[nilay]

A compound containing sulfur (5-ethyl-5-phenyl-2-thiobarbutric acid)mixed with 5 times (of the compound) raney nickel is being refluxed in methanol to convert to hydrocarbon. H2S gas is being liberated.
Could anyone help me out to understand how raney nickel alone is causing desulfurization without hydrogen gas passing? Does the raney nickel itself act as a source of hydrogen? If so, is there a specific commercial name for such a raney nickel? The sulfur from the compound would poison the catalyst, so how the does the reaction at all proceed? Also please suggest a method to scrub H2S or convert H2S to an insoluble sulfide or sulfate form that could be filtered.

Regds/nilaysapre

[sdekivit]

one way to get rid of H2S is to add an excess of iodine. The following reaction will occur:

H2S + I2 --> S + 2H(+) + 2I(-).

About Raney nickle, Biochemistry of Jeremy M. Berg et al states that Raney nickel is used together with a H2-source to stop the reaction and therefore it slows down racemization, because the reaction is reversible. It's a
 Therewfore it's commonly used in combination with hypophosphite

They used it for example in determining whether only the codon-anticodon interactions or not determine the amino acid that will be incorporated in the protein. Here they converted cysteinyl-tRNA to alanyl-tRNA using Raney nickel. Then the used a copolymer containing only U and G and the converted tRNA-molecule was incorporated. So alanine was incorporated instead of the needed cysteine. Thus they concluded that only the codon-anticodon interaction determine the aminoacid that will be incorporated.

[movies]

Raney nickel is very special because it is so useful for these desulfurization reactions.  The hydrogens actually come from the solvent, that's why you have to use a protic solvent.  The mechanism itself is not well understood, but there is some evidence to suggest that it is a radical process.  Raney Ni is sold in several grades based on its activity.  Each vendor seems to have their own system of "rating" their Raney Ni.  Most all of them will perform desulfurization, however.  The catalyst doesn't really get poisoned in these reactions.  I'm not sure if sulfur just has a low affinity for Ni or what.  Raney Ni also contains a certain, variable percentage of aluminum which could also have a function in the mechanism.  Very hand wavy at best.

For scrubbing H₂S I have always been a fan of bubbling through an aqueous bleach solution to oxidize the sulfur, however in this case you'd end up with SO₂ which is not much fun either.

[bharathi]

hi all,
Could someone suggest me options to desulfurise a compound containing C=S to CH2 other than using raney nickel? The compound also contains C=O groups which should remain unaffected in the process of desulfurisation.

Thanks in advance

[movies]

There aren't a whole lot of options here, Raney Ni is really the best and easiest way.  You might be able to do some funky radical chemistry though.

[bharathi]

Hi all,
I am stuck up in synthesizing a desulfurised compound with a safe scalable process.
As described by nilay in his first mail, reductive desulfurisation of the compound 5-ethyl-5-phenyl-2-thiobarbutric acid is to be done to produce  5-ethyl-5-phenyl hexahydropyrimidine-4,6-dione.
I am able to generate this using Raney nickel (10 times loading to 1 time substrate) under anhydrous conditions in Tetrahydrofuran(yield 30%) .
 However ensuring anhydrous conditions with Raney nickel (raney nickel is always available on wet basis in water as it is pyrophoric) on bench/lab scale is manageable, but the same is a cumbersome and risky task on a higher commercial scale.
In an attempt to replace Raney nickel with a suitable alternative, I found a reference suggesting use of nickelchloride and sodium borohydride (10:10 molar ratio to 1 mole of substrate) in methanol with yield of 75% .
However, I am not able to understand the chemistry behind the usage of this combination of reagents.
 I would be grateful if someone could help me out in suggesting the mechanism of this reductive desulfurisation?
Does nickel boride generate insitu and react with the hydrogensulfide gas liberated during the reaction to form nickel sulfide as byproduct?
Someone please help me

Thnx a lot
Bharathi

[movies]

Ah, nickel boride is a good one.  As far as I know, it is generated in situ prior to use.  I think that Aldrich sells Ni₂B however, and that can be used in combination with other reducing agents like (MeO)₂BH.  It has a number of different uses: reduction of alkynes to alkenes, enones to ketones, conjugated dienes to mono-alkenes, quinolines to tetrahydroquinolines, aromatic nitro groups to amines, and of course some sulfide cleavage reactions.

I've never seen anything on the specific mechanism, and it's probably very complicated because of the uncertain nature of the actual Ni/B mixture (Ni₂B just describes the relative stoichiometry, not really the complete composition of the reagent, which is probably a higher order polymeric cluster).  My guess would be that you form a small amount of nickel hydride which then complexes to the sulfur in the substrate and exchanges (H for S) to the nickel sulfide and the reduced substrate.  An additional reduction of the nickel would for a RS-Ni-H complex that could then undergo reductive elimination to the RS-H compound.  I haven't seen any mention of the final S containing species so I don't know if it goes all the way to H₂S or if the S remains adsorbed onto the Ni surface.