Geminal substitution

Fukumoto epoxide rearrangement is worthy of note, in that a stereoselective rearrangement occurs spontaneously during the SAE of a 2-aryl-2-cyclopropylide-neethanol (194), prepared from ketone 193, to give the geminally substituted cydo-... 

Similarly, energy-transfer processes, together with electron transfer and hydrogen abstraction reactions could be induced in poly(organophosphazenes) in an intramolecular way by preparing POPs geminally substituted at the same phosphorus with two different substituent groups. 

The mono-alkylated species N3P3C15R is a good synthon for the generation of geminally substituted products by further reaction with Grignard reagents (Eq. 4) (150)... 

This Pd(0)/formic acid system was effective for the cyclization of substituted 5-allene-l-ynes to give diene 140 (Eq. 25) through initial insertion into the internal 7r-bond of the allene followed by insertion into the alkyne [79]. All of the examples provided were geminally substituted within the backbone to facilitate cyclization. Intramolecular allene-alkyne reductive couplings to generate six-membered rings were not achieved. 

In a similar way as described for the hydroformylation, the rhodium-catalyzed silaformylation can also be used in a domino process. The elementary step is the formation of an alkenyl-rhodium species by insertion of an alkyne into a Rh-Si bond (silylrhodation), which provides the trigger for a carbocyclization, followed by an insertion of CO. Thus, when Matsuda and coworkers [216] treated a solution of the 1,6-enyne 6/2-87 in benzene with the dimethylphenylsilane under CO pressure (36 kg cm"2) in the presence of catalytic amounts of Rh4(CO)12, the cyclopentane derivative 6/2-88 was obtained in 85 % yield. The procedure is not restricted to the formation of carbocycles rather, heterocycles can also be synthesized using 1,6-enynes as 6/2-89 and 6/2-90 with a heteroatom in the tether (Scheme 6/2.19). Interestingly, 6/2-91 did not lead to the domino product neither could 1,7-enynes be used as substrates, while the Thorpe-Ingold effect (geminal substitution) seems important in achieving good yields. 

Hyperconjugation does not seem to have much effect on alkene reactivity towards bromine, since (16) applies whatever the number of alkyl groups on the double bond. However, only cis-olefins are involved in this correlation. To include geminally substituted olefins, an additional term is necessary, as in (19) where d is unity for the pem-disubstituted compounds and zero for... 

In a projected synthesis of verrucarol in which the tetrahydropyranone 175 is envisioned to play a role, the lactone 176 can serve as a viable intermediate (Scheme 6)7S>. Using cyclobutanone annulation methodology, 177 becomes the required intermediate. The geminal substitution of 177 can also be recognized to derive from a spiroannulation approach as previously analyzed. 

A number of additional methods involve the addition of alkynylsilanes to electrophiles with concomitant 1,3-isomerization to afford allenylsilanes geminally substituted with the electrophile moiety. The first of these methods employed a trimethyl-silyl-substituted propargylic silane as the alkynylsilane and various acetals as the electrophile precursors (Table 9.29) [53], The allenylsilanes are formed without contamination by alkynyl isomers. 

The diastereomeric 2-cli loro-2-nitroso-p-menthanc and 3-chloro-3-nitroso-p-menthane (267) epimerize during photolysis143 (equation 119) and can concurrently give the nitrox-ide 269 as detected by ESR spectrometry, which confirms the mechanism for the photolysis of geminally substituted nitroso compounds (equations 120 and 121). 

The canonical structures such as 66 suggest that the pseudo-ortho and pseudo-para positions would be the preferred site(s) for electrophilic substitution. However, the bromination of 4-carbomethoxy- and of 4-acetyl[2.2]paracyclophane gave only the pseudo-geminal substituted product 83b>. 

Fig. 12. Mechanism of the electrophilic substitution leading to pseudo-geminal substitution...

Four other significant homologous series present are gem-6 y substituted w-alkanes. The gem prefix designates geminal-substituted compounds, i.e. two substituents on the same atom of a disubstituted compound. Several research groups have reported the presence of 5,5-diethylalkanes, as well as lesser amounts of other g zw-dialkylalkanes, in sedimentary rocks back to the Precambrian and in hydrothermal fluids. 

Indeed, in a study of reaction rate and efficiency as a function of geminal substitution of the VCP, it was found that heteroatom substitution was not required to activate the VCP for intermolecular [5+2] cydoaddition (Tab. 13.14) [51]. Substitution for -H at the 1-position of the cyclopropane with -Me, -TMS, or - Pr led to an acceleration of the reaction that correlated with size of the substituent. Significantly, good to excellent yields and high, if not complete, regioselectivities are observed with a variety of mono-substituted alkynes and simple, unactivated VCPs. 

With sodium methoxide, arylidene derivatives 209 undergo a rearrangement into photochromic dihetaryl-substituted cyclopentenediones 211. They are easily alkylated to form geminally substituted products 212 in 91-94% yields (07MC301). Their photochemical properties were investigated (07OS975, 08IZV853). 

The addition of LiCl and Cul, and triphenylarsine as ligand are required to suppress side-reactions in the carbonylative cross-coupling of geminally substituted alkenyl triflate in the synthesis of sarcodictyin. Stereochemical configuration of the double bond of organotin compound was completely lost in this reaction (Equation (5)). " ... 

In an interesting adaptation of the cyclocondensation synthesis depicted in eqn (11.26), both the cis and irons isomers of non-geminally substituted [(Ph)(Me)PN]3 have been prepared and separated by using column chromatography on silica gel [eqn (11.27)]. ° ... 

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