Vinyl iodide functional group

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General structure of vinyl iodides

In

drugs
.

Properties

Vinyl iodides are generally stable under

electrophilic center (see figure 1a).[2] In addition, the lone pair on iodide donates into the ╥* of the alkene, which reduces electrophilic character on the carbon as a result of decreased positive charge. Also, this stereoelectronic effect strengthens the C-I bond, thus making removal of the iodide difficult (see figure 1b).[3] In SN1 case, dissociation is difficult because of the strengthened C-I bond and loss of the iodide will generate an unstable carbocation(see figure 1c)[2]

Figure 1.

In cross-

bond dissociation energies of C-I is 57.6kcal/mol, while fluoride, chloride and bromide are 115, 83.7, 72.1 kcal/mol respectively.[4] As a result of having weaker bond, vinyl iodide does not polymerize as easily as its vinyl halide counterparts, but rather decompose and release iodide.[5]
It is generally believed that vinyl iodide cannot survive common
olefin or unsaturated alkane.[6] However, there is evidence in literature, in which a propargyl alcohol's alkyne was reduced in presence of a vinyl iodide using hydrogen over Pd/CaCO3 or Crabtree's catalyst.[7]

Parker's group reduction methods
Parker's group reduction methods

Other applications

Scheme 1. Magnesium-halogen exchange

Besides using vinyl iodides as useful substrates in transition metal cross-

electron withdrawing group can enhance rate of exchange(see Scheme 1b).[8] Also addition of lithium chloride helps enhance magnesium-halogen exchange (see Scheme 1c). It is predicted lithium chloride breaks up aggregates in organomagnesium reagents.[9]

Methods of synthesis

Nomenclature

Vinyl iodides are synthesized by methods such as

Suzuki coupling
, can retain olefin geometry. In synthesis, it is useful to introduce vinyl iodide at various positions to be set up for a coupling reaction at the next synthetic step. Below are various means and methods in introducing and synthesizing vinyl iodides.

Synthesis from alkynes

The common and simplest approach to make vinyl iodide is addition of one equivalent

stereoselectively.[10] As a result, most synthetic methods often involve a hydrometalation
step before addition of I+ source.

α-vinyl iodides

Introducing an α-vinyl iodide from a terminal position of an alkyne is a difficult step. in addition, the vinyl metal intermediate can be mildly

DIBAL-H with N-iodosuccinimide (NIS), selectively favor α-vinyl iodide with little to no byproducts.[11] Also they observed reverse selectivity for β with Ni(PPh3)2Cl2 in their hydroalumination
reactions under same conditions with little or no byproducts. The advantage of this method is that is inexpensive (and commercially available), scalable and one-pot reaction.

Hoveyda'group hydroalumination method
Hoveyda'group hydroalumination method

Another method doesn't involve hydrometalation but hydroiodation with I2/hydrophosphine binary system, which was developed by Ogawa's group.[12]

Ogawa's group Hydroiodation method with I2/hydrophosphine
Ogawa's group Hydroiodation method with I2/hydrophosphine

The hydroiodation proceeds by Markovnikov-type adduct, no reaction is observed without addition of hydrophoshine. In a plausible mechanism proposed by Ogawa's group, the hydrophosphine reacts with HI to form an intermediate complex that coordinate HI to do Markovnikov hydroiodation on the alkene. The advantage of this system is the conditions are mild, can tolerate wide range of functional groups.

Mechanism Proposed by Ogawa's group
Mechanism Proposed by Ogawa's group

β-vinyl iodides

They are generally more methods in making β-vinyl iodides versus α-vinyl iodides using

hydrozirconation)).[13] However, hydrometalation with alkyne with various functional groups often react poorly with side products. The Chong groups have demonstrated using hydrostannation, using Bu3SnH with palladium catalyst with high E stereoselectivity.[13]
They observed using sterically bulky ligands gave higher regioselectivity for β-vinyl iodide. The advantage of this technique is this technique can tolerate a wide range of functional groups.

Chong's group hydrostannation method
Chong's group hydrostannation method

Z selective β-vinyl iodides are slightly more difficult to introduce than E-β-vinyl iodides, often requiring more than one step. Hydroalumination and hydroboration usually proceed by syn fashion, therefore selectively favors E geometry. The Oshima group have demonstrated using hydroindation with HInCl selectively favors Z geometry.[14] They suggested that the reaction proceeds by a radical mechanism. They predict that HInCl adds to alkyne by radical addition in a Z geometry. It does not isomerized to E geometry because of low reactivity of radical InCl2 with intermediate complex (no second addition). If second addition occurs then isomerization will occur through diindium intermediate. They confirm a radical mechanism in a mechanistic study with alkyne and alkene cyclization.

