4,5-Dimethyl-2- phosphinine

The reaction of /3-enamino-A5-phosphanes with dimethyl acetylenedi-carboxylate gives l-aza-4A5-phosphinines (244) [89JCS(P1)2273]. Six-membered ring phosphorus-containing heterocycles were obtained by the reaction of enaminophosphines (245) with ethyl azidoformate (87TL2875). 

Dimethyl-4-phenyl- /l -phosphinine has a planar heterocyclic ring. Whereas pyridine has an almost regular hexagon, all the other... 

Phosphole sulfides reacted with diazoalkanes to give homophospholes which were transformed to phosphinines. Thus, 3,4-dimethyl-l-(methylsulfanyl)phosphole 1-sulfide on heating with ethyl diazoacetate reacted to give the homophosphole which rearranged on treatment with triphenyl phosphite to afford ethyl 4,5-dimethylphosphinine-2-carboxylate in good yield (Table 4). ... 

The first application of phosphinines in catalysis was reported by Zenneck et al. in 1996 in the case of t 6-Fe complexes [46, 98], It was shown that complex 73 could catalyse the cyclotrimerization of dimethyl acetylenedicarboxylate as well as the formation of pyridines from alkynes and nitriles. Importantly, the catalytic activity of this complex was found to be superior to that of the corresponding benzene [Fe(n6-C6FI6)(COD)] complex. Reactions of methylpropargyl ether with butyronitrile in the presence of complex 73 as catalyst in a ratio 620 2,720 1 afforded a mixture of functional benzenes and pyridines. Turn over numbers (TON) for the conversion into pyridines reached 160 and those for the formation of functional benzenes reached 326, thus corresponding to a chemoselectivity of 0.49 (Scheme 26). 

Phosphaalkynes also play a prominent role as dienophiles in Diels-Alder reactions hence the X -phosphinines 13 [23] are formed from cyclic 1,3-dienes such as a-pyrone or cyclopen-tadienones by way of extrusion of CO2 or CO, respectively reactions with anthracene provide an access to the phosphabarrelene series [24]. This type of reaction is also of significance for the construction of phosphorus-carbon cage compounds. The same is true for homo-Diels-Alder reactions which, with 2-phosphabicyclo[2.2.2]octa-2,5-diene as reaction partner, lead to the diphosphatetracyclodecenes 14 [25]. Last but not least, ene reactions with phosphaalkynes as enophiles [25] are also valuable for the construction of polycyclic phosphorus-carbon compounds. The reactions of 9 with 2,3-dimethyl-2-butene (- 15) [26, 27] emphasize this behavior. 

Fig. 6.15 Bond parameters of the phosphabenzene system in 2,6-dimethyl"4-phenyl-X -phosphinine (bond lengths in pm, bond angles in degrees)...

Finally, several imines give products of an aldol-t)fpe condensation rather than Diels-Alder reaction when reacted with (1) in the presence of Lewis acids. Phosphaalkenes react with (1) to give ultimately the aromatic phosphinine and dimethyl azodi-carboxylate affords the expected [4 + 2] adduct in 83% yield. ... 

The first application of a phosphinine-based catalyst in homogeneous catalysis was reported in 1996 by Zenneck et al. The f/ -phosphinine-iron complex 33 was used in the catalytic [2 -h 2 -h 2] cyclotrimerization of the electron-poor alkyne dimethyl acetylenedicarboxylate to give C6(C02Me)6 and in the co-cyclotrimer-ization of methyl propargylether with butyronitrile affording functionalized pyridine derivatives (Scheme 6.8) [60]. Turnover-numbers of up to 160 for the pyridine derivatives and chemoselectivities of up to 1.4 1 (pyridinesiarenes) were observed. Nevertheless, the efficiency of 33 is significantly lower compared to classical CpCo catalysts [61, 62]. 

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