Pentamethylcyclopentadiene

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diolefin with the formula C₅Me₅H (Me = CH₃). 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted as Cp* (to signify the five methyl groups radiating from the periphery of this ligand as in a five-pointed star). In contrast to less substituted cyclopentadiene derivatives, Cp*H is not prone to dimerization.

Synthesis

Pentamethylcyclopentadiene is commercially available. It was first prepared from tiglaldehyde via 2,3,4,5-tetramethylcyclopent-2-enone. Alternatively 2-butenyllithium adds to ethylacetate followed by acid-catalyzed dehydrocyclization:

Organometallic derivatives

Cp*H is a precursor to organometallic compounds containing the C₅Me₅⁻ (Cp*⁻) ligand.

Some representative reactions leading to such Cp*-metal complexes follow:

For the related Cp complex, see cyclopentadienyliron dicarbonyl dimer.

An instructive but obsolete route to Cp* complexes involves the use of hexamethyl Dewar benzene. This method was traditionally used for preparation of the chloro-bridged dimers [Cp*IrCl₂]₂ and [Cp*RhCl₂]₂. Such syntheses rely on a hydrohalic acid induced rearrangement of hexamethyl Dewar benzene to a substituted pentamethylcyclopentadiene prior to reaction with the hydrate of either iridium(III) chloride or rhodium(III) chloride.

.срб

.срб (romanized as .srb) is the Internationalised (Cyrillic) Internet country code top-level domain (IDN ccTLD) for Serbia. It has been active since May 3, 2011.

The Serbian National Register of Internet Domain Names (RNIDS) has initiated on its forum (www.forum.rnids.rs) a public discussion on the Proposal of rules and processes for registering the Cyrillic domain “.срб”.

The Proposal of rules and processes for registering the Cyrillic domain “.срб” is available on the forum of RNIDS. It regulates the registration of the sub-domain “.срб” and the reservation of other domains for the needs of RNIDS, the reservation of the “.срб” domain for the needs of the state, the reservation of the “.срб” domain based on the current .rs domain names and the allocation of the unique code to each and every reserved “.срб” domain.

The Rule book also determines the period for assigning the “.срб” domain according to the priority right, after which the period of free registration of the “.срб” domain will commence, based on the same or similar rules now applicable for the .rs domain.

163693 Atira

163693 Atira, provisional designation 2003 CP₂₀, is an eccentric, stony asteroid, dwelling in the interior of Earth's orbit. It is classified as a near-Earth object and measures about 2 kilometers in diameter. The body was discovered by the Lincoln Near-Earth Asteroid Research project's station at the U.S. Magdalena Ridge Observatory in Socorro, New Mexico, on 11 February 2003.

It is the namesake and the first numbered body of a new subclass of near-Earth asteroids, the Atira (Apohele) asteroids, which have their orbits entirely within that of Earth and are therefore alternatively called Interior-Earth Objects (IEO). As of 2015, there are only 16 known members of the Atira group of asteroids. Atiras are similar to the larger group of Aten asteroids, as both are near-Earth objects and both have a semi-major axis smaller than that of Earth (< 1.0 AU). However, and contrary to Aten asteroids, the aphelion for Atiras is always smaller than Earth's perihelion (< 0.983 AU), which means that they do not approach Earth as close as Atens do in general. The body has an Earth Minimum orbit intersection distance (MOID) of 0.2065 AU (30,890,000 km).