Subject sulfur stabilization

Although the subject of stability of complexes will be discussed in greater detail in Chapter 19 it is appropriate to note here some of the general characteristics of the metal-ligand bond. One of the most relevant principles in this consideration is the hard-soft interaction principle. Metal-ligand bonds are acid-base interactions in the Lewis sense, so the principles discussed in Sections 9.6 and 9.8 apply to these interactions. Soft electron donors in which the donor atom is sulfur or phosphorus form more stable complexes with soft metal ions such as Pt2+ or Ag+, or with metal atoms. Hard electron donors such as H20, NH3( or F generally form stable complexes with hard metal ions like Cr3+ or Co3+. 

The alkylation of sulfur-stabilized anions has been the subject of an excellent recent review633. Anions adjacent to a wide range of sulfur functionalities may be alkylated readily, the most common being sulfoxide and sulfone a-anions. In the synthesis of retinoic acid derivatives and vitamin A634-636, a-sulfonyl anions have been alkylated with an co-acetoxy-containing allyl halide in good yield (equation 96). 

Treatment of any of these compounds with strong base produces an anion (or a lithium derivative if BuLi is used) on what was the methyl group. How does the sulfur stabilize the anion This question has been the subject of many debates and we have not got space to go into the details of all of them. There are at least two factors involved, and the first is evident from this chart of pKa values for protons next to sulfone, sulfoxide and sulfide functional groups. 

Additional material on this subject is provided in the supplemental information for Chapter 25.4 that is available online at http //elsevierdirect.eom/companions/9780120885305. Key topics covered are the role of tectonism in the geologic carbon cycle and how the evolution of pelagic calcifiers in the Phanerozoic led to the development of feedbacks, some stabilizing and some destabilizing, that act on the atmospheric COj reservoir. Also included is a short summary of how the global carbon cycle interacts with the atmospheric O2 and sulfur cycles. 

The stabilization of these heteroaldehydes and -ketones by coordination to transition metals is the subject of this review. Many problems connected with the high reactivity of these heterocarbonyl compounds can be circumvented by using their transition metal complexes. The chemistry of organo-sulfur and organoselenium transition metal complexes in more general terms30,31 and some aspects of thio- and selenoaldehydes and -ketones as... 

The +C(SH)3 cation and the radical dication derived from it have been the subject of high-level calculations.67 The ability of two adjacent sulfur atoms to stabilize cations, anions, and radicals makes these species usefiil for relating bond-breaking and electron-transfer energies.68,69 Electrophilicity parameters for the dithiocarbenium ions (27) have been worked out,70 and the stabilities of the cations (28), (29) and (30) have been estimated using PM3 calculations.71 Cation (31) can be captured by solvent or azide ion, or it may ring close to (32), which subsequently alkylates another (31) cation as shown.72... 

Specifically we wished to measure the rate of reaction of OH with MSA to enable modelling calculations of the stability of MSA in aerosol droplets. The one reported measurement of this rate (2), using pulse radiolysis techniques, 3.2 x 109 M 1 s 1, is fast enough to suggest that this reaction pathway could be an important sink for MSA. This is of interest in explaining an apparent discrepancy that exists between laboratory and field studies of tne oxidation of dimethyl sulfide. Although a number of laboratory studies (6-9 ) show that MSA is the major stable product, and SO2 a minor one, field observation suggest MSA is only a minor (10%) fraction (2) of total non-sea-salt sulfur in marine aerosols. Two possible rationalizations of this are that i) MSA is subject to further reaction in marine aerosols and ii) other reaction pathways of dimethyl sulfide, or perhaps other non-methylated sulfur compounds should be considered. 

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