Stable precursors

The chalcogene heterocycles have been used as stable precursors for sulfur-said selenium-cantaining hetero-l,3-dienes in cycloaddition reactions 3//-l,2,4-Thiaselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-thia-3-aza-buta-1,3-dienes, and 3//-diselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-selena-3-azabuta-l,3-dienes as well as bis(tnfluoro-methyl)-substrtuted nitrile ylides [137]... 

The state of research on the two classes of acetylenic compounds described in this article, the cyclo[ ]carbons and tetraethynylethene derivatives, differs drastically. The synthesis of bulk quantities of a cyclocarbon remains a fascinating challenge in view of the expected instability of these compounds. These compounds would represent a fourth allotropic form of carbon, in addition to diamond, graphite, and the fullerenes. The full spectral characterization of macroscopic quantities of cyclo-C should provide a unique experimental calibration for the power of theoretical predictions dealing with the electronic and structural properties of conjugated n-chromophores of substantial size and number of heavy atoms. We believe that access to bulk cyclocarbon quantities will eventually be accomplished by controlled thermal or photochemical cycloreversion reactions of structurally defined, stable precursor molecules similar to those described in this review. 

The phosphorus-containing hgand precursor H0P(0)(0 Bu)2 was originally reported in 1957 by Goldwhite and Saimders [62]. The potassium salt of the desired phosphate, K0P(0)(0 Bu)2, is used as a stable precursor to the desired acid. This salt is prepared according to the method of Zwierzak and Kluba [63] and can be stored for extended periods at room temperature. A cold (0 °C) aqueous solution of K0P(0)(0 Bu)2 is treated with concentrated HCl to form H0P(0)(0 Bu)2 as a precipitate (Eq. 1). Rapid isolation (filtration) and subsequent crystalhzation affords pure product in good yields (> 80%). The... 

The synthesis of these inhibitors is not always straightforward because some hydro-xybenzyl derivatives that have very good leaving groups (like their aminobenzyl analogues) are unstable. Hence, more stable precursors are often used, such as phenyl ethers30-32 or silyl ethers.33-35 Other functional groups present in the molecule must be sometimes protected (/V-terf-butoxycarbonylation of amides for example).36,37... 

Another example has been provided by Ito et al., who described the use of methanofullerene derivatives as powerful and stable precursors for glycofullerenes.217 Their study was based on the use of [60]fullerenoacetyl chloride (227), obtained from the ferf-butyl [60]fullerenoacetate derivative 226, which had been prepared in 56% yield by treatment of corresponding stabilized sulfonium ylides 225 with C6o-218 Subsequent transformation with p-TsOH in toluene gave [60]full-erenoacetic acid, which was directly converted into the corresponding acyl chloride 227 by using thionyl chloride. Standard ester formation with methyl 2,3,4-tetra-O-benzyI -/<-d-gl ucopyranoside (228) and 4-(dimethylamino)pyridine (DMAP) afforded the desired hybrid derivative 229 in 66% yield. 

Another recent disclosure examined silicon-to-copper transmetallation as a mild means of synthesizing alkenyl-copper reagents from stable precursors. The method requires activation of the silyl group by an allylic alcohol. Again, the silanes in this work are produced by circuitous means but should be accessible by ruthenium-catalyzed hydrosilylation. Treatment of the silyl alcohol with a stoichiometric amount of copper(l) /rz -butoxide results in the C-to-O migration of the silyl group to produce a vinylcuprate shown to be competent for subsequent allylation to produce 1,4-diene products (Scheme 17). 

If the crosslinking reaction is interrupted before the gel point, the molecular weight and functionality distributions of such functional precursors are wider but not basically different from that of polymers of BAf monomer. It was stressed recently that they resemble hyperbranched polymers [24] in a certain respects. The pre-gel polymers are generally not stable because the crosslinking reaction can occur during storage. Stable precursors, e.g. for RAf + R Ba, can be obtained in two ways ... 

The DMS method has not been employed yet for the generation of 117 and 123, since the dibromocarbene adducts of norbomadiene and norbornene rearrange under the usual conditions for the preparation [89]. However, they could be synthesized at -60 °C by taking advantage of tetrabromomethane and methyllithium as a source of the carbene [90] and could prove stable enough to serve as precursors of 117 and 123. On the other hand, the adducts of bromofluorocarbene to norborna-diene and norbornene having the fluorine atom in a cis-orientation should be isol-able at room temperature and hence be usable as stable precursors of 117 and 123. These variations ofthe DMS method were published on the occasion ofthe preparation of cycloadducts of l-oxa-2,3-cyclohexadiene (351) (Section 6.3.6) [35, 91], 1,2,4-cyclohexatriene (162) and 3d2-lJ-f-naphthalene (221) (Section 6.3.4) [35, 92],... 

The linestyles indicate --- stable precursor ions,---transmitted ions, and fTTTTrn non-... 

Since silyl cations are highly reactive and moisture sensitive, the salts (S)-2a and (S)-2b were prepared in situ from the air and moisture stable precursor (S)-5 via a hydride transfer [34, 35] with trityl tetrakis[3,5-bis(trifluoromethyl)phenyl]borate [Tr][TFPB] or trityl tetrakis[pentafluorophenyl]borate [Tr][TPFPB], The authors showed by Si-NMR studies that the desired salts were formed. The silyl salt (5)-2a was then tested in the Diels-Alder reaction as shown in Scheme 5. A good reactivity was found, and the product was obtained in 95% yield with higher than 95% endo selectivity at -40 °C in 1 h. However, only 10% ee was achieved. 

Regardless of the above, an efficient method of DHAP preparation is still essential. Several chemical routes of DHAP synthesis have been described in the literature. Of these routes, those starting from the DHA dimmer [12] or 1,3-dibromoacetone [13] are the most attractive since they provide a stable precursor of DHAP (Scheme 4.4). Their main drawback is the relatively low overall yield. 

Scheme 4.4 Chemical routes to DHAP. (a) Routes from dihydroxyacetone dimer. The stable precursors are converted to DHAP by acid hydrolysis, (b) Route from 1,3-dibromoacetone. The stable precursor is converted to DHAP by treatment with NaOH.

Figure 8 Stable precursors for methylene Meldrum s acid.

Figure 2 The stable precursors of cobalt(0)-carbonyl complexes.

Hydrolytic Decomposition of Phosphoprotein. The kinetic analysis of phosphate liberation revealed that phosphate is not steadily liberated but that an initial lag period occurs which is followed by a transient burst of phosphate liberation123, 177> 192 This burst has been interpreted as resulting from the accumulation of an acid-labile intermediate arising from an acid-stable precursor. The burst is fol-... 

Donor adducts of aluminum and gallium trihydride were the subject of considerable interest in the late 1960s and early 1970s.1 Thin-film deposition and microelectronic device fabrication has been the driving force for the recent resurgence of synthetic and theoretical interest in these adducts of alane and gallane.24 This is directly attributable to their utility as low-temperature, relatively stable precursors for both conventional and laser-assisted CVD,59 and has resulted in the commercial availability of at least one adduct of alane. The absence of direct metal-carbon bonds in adducts of metal hydrides can minimize the formation of deleterious carbonaceous material during applications of CVD techniques, in contrast to some metal alkyl species.10, 11... 

The lower order (L.O.) cyanocuprate (2-Th)Cu(CN)Li has an excellent shelf life, thereby providing a highly stable precursor to higher order cuprates.9 Tetrahydrofuran solutions of this L.O. cuprate are available commercially (from Aldrich Chemical Company, Inc.), thus allowing further simplification of this procedure. 

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