Maze Solving by Chemotactic Droplets

a very interesting paper.
in jacs asap.

Solving maze problems is not only relevant to the everyday issues
of urban transportation1 and to experimental psychology2 but is
also one of the model problems of network and graph theory3 as
well as robotics.4 With the advent of computers, algorithms for
maze solving have become automated, but the solution times still
scale unfavorably with maze size/complexity.5 Several groups have
thus explored the possibility of maze solving by physical, chemical,
or even biological systems: microfluidic networks,6 chemical waves7
or plasmas,8 or microorganisms growing in response to food
gradients within the maze.9 Inspired by the latter example, we
wished to create a system in which an inanimate/chemical construct
would be self-propelled and solve mazes in response to chemical
stimuli. Here we describe one such system comprising small
droplets powered by the combination of acid/base chemistry and
surface tension effects. When subject to a pH gradient within a
maze, these droplets move toward regions of low pH and find the
shortest of multiple possible paths. Taxis in our system is over
distances of several centimeters and derives from the convection
flows developed outside of the droplets.1

preparation of [Me2SSMe+][BF4-] (DMTSF)

what DMTSF can do?
electrophilic sulfenylation reagent capable of reacting with nucleophilic atoms;² reacts with electron-rich alkenes to promote addition reactions,³ cyclizations;⁴ activates dithioacetals,⁵ trithioorthoesters,⁶ and thioglycosides⁷ for carbon–carbon or carbon–heteroatom bond forming reactions). from e-EROS Encyclopedia of Reagents for Organic Synthesis.

Inorganic Chemistry, Vol. 42, No. 8, 2003
Following the published procedure,
solution containing 0.74 mL of methyl disulfide (8.06 mmol) in
7 mL of CH3CN was added dropwise to an equimolar amount of
Me3O+BF4- (1.04 g, 8.06 mmol) dissolved in 8 mL of CH3CN
at 0 °C. After the mixture had been stirred for 2 h at 0 °C, dry
ether was added to precipitate dimethylthiomethylsulfonium fluoroborate
([Me2SSMe+][BF4-]) as a white solid that was stored in
the glovebox at -36 °C (1.1 g; yield, 69%).

Curtius rearrangement

converts acid to carboxylic azides (use DPPA or acid chloride then NaN3) then upon heating rearrange to an isocyanate.

Then the isocyanate can be trapped by alcohol to form a carbamate.

related rxn: Hofmann Rearrangement, Schmidt Reaction.

Condition I used (to amine):

toluene, dppa, tea, 80C 2h,

then water added, 80C, overnight.

turned out the above transformation is not high yielding.
seems trapping the isocyanate by water is not a good idea.

generally, t-BuOH or BnOH was used to make a carbamate. then remove the carbamate by acid or hydrogenation.

2nd condition I used to make amine:
1. toluene, oxalyl chloride 60C. then concentrated invacuo.
2. acid chloride made in step 1 was dissolved in acetone, then added into NaN3 in water at zero degree. then rt. excess water was used to precipitate the product.
3. heated in toluene to 80C for 2 hr.
4. BnOH added. heated for another 2 hr.
isolate product by column.
5. MeOH, acoh, water + pd/c + H2. rt.
2 hr, then celite filtration.
K2CO3 solution was used to neutralize acoh and make free amine( pH>9). then extract the amine to DCM. the crude product is very clean. no column needed.

Oxidation of aldehyde to acid or ester

1. Jones reagent
acidic condition, strong oxidant.

2. I2/MeOH/KOH
forms methylester. mild condition, alkene is untouched.
can't make t-buylester by this way.
tert-aldehyde is more reactive than sec-aldehyde.

3. KMnO4/PH buffer/t-BuOH/water
mild condition, benzyl ether untouched.

4. NaClO2, t-BuOH, PH buffer
mild condition, works for very hindered aldehydes.
t-amlyne used to prevent alkenes from oxidation.