High‐Yield Synthesis of Medium and Large Platinacycloalkanes from Bis(alkenyl) Precursors

Abstract

Metallacycloalkanes are an important class of compounds which have been known for many years to be key intermediates in useful catalytic reactions.[1] For example, metallacyclobutanes are known to be present in the alkene metathesis reaction.[2] More recently, metallacycloheptanes were first implicated, [3] then proven [4] to be involved in ethylene trimerization. There has been interesting discussion [5][6][7] and a belief that the formation of nine-membered and other larger metallacycloalkane rings is unlikely.In 2005, selective tetramerization of ethylene was discovered, which implicated metallacyclononanes as intermediates, [8] and Gibson and co-workers showed that larger metallacycloalkanes were involved in the catalytic formation of higher ethylene oligomers. [9] In spite of the demonstrated importance of medium to large metallacycloalkanes as intermediates, very little is known about such compounds.[10]The elegant synthesis of "molecular gyroscopes" through intramolecular alkene metathesis reactions using the Grubbs catalyst has been described by Gladysz and co-workers.[11]Recently, we introduced a new approach for the synthesis of small metallacycloalkane complexes using ring-closing metathesis (RCM) with the Grubbs catalyst.[12] Herein, we use this route to synthesize new medium to large ring metallacycloalkanes (Scheme 1), that is, exactly the ones that appear difficult to make by conventional methods.The bis(alkenyl)platinum(II) complexes 2 a-d were obtained by the transmetalation reaction of 1-alkenyl Grignard reagents with the corresponding dichloroplatinum(II) precursors 1. Complexes 2 were then readily converted into the platinacycloalkenes 3 using the RCM reaction with the Grubbs catalyst in CH 2 Cl 2 , and were obtained as colorless solids in high yield after recrystallization from CH 2 Cl 2 /diethyl ether. The hydrogenation of these compounds yielded the platinacycloalkanes 4 in high yield. The 31 P{ 1 H} NMR spectra of 4 in C 6 D 6 displayed a singlet at d = 3.4 ppm with Pt satellites ( 1 J Pt-P = 1620 Hz), which is quite similar to the precursor compounds 2 and 3.[13] These and all new compounds described in Scheme 1 were fully characterized. Crystals of 2 c and 3 d (L 2 = dppp = 1,3-bis(diphenylphosphino)propane) suitable for X-ray diffraction were obtained by recrystallization from CH 2 Cl 2 /hexane at T = 0 8C. To our knowledge, this is the first example of a structurally characterized larger metallacycloalkene compound reported in the literature.The molecular structures of 2 c and 3 d are illustrated in Figures 1 and 2, respectively. The structural analysis confirms that 3 d is the RCM product. The most striking structural features of complexes 2 c and 3 d are the differences in some of the bond angles and bond lengths. The PtÀC and PtÀP bond lengths in the two compounds are almost identical and comparable to the literature values [14] (2.118(3)-2.110(3) and 2.263(9)-2.293(7) , respectively), while the C = C distances range from 1.229(7) to 1.341(7) . The C = C distance in 3 d is comparati...