PDMS thimbles

Alternate Names PDMS dimethicone polymerized silox-ane silicone Sylgard 184 trimethylsUoxy-terminatedpolydi-methylsiloxane. 

Physical Data colorless free-flowing liquid average molecular weight is supplier dependent d 0.98 g 

Solubility most chlorinated, hydrocarhon, or ethereal solvents. Insoluble in MeOH, EtOH, glycerol, and water. Limited solubility in DMF and DMSO. 

Form Supplied in PDMS thimbles are fabricated from commercially available Sylgard 184 - a two-component prepolymer of PDMS. Typical impurities include residual Pt catalyst. This polymer has a high concentration of silica nanoparticles. 

Preparative Methods reaction between dimethylchlorosilox-ane and water yields PDMS. The anionic polymerization of hexamethylcyclotrisiloxane yields PDMS. PDMS thimbles are fabricated by curing Sylgard 184 in an oven. 

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Other Cascade Reactions with Incompatible Catalysts - Polydimethylsiloxane (PDMS) Thimbles for Generic Site Isolation... 

Figure 5.2 The use of hollow PDMS thimbles to achieve site separation ofGrubbs catalyst and an osmium dihydroxylation catalyst [34], The solution of the Grubbs catalyst was placed on the interior of the PDMS thimble in which a metathesis reaction was then performed. After...

The same approach has also been used in a reaction cascade involving 4-dimethy-laminopyridine (DMAP) and an acid catalyst [35], These two catalysts are mutually incompatible as the add quenches the DMAP, but site isolation using a PDMS thimble enables the cascade to proceed successfully (Figure 5.3). 

PDMS thimble contained in glass vial. The substrate, an acetal, is within the PD MS thimble and undergoes an acid-catalyzed transformation into an aldehyde. The aldehyde then diffuses to the exterior and undergoes the Baylis—Hillman reaction catalyzed by DMAP to give the product, (b) Various acid and base... 

Handling, Storage, and Precautions PDMS thimbles may be stored for over a year under ambient conditions. No specific precautions are necessary. 

Differential Flux of Reagents and Catalysts Through PDMS Thimbles. Flux of a molecule through a polymeric membrane depends on several factors, but two of the most important are the partition coefficient of a molecule from the solvent into the polymer and the rate of diffusion of a molecule in the polymer matrix. Simply, a molecule must be soluble in PDMS and possess a reasonable rate of diffusion within PDMS to flux through it. Numerous reviews cover these concepts with mathematical rigor. ... 

Preparation of PDMS Thimbles. The thimbles are readily fabricated from commercially available PDMS kits (Sylgard 184) from Dow Corning. The kit is a two-component mixture that is mixed in a 10 1 ratio to yield a viscous prepolymer of PDMS. A Pt catalyst found in one component adds a terminal [Si]-H bond across a [Si]-CH=CH2 to yield cross-links in the polymer matrix of the form [Si]-CH2CH2-[Si]. The cross-linking chemistry is robust even when the two components are mixed at ratios distant from 10 1. Importantly, amines inhibit the cross-linking reaction through coordination to the Pt catalyst. 

Molecules that possessed very low flux through the walls of PDMS thimbles included ionic liquids, p-toluenesulfonic acid, and polystyrene. Others have shown that ionic liquids have no solubility within PDMS and our results support this. For instance, the ionic liquid shown in eq 2 did not partition into PDMS. In addition, p-toluenesulfonic acid did not flux through PDMS when DMF was used as the solvent. The negligible flux was probably due to its deprotonation and the fact that ionic molecules have little to no solubility in PDMS. Finally, it was well known that polymers do not flux through polymeric membranes. When polystyrene (MW = 18000 g mol ) was added to the interior of a thimble with CH2CI2 on the interior and exterior, it did not flux through the walls of the thimble even after 3 days. ... 

In summary, a wide variety of organic molecules with various functional groups flux through PDMS thimbles, but care should be taken to consider the solvent, thickness of the walls, temperature, and the presence of any ionic functional group. 

