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Natural Science Forum / Chemistry / Organic Synthesis / December 2003Tertiary Alchol Dehydration
Are there any mechanisms/syntheses where a tertiary alcohol is dehydrated in an non-carbocationic pathway? For instance, instead of the "usual" R CH3 | | C - C - OH -> R = C (CH3)2 | | R' CH3 I'm trying to get to R | C - C = CH2 | | R' CH3 A potential caveat is a double bond elsewhere in the molecule that needs to be preserved. Additionally, R and R' form "opposite ends" of a ring structure so stearic hindrance is also a problem. I've looked at perhaps going to an alkyl halide then trying sn SN2 to remove the halide, but haven't found a path for that either. I've checked a few sources, including the orgsyn.com online library, to no avail. Does anyone out there have any pointers or suggestions I could check out? Thanks in advance. John Wilman jwilman3@comcast.net  SignaturePaul J. Franklin(moderator - sci.chem.organic.synthesis) http://organicworldwide.net/sci.chem.organic.synthesis Georgia State University <chepjf@panther.gsu.edu> Atlanta, GA > Are there any mechanisms/syntheses where a tertiary alcohol is > dehydrated in an non-carbocationic pathway? For instance, instead of > the "usual" > R CH3 > | | > C - C - OH -> R = C (CH3)2 > | | > R' CH3 > I'm trying to get to > [quoted text clipped - 3 lines] > | | > R' CH3 > A potential caveat is a double bond elsewhere in the molecule > that needs to be preserved. Additionally, R and R' form "opposite > ends" of a ring structure so stearic hindrance is also a problem. > I've looked at perhaps going to an alkyl halide then trying sn > SN2 to remove the halide, but haven't found a path for that either. > I've checked a few sources, including the orgsyn.com online > library, to no avail. Does anyone out there have any pointers or > suggestions I could check out? Thanks in advance. 1) Make the tosylate or triflate, kinetically eliminate to olefin with hindered base - as long as there is no accessible hydrogen to give a more substituted olefin. Don't hit it so hard as to foster isomerization. 2) Methyleneate the ketone (Wittig is probably too bulky) rather than make the tertiary alcohol. 3) Mild UV or thermal olefination crackings whose elimination goes through a cyclic transition state (e.g., xanthates). SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) "Quis custodiet ipsos custodes?" The Net! -- Paul J. Franklin(moderator - sci.chem.organic.synthesis) http://organicworldwide.net/sci.chem.organic.synthesis Georgia State University <chepjf@panther.gsu.edu> Atlanta, GA > Are there any mechanisms/syntheses where a tertiary alcohol is > dehydrated in an non-carbocationic pathway? For instance, instead of > the "usual" > R CH3 > | | > C - C - OH -> R = C (CH3)2 > | | > R' CH3 > > I'm trying to get to > > R > | > C - C = CH2 > | | > R' CH3 > > A potential caveat is a double bond elsewhere in the molecule > that needs to be preserved. Additionally, R and R' form "opposite > ends" of a ring structure so stearic hindrance is also a problem. > > I've looked at perhaps going to an alkyl halide then trying sn > SN2 to remove the halide, but haven't found a path for that either. > > I've checked a few sources, including the orgsyn.com online > library, to no avail. Does anyone out there have any pointers or > suggestions I could check out? Thanks in advance. 1) Make the tosylate or triflate, kinetically eliminate to olefin with hindered base - as long as there is no accessible hydrogen to give a more substituted olefin. Don't hit it so hard as to foster isomerization. 2) Methyleneate the ketone (Wittig is probably too bulky) rather than make the tertiary alcohol. 3) Mild UV or thermal olefination crackings whose elimination goes through a cyclic transition state (e.g., xanthates). SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) "Quis custodiet ipsos custodes?" The Net! -- Paul J. Franklin(moderator - sci.chem.organic.synthesis) http://organicworldwide.net/sci.chem.organic.synthesis Georgia State University <chepjf@panther.gsu.edu> Atlanta, GA -- Paul J. Franklin(moderator - sci.chem.organic.synthesis) http://organicworldwide.net/sci.chem.organic.synthesis Georgia State University <chepjf@panther.gsu.edu> Atlanta, GA You can try to use microwave-chemistry... Olivier Lohio > Are there any mechanisms/syntheses where a tertiary alcohol is > dehydrated in an non-carbocationic pathway? For instance, instead of > the "usual" > R CH3 > | | > C - C - OH -> R = C (CH3)2 > | | > R' CH3 > I'm trying to get to > R > | > C - C = CH2 > | | > R' CH3 > A potential caveat is a double bond elsewhere in the molecule > that needs to be preserved. Additionally, R and R' form "opposite > ends" of a ring structure so stearic hindrance is also a problem. > I've looked at perhaps going to an alkyl halide then trying sn > SN2 to remove the halide, but haven't found a path for that either. > I've checked a few sources, including the orgsyn.com online > library, to no avail. Does anyone out there have any pointers or > suggestions I could check out? Thanks in advance. > John Wilman > jwilman3@comcast.net SignaturePaul J. Franklin(moderator - sci.chem.organic.synthesis) http://organicworldwide.net/sci.chem.organic.synthesis Georgia State University <chepjf@panther.gsu.edu> Atlanta, GA > Are there any mechanisms/syntheses where a tertiary alcohol is > dehydrated in an non-carbocationic pathway? For instance, instead of > the "usual" > R CH3 > | | > C - C - OH -> R = C (CH3)2 > | | > R' CH3 > > I'm trying to get to > > R > | > C - C = CH2 > | | > R' CH3 > > A potential caveat is a double bond elsewhere in the molecule > that needs to be preserved. Additionally, R and R' form "opposite > ends" of a ring structure so stearic hindrance is also a problem. > > I've looked at perhaps going to an alkyl halide then trying sn > SN2 to remove the halide, but haven't found a path for that either. > > I've checked a few sources, including the orgsyn.com online > library, to no avail. Does anyone out there have any pointers or > suggestions I could check out? Thanks in advance. 1) Make the tosylate or triflate, kinetically eliminate to olefin with hindered base - as long as there is no accessible hydrogen to give a more substituted olefin. Don't hit it so hard as to foster isomerization. 2) Methyleneate the ketone (Wittig is probably too bulky) rather than make the tertiary alcohol. 3) Mild UV or thermal olefination crackings whose elimination goes through a cyclic transition state (e.g., xanthates). SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) "Quis custodiet ipsos custodes?" The Net! -- Paul J. Franklin(moderator - sci.chem.organic.synthesis) http://organicworldwide.net/sci.chem.organic.synthesis Georgia State University <chepjf@panther.gsu.edu> Atlanta, GA |
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