Sulfur divalent, displacements

In its outer electronic shell divalent sulfur has two s and four p electrons, and it also has five empty 3d orbitals. A bimolecular nucleophilic displacement reaction on sulfur might then occur in a single step or an intermediate such as 7, in which the sulfur accepts the pair of electrons of the entering Lewis base into one of its... 

FECH (also known as heme synthase) is an iron-sulfur protein located in the inner mitochondrial membrane. This enzyme inserts ferrous iron into protoporphyrin to form heme During this process, two hydrogens are displaced from the ring nitrogens. Other metals in the divalent state will also act as substrate, yielding the corresponding chelate (e.g., incorporation of Zn into protoporphyrin to yield zinc protoporphyrin). In iron-deficient states Zn successfully competes with Fe in developing red cells so that the concentration of zinc protoporphyrin in erythrocytes increases. Furthermore, other dicarboxylic porphyrins will also serve as substrates (e.g., mesoporphyrin). 

Rhodanese is present in the mitochondria, particularly of liver and kidney cells. A double-displacement mechanism has been proposed for its biochemical action. The steps are as follows The free enzyme reacts with a sulfane sulfur-containing compound (a sulfane sulfur is one that is divalent and covalently bonded only to other sulfurs), cleaving the S-S bond of the donor substrate (e.g., SSOj ) to form the sulfur-substituted enzyme. The latter reacts with the cyanide (a thiophilic acceptor) to form thiocyanate in an essentially irreversible reaction. 

The displacement on the trithionate ion 8(803) takes place at the central divalent sulfur (94) via a transition state resembling that of the 8n2(C) reaction. The increasing effectiveness of the bases CN < PhjP < PhS < EtS has been established. 

Step 1 Make a new bond between a nucleophile and an electrophile and simultaneously break a bond to give stable molecules or ions Reaction is initiated by nucleophilic attack of the divalent sulfur atom of methionine on the carbon of cyanogen bromide displacing bromide ion. The product of this nucleophilic displacement is a sulfonium ion. 

P, Si, some metals) can be prepared in chiral form and do invite the use of standard techniques.( ) However, many important displacement reactions take place at mono- or divalent reaction centers, where there is no possibility of using a chiral probe of mechanism. In this paper, we report the results of experiments designed to probe the preferred geometry and mechanism of radical displacements at divalent sulfur, a reaction center characterized by a plane of symmetry. Further, the technique is general and can presumably be used to study the geometries of other displacement reactions at centers which are inherently achiral or cannot be easily prepared in chiral form.(2)... 

We conclude that we are dealing with a free radical displacement at divalent sulfur (eq. 3). We arrived at a similar conclusion in a study of the prototype of the system described in this paper.( ) Other free radical displacements at the divalent sulfur of sulfides are known.(11-17)... 

In the present article, the displacement reactions at divalent sulfur are discussed. Since such an atom is not optically active, the stereochemistry can only be inferred by analogy with the Sn2 reaction on carbon compounds. In general a compound of divalent sulfur, X—S—Y, will react with a nucleophile Z to form three types of products. The stoichiometry can be represented as follows ... 

---