Home >
Community >
How can the trityl tetrafluoroborate function as a Lewis-acid catalyst
Upvote
27
Downvote
+ Catalysis
+ Chemistry
Posted by
Madbouly Alghobary
How can the trityl tetrafluoroborate function as a Lewis-acid catalyst
One point is that you're assuming that the "p" orbital in the center is the Lewis acidic site. This may not be the case, you can draw a resonance structure that breaks aromaticity but puts the cation at the para position of the phenyl ring. For example, the ortho and para positions are known hydride acceptors (link).
Another issues to consider is that the phenyls are bulky enough that they can't even remain coplanar.
So we can return to your issue of reversibility by looking at trityl via these two properties:
If the Lewis acidic site is via one of the phenyl rings, then the resultant adduct is destabilized by breaking aromaticity. It may be kinetically accessible and allow the trityl ion to serve as a Lewis acid, but the adduct is not stable.
If you do create an adduct with the central carbon, this adduct is greatly destabilized by steric repulsion. Enough so that fragmentation becomes likely.
In both cases, there is a kinetically accessible Lewis acid/Lewis base adduct, but this adduct has some kind of feature that makes it thermodynamically less stable. Therefore, the trityl ion can serve a catalytic role as a Lewis acid.
The exact mechanism of action (1) or (2) is likely dependent on the exact reaction you're looking at.
One point is that you're assuming that the "p" orbital in the center is the Lewis acidic site. This may not be the case, you can draw a resonance structure that breaks aromaticity but puts the cation at the para position of the phenyl ring. For example, the ortho and para positions are known hydride acceptors (link).
Another issues to consider is that the phenyls are bulky enough that they can't even remain coplanar.
So we can return to your issue of reversibility by looking at trityl via these two properties:
If the Lewis acidic site is via one of the phenyl rings, then the resultant adduct is destabilized by breaking aromaticity. It may be kinetically accessible and allow the trityl ion to serve as a Lewis acid, but the adduct is not stable.
If you do create an adduct with the central carbon, this adduct is greatly destabilized by steric repulsion. Enough so that fragmentation becomes likely.
In both cases, there is a kinetically accessible Lewis acid/Lewis base adduct, but this adduct has some kind of feature that makes it thermodynamically less stable. Therefore, the trityl ion can serve a catalytic role as a Lewis acid.
The exact mechanism of action (1) or (2) is likely dependent on the exact reaction you're looking at.
Im a little unconvinced by (2). Trityl protected alcohols are readily formed and require moderately forcing conditions to remove. I will however look for some literature on the phenyl group being the LA centre (intuitively this makes me uncomfortable but I can maybe buy into that argument)More
I think it depends on how hindered the alcohol is. I gave away my copy of Wuts/Greene, but you might want to check there for the types of alcohols that can be protected. My guess is that tertiary and maybe secondary alcohols are not effectively protected with trityl.More
One point is that you're assuming that the "p" orbital in the center is the Lewis acidic site. This may not be the case, you can draw a resonance structure that breaks aromaticity but puts the cation at the para position of the phenyl ring. For example, the ortho and para positions are known hydride acceptors (link).
Another issues to consider is that the phenyls are bulky enough that they can't even remain coplanar.
So we can return to your issue of reversibility by looking at trityl via these two properties:
If the Lewis acidic site is via one of the phenyl rings, then the resultant adduct is destabilized by breaking aromaticity. It may be kinetically accessible and allow the trityl ion to serve as a Lewis acid, but the adduct is not stable.
If you do create an adduct with the central carbon, this adduct is greatly destabilized by steric repulsion. Enough so that fragmentation becomes likely.
In both cases, there is a kinetically accessible Lewis acid/Lewis base adduct, but this adduct has some kind of feature that makes it thermodynamically less stable. Therefore, the trityl ion can serve a catalytic role as a Lewis acid.
The exact mechanism of action (1) or (2) is likely dependent on the exact reaction you're looking at.
One point is that you're assuming that the "p" orbital in the center is the Lewis acidic site. This may not be the case, you can draw a resonance structure that breaks aromaticity but puts the cation at the para position of the phenyl ring. For example, the ortho and para positions are known hydride acceptors (link).
Another issues to consider is that the phenyls are bulky enough that they can't even remain coplanar.
So we can return to your issue of reversibility by looking at trityl via these two properties:
If the Lewis acidic site is via one of the phenyl rings, then the resultant adduct is destabilized by breaking aromaticity. It may be kinetically accessible and allow the trityl ion to serve as a Lewis acid, but the adduct is not stable.
If you do create an adduct with the central carbon, this adduct is greatly destabilized by steric repulsion. Enough so that fragmentation becomes likely.
In both cases, there is a kinetically accessible Lewis acid/Lewis base adduct, but this adduct has some kind of feature that makes it thermodynamically less stable. Therefore, the trityl ion can serve a catalytic role as a Lewis acid.
The exact mechanism of action (1) or (2) is likely dependent on the exact reaction you're looking at.
More
VOTE
VOTE
VOTE