Site Isolation of Water from L1A1H4. Water was successfully added to the interior of a PDMS thimble and site isolated from LiAlHa that was added to the exterior to complete a two-step, one-pot reaction as shown in eq 3. In this reaction, 74 equiv of water was added for every 1.25 equiv of L1A1H4, so control reactions where water and L1A1H4 were not site isolated by the walls of a PDMS thimble failed to yield the product. 

The use of butyl lithium was problematic because it rapidly reacted with the walls of the PDMS thimbles such that holes formed and water leaked out to quench any remaining butyl lithium. 

The Grubbs catalyst was recycled in a cascade sequence as shown in eq 11. The Grubbs catalyst was recycled as described before, but in these reactions the extracted product was epoxidized with m-CPBA after removal from the reaction vessel containing the PDMS thimble. The isolated 3delds of the product ranged from 60 to 86% through five cycles. 

Site Isolation of Polymeric Catalysts. A common method to site isolate catalysts from the products of a reaction is to attach the catalyst to a polymer that can be precipitated at the end of a reaction. " Catalysts attached to a polymer are generally not site isolated from other catalysts attached to a different polymer such that only one catalyst can be added to a reaction mixture. The use of PDMS thimbles solves this problem. ... 

It is well known that pol)miers do not flux through polymeric membranes. In experiments with PDMS thimbles and CH2CI2 as the solvent, the polymers shown in eqs 18 and 19 did not flux through the membranes even after 3 days. No evidence of the pol)miers was found on the exterior of the thimbles, which can be contrasted with the rapid flux of small molecules under identical conditions (eq 1). 

Striking differences in yields were observed in reactions mn with or without PDMS thimbles. The acid catalyst was either PTSA or commercially available beads with PTSA bonded to the backbone (eq 21). The basic catalyst was either DMAP, commercially available beads with DMAP bonded to the backbone (eq 22), or a linear polymer (eq 18). Six reactions were completed with either PTSA or the beads with PTSA and each of the three different bases. In reactions with PDMS thimbles to site isolate the acid from the base, the isolated yields of the final product were 71-93%. The lowest yields were when DMAP was the basic catalyst. This result was probably due to the flux of DMAP through the thimhles that quenched some of the acid on the interior of the thimhle. 

A series of cascade reactions with an acid catalyst on the interior of a PDMS thimble and a base catalyst on the exterior were completed (eq 20). In these reactions, the acetal was added to the interior of a thimble with an acid catalyst and methyl acrylate was added to the exterior with a basic catalyst. The solvent on the interior and exterior of a thimble was 7 1 (v/v) DMF H20. The reactions were heated to 70 °C for 72 h and then product was isolated. 

In five of the six reactions without thimbles to site isolate the acid and base catalysts, the isolated yields of the product ranged from 0 to 15%. Only in the reaction with polymeric beads with PTSA and beads with DMAP did the isolated yield reach 50%. In summary, the PDMS thimbles successfully site isolated polymeric catalysts from one another and this approach should work for other polymeric catalysts. 

Wacker-Tsuji oxidation of olefins (eq 23) and Pd-mediated homocoupling of aryl boronic acids (eq 24) were completed on the interior of PDMS thimbles. In each of these reactions, the catalyst was PdCl2 and a polar protic solvent was used on the interior of the thimbles. The products were fiuxed to the exterior of the thimbles by the addition of CH2CI2 or hexanes to the exterior. Isolated yields were 56-93%. 

The reason for the low flux of PdCl2 was that it was soluble only in polar protic solvents. Its solubility in PDMS was negligible, so it did not flux through the walls of the thimbles. When phosphines were added to the reaction mixture to coordinate to PdC and render it less polar, the phosphines and Pd readily fluxed through the walls of the PDMS thimbles. This method demonstrates that catalysts that do not partition into PDMS will remain site isolated by it. 

Final Notes. Small molecules, organometallic catalysts, inorganic catalysts, and polymeric catalysts were all successfully site isolated at levels up to >99.998% using PDMS thimbles. When site isolating catalysts or reagents, the most important parameters to consider are the solubilities and rates of diffusion within the swollen PDMS matrix and the solubilities in the organic solvents on the interior and exterior of thimbles. 

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