12-05-02 20:10
No 386550
Almost all the procedures I can find for converting benzyl chlorides to nitriles seem to use NaCN. The highest yielding and generally recommended variations seem to be the ones using NaCN in DMSO, such as in this post by G_Pig:
A slurry of 49g NaCN in 400mL DMSO is chilled in an ice bath and 0.7 mol subst. benzyl Cl added dropwise over 45 minutes, using the ice bath to maintain a temp around 35-40C. Don't keep it too cold, it SHOULD rise to at least 30C or you are chilling too much. After complete addition, remove ice and stir for an additional 90-120 mins. At this point, most of the solid NaCN should be consumed and the solution will be very cloudy and redish. Slowly add 300mL H2O with stirring. Cool the solution to RT. Extract with Et2O, wash extracts with H2O, Sat. NaCl; Dry (MgSO4). Evap. the solvent to get the acetonitrile as an offwhite oil.[/I]
Can KCN be directly substituted for NaCN in this rxn when done on 2,5-dimethoxybenzyl chloride? Would there be any difference in yields?
Thanks,
psygn
(Master Whacker)
12-05-02 23:37
No 386601
KCN is not nearly as soluble as NaCN in DMSO so a larger amount of DMSO will have to be employed when using the potassium salt. If you are planning on running the rxn. at the same mole scale as the above procedure, I would use about a liter of dry DMSO. For highest yields, use powdered KCN and allow rxn to run twice as long as if NaCN was being used.
(Chief Bee)
12-05-02 23:49
No 386608
The reaction with KCN is much slower, and gives lower yield. See JOC 25(6), 877-879 (1960)
See below for a direct comparison:
1-Chlorobutane, NaCN, 140°C, 15 min, 93%
1-Chlorobutane, KCN, 120-140°C, 10h, 69%
2-Chlorobutane, NaCN, 120-145°C, 3h, 64%
2-CHlorobutane, KCN, 120-138°C, 24h, 42%
(Stranger)
12-06-02 02:35
No 386660
Thanks for the replies :)
Is there any way of generating NaCN in situ from KCN and a sodium salt, producing NaCN and a potassium salt that wouldn't interfere with the rxn? Is there any other way of converting KCN to NaCN apart from via HCN?
Thankx :D
(Moderator)
12-07-02 05:09
No 387038
Add NaI or KI to generate the more reactive benzyliodide in-situ (Finkelstein reaction).
(Hive Addict)
12-07-02 12:40
No 387116
And one would need more KCN than NaCN as K atomic weight is almost twice as large as Na's
For those about to synth,we salute you
(Stranger)
12-07-02 23:44
No 387248
(Rated as: excellent)
If you can buy the resin amberlyst A26 you can create a Polymer-Supported Cyanide.
Polymer-Supported Cyanide and Thiocyanate Reagents:
Amberlyst A26, chloride form (100 g) is stirred for 6 h with potassium cyanide (42 g) in water (450 ml) or with potassium thiocyanate (60 g) in water (400 ml) as appropriate. The resin is filtered off and washed successively with water. tetrahydrofuran, and ether before drying under vacuum at 40°C 12 torr. To estimate the activity of the polymer-supported cyanide: a sample of dry resin (1.00 g) is stirred for 12 h with potassium iodide (830 mg) in water (10 ml). The resin is filtered off, washed with water, and the combined filtrate and washings titrated against silver nitrate solution. The activity is generally 3.2-3.6 mmol/g.
1,12-Dicyanododecane; Typical Procedure:
A mixture of 1.12-dibromododecane (328 mg. 1.0 mmol), polymer-supported cyanide (3.6 mmol), and benzene (1o ml) is stirred and heated under reflux for 18 h. The resin is filtered off and washed with benzene. The combined filtrate and washings are evaporated to dryness to give the crystalline product. Yield: 220 mg (100%) mp 33-35 °C; Lit.'" m.p. 35.5-36 °C; satisfactory I.R. and H.N.M.R. spectra obtained.
NB: They have got a 87% yield with benzyl-chloride, at reflux 3h in benzene with 1.2 mol of reagent.
They say that toluene can be used too.
The resin can be regenerated and reused.
There are other reagents to support cyanide, like alumina as well as other resins.
Ref: Synthesis, 1980, 299-300