DIMET

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

Many novel cluster compounds have now been prepared in this way, including mixed metal clusters. Further routes involve the oxidative fusion of dicarbon metallacarborane anions to give dimetal tetracarbon clusters such as (103) and (104) O (jjg insertion of isonitriles into inetallaborane clusters to give monocarbon meiallacarboranes such as (105) and the reaction of small ii/t/o-carboranes with alane adducts such as Et3NAlH3 to give the commo species (106) ... 

Metalated epoxides can react with organometallics to give olefins after elimination of dimetal oxide, a process often referred to as reductive alkylation (Path B, Scheme 5.2). Crandall and Lin first described this reaction in their seminal paper in 1967 treatment of tert-butyloxirane 106 with 3 equiv. of tert-butyllithium, for example, gave trans-di-tert-butylethylene 110 in 64% yield (Scheme 5.23), Stating that this reaction should have some synthetic potential , [36] they proposed a reaction pathway in which tert-butyllithium reacted with a-lithiooxycarbene 108 to generate dianion 109 and thence olefin 110 upon elimination of dilithium oxide. The epoxide has, in effect, acted as a vinyl cation equivalent. 

Primary radicals are unstable, lowest members such as dimet peroxide are shock sens and dangerous expls sensitivity lessens with increasing mw polymeric peroxides (copolymers of olefins and 02) explode on heating... 

Dimetal derivatives 283 of alkyl and benzyl phenyl sulfones were found to react with aldehydes or ketones to give a, /J-unsaturated phenyl sulfones 284 in good yields366. 

The bis( U-oxo)dimetal [M2( tt-0)2] " core (Scheme 12d) has been proposed as a common motif for oxidation chemistry mediated by manganese, iron, and copper... 

Note It is reported that the use of chlorobenzene as solvent is essential when the agent is to be used to detect aromatic amines [1]. In the case of steroids, penicilt diuretics and alkaloids the reaction should be accelerated and intensifled by spr afterwards with dimethylsulfoxide (DMSO) or dimethylformamide (DMF), indeed i step makes it possible to detect some substances when this would not otherwise be p sible [5,9-11] this latter treatment can, like heating, cause color changes [S, 9]. Peni lins and diuretics only exhibit weak reactions if not treated afterwards with DMF 11]. Steroids alone also yield colored derivatives with DMSO [9]. Treatment afterws with diluted sulfuric acid (c = 2 mol/L) also leads to an improvement in detection s sitivity in the case of a range of alkaloids. In the case of pyrrolizidine alkaloids i possible to use o-chloranil as an alternative detection reagent however, in this cas is recommended that the plate be treated afterwards with a solution of 2 g 4-(dimet] amino)-benzaldehyde and 2 ml boron trifluoride etherate in 100 ml anhydrous etha because otherwise the colors initiaUy produced with o-chloranil rapidly fade [12]. 

Scheme 10 Diaza-Cope rearrangement of chiral 2,5-diaza-1,5-dienes and homolytic C - C bond cleavage of dimetal piperazides cause loss of stereochemical pimity...

Reaction of ButadiynediyI dimetal Complexes with Fc2(CO)9 Formation of Various Complexes with the C4 bridge... 

The interaction of butadiynediyl dimetal complexes [Fp -C -CsC-M, Fp =FeCp (CO)2, M= Fp, Rp, SiMea, Rp= RuCp(CO)2] with diiron nonacarbonyl, Fe2(CO)9, results in the formation of a mixture of products, as is also observed in the case of their interaction with organic acetylenes. Interesting polymetallic complexes, propargylidene-ketene compounds, zwitterionic cluster compounds, and pa-p -propargylidene-cyclobutene compoimds were isolated from the reaction mixtures and successfully characterized. The product distributions were found to be dependent on the metal fragment (M) at the other end of the C4 rod. The results of the reaction are described... 

Monocaesium acetylide or monopotassium acetylide, and the ammoniate of monolithium acetylide all ignite and incandesce in unheated sulfur dioxide. The dimetal derivatives including sodium acetylide appear to be less reactive, needing heat before ignition occurs. 

The catalytic cycles that have been documented, namely alkyne eyelotrimerization and olefin isomerization, demonstrate that addition and elimination from dimetal centers can occur readily in the presence of metal-metal bonds and alkoxide 1igands. 

It is becoming increasingly apparent that a very extensive chemistry is associated with the dimetal species (1), and (3) - (13). Three types of reaction may be identified (i) aSTdition of other metal ligand fragments, discussed in the next Section, (ii) replacement of peripheral ligands on the metal centres, and (iii) reactions at the bridging carbon atom. One example of (ii) and (iii) will suffice to illustrate the scope and potential for new chemistry. 

The PR3 ligands transoid to the y-CR group in (lj are readily displaced by CO gas (1 bar), affording initially the dimetal compounds (1 ), which subsequently in solution yield the trimetal complexes (lj). Evidently compounds ( ) partially dissociate into [(n-C5H5)(0C)2W =CRJ and [Pt(C0)(PR3)], and the latter adds to undissociated (14) to give the complexes (15) (10). 

We referred earlier to the significance of reactions at the alkylidyne carbon atoms of the dimetal species. Our studies in this area are in a preliminary stage, but Schemes 1 and 2 summarise some chemistry at the bridged carbon centres for the compounds (1 ) and (3,)(12). It will be noted that protonation of the neutral bridged al ylidyne compounds yields cationic alkylidene species in which one C—C bond of the tolyl group is n2 co-ordinated to tungsten, a feature revealed by both n.m.r. and X-ray diffraction studies. 

Clusters of Class B may be prepared by adding appropriate metal ligand fragments to the dimetal species. Thus treatment of (3) with [Fe2(C0)9l affords (34), and similarly (4) reacts with [Fe2(C0),J to give (35)(9). 

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