Selenenylation

Electrophilic selenenylation has important synthetic applications. Much of the research emphasis has been on the development of convenient reagents. The selenides, per se, are not usually the desired final product. Selenenyl substituents can be removed both reductively and oxidatively. In some cases, the selenenyl substituent 

Tiecco, Top. Curr. Chem., 208, 7 (2000) T. G. Back, Organoselenium Chemistry A Practical Approach, Oxford University Press, Oxford, 1999 C. Paulmier, Selenium Reagents and Intermediates in Organic Chemistry, Pergamon Press, Oxford, 1986 D. Liotta, Organoselenium Chemistry, Wiley, New York, 1987 S. Patai, ed.. The Chemistry of Organic Selenium and Tellurium Compounds, Vols. 1 and 2, Wiley, New York, 1987. 

As shown in Table 5.5, alkyl substimtion enhances the reactivity of alkenes, but the effect is very small in comparison with halogenation (Table 5.2). Selenenylation seems to be particularly sensitive to steric effects. Note than a phenyl substiment is rate retarding for selenenylation. This may be due to both steric factors and alkene stabilization. The Hammett correlation with a gives a p value of —0.715, also indicating only modest electron demand at the TS. ° Indeed, positive values of p have been observed in some cases.  

Terminal alkenes show anti-Markovnikov regioselectivity, but rearrangement is facile.The Markovnikov product is thermodynamically more stable (see Section 3.1.2.2). 

Styrene, on the other hand, is regioselective for the Markovnikov product, with the nucleophilic component bonding to the aryl-substituted carbon as the is the result of weakening of the bridging by the phenyl group. 

---

In some reactions intramolecular chalcogen nitrogen interactions may lead to stereochemical control. For example, selenenyl bromides react with C=C double bonds, providing a convenient method of introducing various functional groups. The reaction proceeds readily, but affords a racemic mixture. The modified reagent 15.22 contains a chiral amine in close interaction with the selenium atom. It reacts with olefins affording up to 97% ee of isomer A (Scheme 15.2). ... 

Terminal alkenes react with selenenyl halides with Markovnikov regioselectivity.64 However, the (J-selenyl halide addition products readily rearrange to the isomeric products.65... 

Chiral selenenylating reagents have been developed and shown to be capable of effecting enantioselective additions and cyclizations. The reagent 4, for example, achieves more than 90% enantioselectivity in typical reactions.104... 

II and 12 indicate, the selenenylation of ketones can also be effected by reactions of enol acetates or enol silyl ethers. 

Pentenyl amides such as 15A cyclize to lactams 15B on reaction with phenyl selenenyl bromide. The 3-butenyl compound 15C, on the other hand, cyclizes to an imino ether 15D. What is the basis for the differing reactions ... 

The same substrate 170 reacts with alkyl(phenyl)selenenyl chlorides to give predominantly 2,5-dihydro-l,2-oxaphosphole-2-oxide derivatives 175 (Scheme 70) [150],... 

Methylated organo-selenium has been determined by GC/MS or fluorine-induced chemiluminescence to determine DMSe, DMDSe, and DMSeS. This last compound, dimethyl selenenyl sulfide, was mistakenly identified as dimethyl selenone (CH3Se02CH3) in earlier work with bacteria.181,182 However, much recent work with many microorganisms have shown ample evidence of DMSeS production from Gram-negative bacteria,181,183 phototrophic bacteria,167,184 phytoplankton185 and in B. juncea detailed above. SPME with microwave-induce plasma atomic emission spectrometry was recently used to... 

Association of Sulfenyl, Selenenyl and Tellurenyl Halides with Chelating Substituents... 

Donors can stabilise selenenyl iodides, when competing coordination of the same donor with molecular iodine is avoided. 

Internal amino64 or imino65 67 chelate functions can achieve this type of selenenyl iodide stabilisation. 2-(4,4-Dimethyl-2-oxazolinyl)benzeneselenenyl iodide66 exhibits weak iratermolecular Se- -I contacts (372.5 pm) leading to a centrosymmetric dimer with longer N -> Se bonds (213.3 pm) and shorter Se-I bonds (277.7 pm) [Figure 19(a)] compared to the strictly monomeric 4-ethyl-substituted isomer (Se-N 207.4).65 The structure is related to the dimer of the T-shaped cyano derivative [Figure 19(b)].68... 

Scheme 4.5. Trapping of a vinyl zircono-cene with an aryl selenenyl halide.

The addition of phenyl sulfenyl choride and phenyl selenenyl chloride to glycals has been investigated, which provides another entry to the 2-deoxy-P-glucosides. As summarized by Roush et al. [156] (Scheme 5.53), the method gives the best selectiv-... 

The addition of selenenyl derivatives to olefins has been shown to be of mechanistic interest and synthetic utility because of the versatility of the selenium functionalities28,133. The possibility of modifying double bonds with seleno derivatives has been applied also to conjugated systems in order to obtain arylseleno dienes, or electron-deficient dienes, both being useful synthetic intermediates or building blocks. 

It is noteworthy that 108 reacts in AcOH with benzenesulfenyl chloride to give a 1 1 mixture of the sulfur analogues of 138 and 140, but when the reaction is carried out in the presence of LiClC>4 a complex mixture of at least five products was detected. From this comparison the authors suggest that areneselenenylation is much less affected by the solvent than arenesulfenylation, and if the reaction profiles for the two product-forming processes are assumed to be similar, the difference in product distributions can be interpreted in terms of a more efficient bridging ability of selenium than that of sulfur. In the addition of selenenyl derivatives, the solvent-dependent product distribution has also... 

The larger (Z,Z)-l,5-cyclononadiene (169) reacts141 stereoselectively with PhSeCl in AcOH to give the substituted hydrindan 170 (equation 138). In consideration of the anti addition mode of selenenyl reagents to double bonds, the transannular reactions of 169 have been rationalized on the basis of the two reaction intermediates, 171 or 172, which are liable to place the PhSe- and AcO- groups in a cis- 1,4-relationship and trans to the bridgehead hydrogen (equation 139). The preferential formation of 170 has thus been attributed to the fact that the pathway via 172 should involve a boat transition state. 

---