Pentamethyl group 15 compounds

The reaction of HFA with 2,2,3,4,4-pentamethyl-A -phosphetane has been investigated (204). A crystalline 2 1 adduct has been obtained. NMR studies indicate two isomers of the dioxaphospholane 60e relative to the position of the 3-methyl group. Compound 60e slowly decomposes to yield diastereo-meric phosphites 62 with the same isomeric composition. Further action of HFA affords 6O0 (204). 

With the exception of nitrogen, all of the Group Vb elements are expected to form pentacoordinate compounds in their 5+ oxidation state, and this is, indeed, the case with some of the halides, alkoxides, etc. It was not until the pioneering work of Georg Wittig and his collaborators, however, that the first examples of pentaorganyls of these borderline elements between metals and metalloids were reported (95-99, 102, 104). In this early investigation, a complete set of the pentaphenyls could be obtained and characterized (95-99, 102), but apart from the pentamethyl-antimony case, all attempts for the preparation of pentaalkyl derivatives failed (104). 

The hydrolysis rates of 1,3-dioxolanes are decreased by substitution of four methyl groups at the positions 4 and 5. 2,4,4,5,5-Pentamethyl-l,3-dioxolane reacts 6.5 times more slowly than 2-methy 1-1,3-dioxolane [161]. The AS value is decreased by 9 eu. A Hammett p value of—2.0 has been found for the substituent effect on the hydrolysis of 2-phenyl-4,4,5,5-tetramethyl-l,3-dioxolane in dilute aqueous hydrochloric acid [167]. The data obey the simple Hammett equation, and it is not necessary to apply o+ values. For the reaction of the unsubstituted 2-phenyl-4,4,5,5-tetramethyl compound, the solvent isotope effect is kH/kD = 0.42, and AS is —14.2 eu. General catalysis by formic acid has been observed in the hydrolysis of the p-methoxy compound. However, the rate is not significantly increased by addition of strong nucleophiles. 

C42H5 ClN4NiO,7, Mr 1006.04, uv, (pentamethyl ester, F 430 M in HjO) 431,418,296,274 nm. F. belongs to the structural class of the hydroporphyrins. Occurrence F. (coenzyme F 430), is the prosthetic group of methyl-coenzyme M reductase, which catalyzes the last step of methane formation in methano-genic bacteria. The mechanism of formation of methane from methyl-coenzyme M (H3C-S-CH2-CH2-SOJ) is still unknown. However, the central nickel atom in F. most certainly participates in the reduction. Biosynthesis F. is formed from 5-amino-4-oxova-leric acid via the normal biosynthetic route for porphi-noid compounds. The 2- and 7-methyl groups originate from 5-adenosylmethionine. 

More or less contemporaneously, Rama Rao et al. reported isolation of two compound from Moms alba which they called albanol A and B and assigned formulae 27 and 136 to them, on the basis of an X-ray analysis of albanol A pentamethyl ether (27 a) (115). Mulberrofuran G proved to be identical with albanol A by direct comparison. Our group has also isolated albanol B (136) from the Japanese cultivated mulberry root bark 112). 

Probably the first report on the use of a metaUocene in zeolite synthesis dates back more than ten years [124]. In the past few years, however, essentially two groups reported on the influence (i.e. structure directing properties) of cobalt metallocene compounds in the synthesis of zeolites and zeolite-like materials [e.g. 125-128]. In none of these studies had a new framework topology been synthesized. However, very recently Balkus and coworkers reported the synthesis of the first fourteen-membered ring zeolite, which they called UTD-1 (University of Texas at Dallas No. 1) [129-132]. UTD-1 was synthesized with a quite unusual template, viz. with bis(pentamethyl-cyclopentadienyl)cobalt(III) hydroxide [129-132]. Both the essentially pure-silica version and high-silica derivatives have been prepared. A typical synthesis of the all-silica version com-... 

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