molecular species. 

Ozawa conditions Herrmann s catalyst, phosphine ligand CS2CO3, THF, 125 C, 24 h Fagnon conditions Pd(OAc)2, PivOH, K2CO3 DMAc, 95°C, 72 h... 

The scope of the cyclization reaction has been extended so as to permit the synthesis of flve-membered rings, such as benzofurans and indoles, by using Herrmann s catalyst Unfortunately, however, the regioselectivity observed in the formation of benzo-fused flve-membered rings has been uniformly low (Scheme... 

Several catalysts have been developed that allow efficient Heck reactions of reluctant substrates under reasonable conditions and with high turnover numbers. One such catalyst is Herrmann s catalyst 5.58, a palladacycle derived from (o-tol)3P 5.57 (Schemes 5.17 and 5.18). Palladium complexes with t-BusP also provide highly active catalysts (Scheme 5.19). ... 

FIGURE 6.18 Synthesis of radiolabeled 2-p-carborane (198,199), 9-o-carborane (200,201), 3-o-carborane (202, 203), 9-m-carborane (204,205), l-phenyl-S- l-o-carborane (206), and l,2-diphenyl-3- 25l-o-carborane (207) using Herrmann s catalyst (208). 

Using the procedure described above for radioiodination with Herrmann s catalyst (208), Sjoberg et al. also synthesized radiobrominated 199,201,203, and 205 (Figure 6.18) from the corresponding cold iodine precursors (Winberg et al., 2003, 2005). Compound 205 was obtained in 64% whereas 199,201, and 203 were radiolabeled with more than 90% yields. Maximum yields were obtained at 110°C temperature and 40 min reaction time. 

Palladacycles As mentioned previously, the use of P(o-Tol)3 as a ligand was pioneered by Spencer, however it was Herrmann who demonstrated its true potential. He discovered that treatment of Pd(OAc)2 with P(o-To1)3 in toluene actually affords a cyclometallated palladacycle 97, now known as Herrmann s catalyst. When this complex was employed in the Heck reaction, efficient coupling of aryl bromides and activated aiyl chlorides was observed at high temperatures. 

This reductive Heck reaction has become a useful tool in cyclization reactions for complex molecule synthesis. In his protecting group free synthesis of ambiguine H (119), Baran successfully applied this strategy. Slow addition of Herrmann s catalyst 91 to substrate 117 provided intermediate 118 in a reliable 65% yield. This chemistry proved both robust and scalable, providing gram quantities of 118. 

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