Oshima's group hydroindation method
Oshima's group hydroindation method

Substitution

Substitution is perhaps most useful method in introducing vinyl iodide into the molecule. Halogen-exchange can be useful since vinyl iodides are more reactivity than other

regiochemistry and stereochemistry
is currently unexplored.

Buchwald's group halogen exchange method
Buchwald's group halogen exchange method

Halogen-exchange can also be done with zirconium derivatives that retain

olefin’s geometry[16]

Marek's group Halogen exchange method
Marek's group Halogen exchange method

The Marek group have further investigated using zirconium catalyst on E or Z

alkoxy group at the β position to form intermediate vinyl zirconium complex. The E geometry selectivity is not cause by sterics but rather the reaction itself is not concerted. In a mechanistic study, they observed isomerization, which suggest E geometry product is more favored than Z geometry. The difference of results between halogen exchange and E-vinyl ether reaction is that only when there is a presence of an oxonium intermediate, is isomerization
observed.

Marek's group zirconium vinyl iodide synthesis from vinyl ether
Marek's group zirconium vinyl iodide synthesis from vinyl ether
Scheme 2.

An interesting substitution reaction is vinyl boronic acid to vinyl iodide done by Brown's group.

ICl
which is more polar than I2, in which, they observed similar results (see scheme 2b).

Radical substitution of carboxylic acid to iodide is demonstrated by a modified Hunsdiecker reaction.[19] Homolytic cleavage of O-I bond generates CO2 and vinyl radical. Vinyl radical recombines with iodide radical to form vinyl iodide.

Modified Hunsdiecker reaction
Modified Hunsdiecker reaction

Iododesilylation

Iododesilylation is a substitution reaction of

silyl
group for iodide. The advantages of iododesilylation are that it avoids toxic tin reagent and intermediate vinyl silyl are stable, nontoxic and easily handled and stored. Vinyl silyl can be made from terminal alkyne or other methods.

The Kishi's group reported a mild preparation of vinyl iodide from vinyl silyl using NIS in mixture of acetonitrile and chloroacetonitrile.[20] They observed retention of olefin geometry in some vinyl silyl substrates while inversion in others. They reasoned that the R group's size had an effect on the geometry of the olefin. If the R group is small, the solvent acetonitrile can participate in the reaction leading to inversion of the olefin's geometry. If the R group is big, the solvent is unable to participate, leading to retention of olefin's geometry

Kishi's group iododesilylation method
Kishi's group iododesilylation method

Zakarian's group then decided to run the reaction in

acetonitrile. In addition, they observed accelerated reaction rate because HFIP activate NIS by hydrogen bonding
.

Zakarian's group HFIP iododesilylation method
Zakarian's group HFIP iododesilylation method

Unfortunately, iododesilylation under those conditions (above) can potentially yield multiple byproducts in highly functionalized molecules with oxygen

radical reactions producing HI and I2 help facilitate cleavage in alcohol's protecting group and may add into other alkene bonds.[22] They experimented with the use of silver additives such as silver acetate and silver carbonate in which the silver can react with the excess iodide to form silver iodide
. They achieved better conversions with these conditions.

Name reactions

Some famous vinyl iodide synthesis methods involve conversion of

Cortistatin A by Shair.[26]
Another method is the
HMPA.[28]

Stork-Zhao Olefination
Stork-Zhao Olefination

Below is example of employing both Takai olefination and Stork-Zhao olefination in total synthesis of (+)-3-(E)- and (+)-3-(Z)-Pinnatifidenyne.[29]

Employment of Takai and Stork-Zhao olefination by Kim's group
Employment of Takai and Stork-Zhao olefination by Kim's group

Elimination method

Vinyl iodides are rarely by made an elimination reaction of vicinal diiodide because it tends to decompose to alkene and iodide.[30] The Baker group have shown using decarboxylation, elimination can occur.[31]

Baker's group elimination method
Baker's group elimination method

See also

  • List of functional groups
  • Group contribution method

References

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    .Takami, Kazuaki, et al. "Triethylborane-mediated hydrogallation and hydroindation: Novel access to organogalliums and organoindiums." The Journal of Organic Chemistry 68.17 (2003): 6627-6631.
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    .Danishefsky, Samuel J., et al. "Total synthesis of baccatin III and taxol." Journal of the American Chemical Society 118.12 (1996): 2843-2859
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